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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
This textbook provides an integral and integrated treatment of industrial-relevant problems for students of both chemistry and chemical engineering.
As such, this work combines the four disciplines of chemical technology - chemistry, thermal and mechanical unit operations, chemical reaction engineering and general chemical technology - and is organized into two main parts. The first covers the fundamentals, as well as the analysis and design of industrial processes, while the second section presents 20 concrete processes, exemplifying the inherent applied nature of chemical technology. These are selected so that they all differ with respect to at least one important aspect, such as the type and design of the reactor, the chemistry involved or the separation process used. As a result, readers will recapitulate, deepen and exercise the chemical and engineering principles and their interplay, as well as being able to apply them to industrial practice.
Instructive figures, rules of thumb for swift but reliable estimating of parameters, data of chemical media, and examples utilizing data from industrial processes facilitate and enhance the study process. A small general survey of selected modern trends, such as multifunctional and micro reactors, or new solvents for homogeneous catalysis, such as ionic liquids, point out to the reader that this is not a concluded discipline,
but a developing field with many challenges waiting to be solved.
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Seitenzahl: 1832
Veröffentlichungsjahr: 2013
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Contents
Cover
Related Titles
Title Page
Copyright
Dedication
Preface
Notation
Chapter 1: Introduction
1.1 What is Chemical Technology?
1.2 The Chemical Industry
Chapter 2: Chemical Aspects of Industrial Chemistry
2.1 Stability and Reactivity of Chemical Bonds
2.2 General Classification of Reactions
2.3 Catalysis
Chapter 3: Thermal and Mechanical Unit Operations
3.1 Properties of Gases, Liquids, and Solids
3.2 Heat and Mass Transfer in Chemical Engineering
3.3 Thermal Unit Operations
3.4 Mechanical Unit Operations
Chapter 4: Chemical Reaction Engineering
4.1 Main Aspects and Basic Definitions of Chemical Reaction Engineering
4.2 Chemical Thermodynamics
4.3 Kinetics of Homogeneous Reactions
4.4 Kinetics of Fluid–Fluid Reactions
4.5 Kinetics of Heterogeneously Catalyzed Reactions
4.6 Kinetics of Gas–Solid Reactions
4.7 Criteria used to Exclude Interphase and Intraparticle Mass and Heat Transport Limitations in Gas–Solid Reactions and Heterogeneously Catalyzed Reactions
4.8 Kinetics of Homogeneously or Enzyme Catalyzed Reactions
4.9 Kinetics of Gas–Liquid Reactions on Solid Catalysts
4.10 Chemical Reactors
4.11 Measurement and Evaluation of Kinetic Data
Chapter 5: Raw Materials, Products, Environmental Aspects, and Costs of Chemical Technology
5.1 Raw Materials and Energy Sources
5.2 Inorganic Products
5.3 Organic Intermediates and Final Products
5.4 Environmental Aspects of Chemical Technology
5.5 Production Costs of Fuels and Chemicals Manufacturing
Chapter 6: Examples of Industrial Processes
6.1 Ammonia Synthesis
6.2 Syngas and Hydrogen
6.3 Sulfuric Acid
6.4 Nitric Acid
6.5 Coke and Steel
6.6 Basic Chemicals by Steam Cracking
6.7 Liquid Fuels by Cracking of Heavy Oils
6.8 Clean Liquid Fuels by Hydrotreating
6.9 High Octane Gasoline by Catalytic Reforming
6.10 Refinery Alkylation
6.11 Fuels and Chemicals from Syngas: Methanol and Fischer–Tropsch Synthesis
6.12 Ethylene and Propylene Oxide
6.13 Catalytic Oxidation of o-Xylene to Phthalic Acid Anhydride
6.14 Hydroformylation (Oxosynthesis)
6.15 Acetic Acid
6.16 Ethylene Oligomerization Processes for Linear 1-Alkene Production
6.17 Production of Fine Chemicals (Example Menthol)
6.18 Treatment of Exhaust Gases from Mobile and Stationary Sources
6.19 Industrial Electrolysis
6.20 Polyethene Production
References
Index
Related Titles
Baerns, M., Behr, A., Brehm, A., Gmehling, J., Hinrichsen, K.-O., Hofmann, H., Onken, U., Palkovits, R., Renken, A.
Technische Chemie
2013
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Arpe, H.-J., Hawkins, S
Industrial Organic Chemistry
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Hagen, J.
Industrial Catalysis
A Practical Approach
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Green, M. M., Wittcoff, H. A.
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Froment, G. F. / Bischoff, K. B. / De Wilde, J.
Chemical Reactor Analysis and Design
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Moulijn, J. A., Makkee, M., van Diepen, A.
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
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Cover Design Adam-Design, Weinheim
Typesetting Thomson Digital, Noida, India
Print ISBN: 978-3-527-30446-2
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To our wifes Christina and Talke and our childrenAntonia, Friederike, Jonathan, Karolin, Lukas, and Theresa.
Preface (and Guidelines how to Use this Textbook)
This textbook tries to marry the four disciplines of chemical technology, namely, chemistry (key reactions, catalysis), thermal and mechanical unit operations (distillation, absorption/adsorption, mixing of fluids, separation of solids from fluids, etc.), chemical reaction engineering (thermodynamics, kinetics, influence of heat and mass transfer, reactor modeling), and general chemical technology, that is, the pedigree of routes from raw materials via intermediates to final products and environmental aspects chemical technology.
The development and understanding of chemical processes relies on knowledge of all four disciplines. This book is an approach to integrating these disciplines and to enlivening them by problems and solutions of industrial practice. The book intends to enable students of chemical engineering as well as of chemistry (especially those with a focus on technical chemistry) to understand industrial processes and to apply these fundamental disciplines for the design of reactors, including pre-and post-treatment of feedstocks and products.
We emphasize that the depth of specialist literature cannot and should not be reached; but students who want to study certain aspects in more detail will find further references.
The book is organized into three main parts:
The first half provides a comprehensive examination of the fundamental disciplines of chemical technology: After a short introduction, the subsequent chapters survey chemical aspects (Chapter 2), thermal and mechanical unit operations (Chapter 3), and chemical reaction engineering (Chapter 4).Chapter 5 gives an overview of raw materials and energy sources (fossil fuels, renewable energy), in which economical, ecological and social aspects of energy consumption are also covered. Thereafter, the routes from fossil fuels (natural gas, oil and coal) to fuels, petrochemicals, bulk chemicals, and final products are described, and the main inorganic intermediates and final products are also given. Finally, some main environmental aspects of chemical technology (air and water pollution), and the costs of manufacturing fuels and chemicals are discussed.In the last part (Chapter 6), 20 industrial chemical processes and their design are analyzed, exemplifying the inherent applied nature of chemical technology. By this means the reader can recapitulate, deepen, and exercise the chemical and engineering principles and their interplay (hopefully learned in Chapters 1–5), and will extend them to industrial practice. The processes have been selected such that they all differ with respect to at least one important aspect like the type and design of the reactor, the chemistry involved, or the separation process used.The book is supplemented by a brief survey of selected modern trends such as microreactors, and new solvents for catalysis like ionic liquids, which should convince the reader that chemical technology is not a “completed” discipline, but a developing field with huge future challenges such as, for example, with regard to solving the energy problems for generations to come.
Several chapters are based on Bachelor and Master Courses the authors have taught students of chemistry and of chemical engineering at different Universities for many years (Aachen, Bayreuth, Erlangen, Karlsruhe). We have tried to consider the challenges specific to instructing chemists and engineers in chemical technology; for example, the problems both groups have in integrating the different disciplines: According to our experience, chemists tend to be too anxious with regard to chemical engineering methods (and most notably with the mathematics involved). In contrast, engineers often feel uncomfortable if chemical aspects have to be examined and come to the fore.
We hope that students both of (technical) chemistry and chemical engineering will appreciate this book, and that chemical engineers will acquire a sufficient feeling for chemistry and, likewise, chemists for the principles of chemical engineering.
To facilitate learning, the reader will find many instructive figures, examples, and rules of thumb for estimations of parameters and data of chemical media, many examples utilizing data from industrial processes, and in some cases partly the results of the authors' research. Complicated mathematical operations will only be used if mandatory. Numerous literature references are cited to guide the reader, where certain aspects are documented in more detail.
To simplify consultation of this textbook, several equations are accentuated by two types of exclamation marks:
To illustrate certain aspects in more detail and to facilitate the use of the derived equations, several insertions marked either as “topics” or “examples” have been added.
At the end of each main chapter, a summary with “take-home messages” is given.
This book cannot, and is not intended to, compete with specialized textbooks, but hopefully gives a comprehensive and integrated outline of the fascinating subject of chemical technology and all its facets. It intends to be of value to all students of chemical engineering and technical chemistry, as well as to researchers and people from industry needing a concise book that covers all main aspects of industrial chemistry.
A book such as this could not have appeared without the sustainable help of a number of people. Only a few of them can be mentioned by name.
Our understanding of chemical technology owes much to having been fortunate in working and discussing the subject with Professor Wilhelm Keim, Professor Kurt Hedden, and Prof. Gerhard Emig, and we are grateful to all three of them.
We would like to thank our students, who followed our courses and/or did their PhD thesis in our institutions. They provided us with plenty of feedback.
We express special gratitude to Dr. Christoph Kern, Dr. Wolfgang Korth, and Professor Bastian Etzold for fruitful discussions, ideas and critiques, Michael Gebhardt and Dr. Stephan Aschauer for all the work and care invested in preparing numerous figures, Dr. Eva Öchsner and Dr. Sebastian Willmes for their assistance in preparing Chapters 6.9, 6.11.2, 6.12, 6.15, Prof. Udo Kragl for his assistance in preparing Chapter 2.3, and Markus Preißinger and Andreas Hofer for proofreading.
We would also like to thank the production team at Wiley-VCH, particularly Waltraud Wüst and Karin Sora.
Finally, we would like to express our appreciation to our wives and children, who witnessed the writing of this book in so many evening and weekend hours, continuously encouraged us, and patiently allowed us to spend a considerable amount of time during the last eight years on the preparation of this book.
If you like this book, please recommend it to others. If you have suggestions for improvements or discover faults (inevitable despite of all our efforts) please send us an e-mail1).
Prosit! (Latin: it may be useful)Andreas Jess ([email protected])Peter Wasserscheid ([email protected])Bayreuth/Erlangen, October 2012
Notation
The International System of Units (abbreviated SI from the French Système international d'unités) developed in 1960 is the modern form of the metric system. This system is nowadays used in many countries both in everyday life and in science. Unfortunately, the popular use of SI units is still limited in important countries like the USA and the UK, although this may lead to mathematical mismatches with disastrous consequences (see Section “Critical units” at the end of this chapter).
As listed in Table 1, the SI defines seven base units, namely, meter, kilogram, second, ampere, kelvin, mole, and candela. All other units can be derived from these base units. Frequently used SI derived units are newton (N), the unit of force (1 N = 1 kg m s−2), pascal (Pa), the unit of pressure (1 Pa = 1 N m−2 = 1 kg m−1 s−2), joule (J), the unit of energy (1 J = 1 N m = 1 kg m2 s−2), and watt (W), the unit of power (1 W = 1 J s−1 = 1 N m s−1 = 1 kg m2 s−3).
Table 1 The seven base units of the SI.
Throughout this book, all equations (and the respective symbols listed in Table 2) are related to SI units. As a consequence and a general rule for this book: insert all variables in SI units into the equations and you will always get the correct result of a certain quantity in SI units. Nevertheless, the results of calculations are sometimes given in “handier” units, for example, with a prefix such as kJ or MJ instead of 1000 J or 1 000 000 J, or the well-known unit bar (= 105 Pa) is used for the pressure instead of Pa.
Table 2 Symbols and abbreviations used in this book.
Simple abbreviations or subscripts such as A and B to denote the components as well as n and m as variables for reaction orders, or integration constants, are subsequently not listed.
Comments on the Symbols Used in this Book
Throughout this book, we have tried to use available standards for all our symbols. Hence, most of our symbols agree with common practice. Unfortunately, there is yet no standard set of symbols in chemical engineering. Most notably, there are still differences between European and American practice. For the reader's convenience, Table 3 summarizes some important deviations of our symbols from the practice of others.
Table 3 Meaning and definition of symbols.
Presentation of Measured Values and Confidence Limits
Presentation of Measured Values
In general, the result of a measurement is represented exactly by the measured (mean) value x, the measurement error Δx, and the unit [x]:
For example the measured value of a length is correctly represented by:
Nevertheless, in most cases, the measured value is only given with the respective unit. For the above given example, a value of L = 5.81 m would indicate that the true value is somewhere between 5.805 and 5.815 m. Thus a rough indication and crude way to represent the error is also provided by the number of significant figures (also called significant digits). Rounding to significant digits is a more general technique than rounding to n decimal places, since it handles numbers of different scales in a uniform way.
For example, if rounding to 3 significant figures:
19.341 becomes 19.30.034 567 8 becomes 0.034 634 560 becomes 34 600.One issue with rounding to significant digits is that the value of n is not clear if the last digit(s) is (are) zero. For example in the final example above, n could be anything from 3 to 5, that is, the value may lie in between 34 500 and 34 700.
The number of significant digits is particularly important with regard to spurious accuracy, as revealed by the following example. In September 2011, the Kenyan long-distance runner Patrick Makau broke the marathon world record in 2:03:38 h (7418s). If we use a calculator, which is usually equipped with ten decimal places, and divide the distance (42,195 m) by the time, we get an average speed of 5.688 190 887 m s−1 which is equivalent to 5 688 190 887 nm s−1. Quite evidently, the speed of the runner is not known with an accuracy of nm s−1, and so it is probably more sensible to report it only to four significant digits (5.688 m s−1) as the time is also only measured to four significant digits (s).
Experimental results are also frequently evaluated by a pocket calculator or a computer, for example, the electrical resistance of a wire may be derived by dividing the adjusted voltage by the measured current. As a general rough rule as to how to present experimental results we may state that the last but one digit should be really accurate and be secured by the experimental method used. In other words, you are not accountable for the last but for the last but one digit.
A general rule of how to handle additions, subtractions, multiplications, and divisions of experimental values is that the number of significant digits of the result of such operations is never greater than the smallest value of all significant figures. For example, the product of 1.142 and 2.345 678 should be given as 2.679 (and not as 2.6 787 642 764 . . .), or the sum of 1.142 and 2.345 678 should be given as 3.488 (and not as 3.487 678).
Mean Value and Confidence Limits
Usually, we carry out experiments where we measure the value of a certain quantity n times. Now we want to know the mean value μ and the confidence interval. The confidence limits for µ are given by:
The factor t depends on the significance level and on the number of measurements (Table 4). For the significance level, values of 5% or 1% are typically chosen, which is equivalent to a confidence coefficient of 95% and 99%, respectively. In most cases, the confidence coefficient is set to 95%.
Table 4 The “t” table.
Example: The reaction rate (at constant reaction conditions, i.e., constant concentration, temperature, etc.) is measured 10 times (n = 10, Table 5). The confidence coefficient is set to 95%, and the t value is then 2.262 (see Table 4 above). Thus we get:
Table 5 Example of how to determine the mean value and confidence interval.
Number of measurementMeasured reaction rates x (e.g., in mol m− s−1)x212.054.2022.335.4331.983.9242.054.2052.295.2462.104.4171.973.8881.963.8492.214.88101.943.76n = 10 = 2.088 = 43.78 = 436.0Here we obtain for the standard deviation:
and thus we, finally, have:
In other words, at 95% confidence, we have a true mean value of the reaction rate lying between the 2.189 and 1.987.
Problem of Outlier
It sometimes occurs in a series of n + 1 measurements that one value (xn+1) lies far from the other values. A criterion as to whether this so-called outlier can be omitted is:
The value of k depends on the number of measurements n. For n > 10, k = 4, and for lower values of n, k increases (e.g., for n = 4, k is about 7).
For the given example (with k = 4 and s = 0.141) we get:
and thus if the value of the rate of the outlier would be more than 2.65 (= 2.088 + 0.564) or less than 1.54 (= 2.088 − 0.564) it can be omitted.
Critical Units
Conversions from one unit to another are very important. Two examples may illustrate this (found and adopted in/from S. S. Zumdahl (2009) Chemical principles. Brooks/Cole, Belmont, USA).
If you ask the National Aeronautics and Space Administration (NASA), such conversions are very important. In 1999 NASA lost the $ 125 million Mars Climate Orbiter just as it was approaching the red planet. The problem? Unit conversion. NASA's scientists and engineers in Pasadena worked in metric units, and assumed that the thrust data for the rockets on the orbiter they received from Lockhead Martin Aeronautics in Denver, which built the spacecraft, were in metric units. In reality, the units were imperial (pounds, miles, etc.), and the lack of conversion meant that the orbiter approached Mars from an altitude of 60 kilometers instead of 150 kilometers. As the result the friction from the atmosphere caused the craft to burn up. The root cause of the failure was that the flight system written to take thrust instructions used the metric unit newton (N), while the software on the ground, which generated those instructions used the imperial measure pound-force (lbf). As a result, the thruster firings were underestimated by a factor of 4.45 (1 lbf = 4.45 N).The Gimli Glider is the nickname of an Air Canada aircraft that was involved in an aviation incident in 1983. At an altitude of 12,500 m is ran out of fuel halfway through its flight from Montreal to Edmonton via Ottawa The crew was able to glide the aircraft safely at a former Canadian Air Force base in Gimli, Manitoba. The subsequent investigation revealed that someone pumped 22,300 pounds of kerosene into the aircraft instead of 22,300 kilogramms.The moral of both stories: remember to watch your units!
Piping and Instrumentation Symbols Used in Flow Schemes (Table 6)
Table 6 Flow scheme symbols.
ValveHeat exchangerPumpCompressorMultistage compressorFilterCycloneTurbineGeneratorOvenCombustor furnace chamberCondenserSeparator (gas/liquid)Column with traps (distillation, absorption)Distillation columnAbsorption/extraction columnFixed-bed reactor adsorption columnSymbols Used in Measuring and Control Technology
Measurement and control devices in flow schemes of chemical plants are denoted by a combination of up to four letters:
First letter (measured variable):
Supplement letter:
Consecutive letters (measured data processing):
Examples:
1
Introduction
1.1 What is Chemical Technology?
The field of chemical technology stands between:
classical chemistry, which is the science concerned with the composition, behavior, and properties of matter, and with the changes it undergoes during chemical reactions andchemical engineering, which is the branch of engineering that deals with the application of chemistry, physics, life sciences, and mathematics to processes of converting raw materials or chemicals into more useful or valuable forms or to environmental processes, and involves the design of large-scale processes and also of laboratory-scale plants. Modern chemical engineering is also concerned with the design and synthesis of new materials, new techniques such as nanotechnology, and is one of the major disciplines related to energy technology such as the development of efficient fuel cells, energy storage systems, and the use of solar energy and other renewables.In the chemical industry, natural scientists (primarily chemists, but also biologists and physicists), engineers, and also business men form a team, and the following questions may, for example, be important:
What is the amount and purity of the product that the client would like to have?What apparatus are suitable to produce a certain chemical?How can the heat be provided for an endothermic reaction and how can we cool a reactor in the case of an exothermic reaction?Is the process safe and economic?How can a current process be improved?What type of equipment is needed to separate a reaction mixture?Which catalyst is the best and for how long is the catalyst still active?Is the process environmentally benign?Chemical technology should give answers to all these questions, and relies mainly on knowledge of the following four key disciplines and on their application and integration:
This book covers all four disciplines: chemical aspects in Chapter 2, thermal and mechanical unit operations (Chapter 3), reaction engineering (Chapter 4), and general chemical technology (Chapter 5). In addition, 20 industrial processes are inspected in detail (Chapter 6).
1.2 The Chemical Industry
For all industrialized countries the chemical industry is an important part of the economy. However, compared to the oil, gas, and coal industries – which are equally reliant on chemical technology – the chemical industry is relatively small. In 2011, six of the ten (and ten of the 20) most important companies by revenue were primarily oil and gas companies, and the biggest chemical company (BASF) was ranked only 62 (Table 1.2.1). Thus the chemical industry, which produces chemicals ranging from base chemicals to fine chemicals mainly from crude oil derivatives, such as naphtha and liquefied petroleum gases (LPG), is still has a “free ride” in terms of energy consumption, which is still mainly driven by crude oil.
Table 1.2.1 List of the 20 most important companies by revenues in 2011. Data from http://en.wikipedia.org/wiki/List_of_companies_by_revenue (accessed on 04.09.2012).
The ten largest chemical companies (without pharmaceuticals) by sales and a geographic breakdown of world chemicals sales are listed in Tables 1.2.2 and 1.2.3, respectively. In recent years the role of the chemical industry in the European Union (EU-27) and in North America has decreased; for example, in 2000 the EU-27 share of the global production of chemicals was about 29%, whereas the value for 2010 is only 21%. The share of Asia (without Japan) has increased in this period from 21% to 42%. Table 1.2.4 lists the top ten pharmaceutical companies.
Table 1.2.2 The 10 largest chemical companies by sales in 2007 and 2010 (without pharmaceuticals. Data for 2007 from Behr, Agar, and Joerissen [2010] and for 2010 from International Chemical Information Service, www.icis.com (accessed on 04.09.2012).
Table 1.2.3 Geographic breakdown of world chemicals sales in 2010 (production of chemicals excluding pharmaceuticals; data from www.cefic.org/facts-and-figures, accessed 18.09.2012).
Table 1.2.4 The 10 largest pharmaceutical companies by 2011 sales. http://de.wikipedia.org/wiki/Pharmaunternehmen-Gro.C3.9Fe_Pharmaunternehmen (accessed on 04.09.2012).
CompanyCountrySales in 2011 in billion US $PfizerUSA58NovartisSwitzerland54Merck & Co.USA41Sanofi-AventisFrance37Hoffmann-La RocheSwitzerland35GlaxoSmithKlineUnited Kingdom34AstraZenecaUnited Kingdom34Johnson & JohnsonUSA24AbbottUSA22Eli LillyUSA22Table 1.2.5 gives the annual global production of important chemicals in 2003. In general, the structure of the chemical industry is characterized by a small number of base chemicals such as ammonia, ethylene, and chlorine, which are further converted into many intermediates such as ethylenoxide, styrene and vinyl chloride and finally into a huge number of chemical consumer goods such as pharmaceuticals or polymers (Table 1.2.5).
Table 1.2.5 World production of important chemicals in 2003 (Baerns et al., 2006).
ProductMillion tonnesBillion €Inorganic base chemicalsSulfuric acid (100%)170Ammonia111Nitrogen-fertilizers (counted as N)90Chlorine43Phosphor-fertilizers (counted as P2O5)37Potassium salts (counted as K2O)28Organic chemicalsEthylene100Propylene56Benzene32Methanol29Polymers202Pharmaceuticals466Pesticides25Today, bulk chemicals are increasingly produced in Asia and in the Middle East and not in Europe, Japan, and North America. For example, in Germany, the most important chemicals are fine chemicals and pharmaceuticals, with a share of 46%, whereas the role of organic and inorganic base chemicals is comparatively small (26%) (Table 1.2.6).
Table 1.2.6 Important products of the German chemical industry for 2007 (Behr, Agar, and Joerissen,).
ProductsShare of total production value (%)Organic base chemicals18Inorganic base chemicals8Polymers and rubber18Chemical fibers2Fine and specialty chemicals26Pharmaceuticals20Soaps, detergents, and cosmetics8Global sales in the oil and gas industry are of the same order of magnitude as those of the world's chemical and pharmaceutical industry (Table 1.2.7). If the global oil and gas consumption and the respective average prices are taken as an estimation of sales, we obtain values of these two businesses of €2000 and €800 billion a−1, respectively, compared to sales for the global chemical and pharmaceutical industry of €2500 billion a−1 (basic chemicals, life sciences, fine chemicals, and consumer products, see Tab. 1.2.7). Within the chemical and pharmaceutical industry, the share of the sales of basic chemicals (including polymers) is 36% followed by life science products (mainly pharmaceuticals) with 30%, and fine chemicals and consumer products with 23% and 11%, respectively.
Table 1.2.7 Sales of the oil & gas industry (only oil and gas business) and sales of the chemical and pharmaceutical industry in 2008 (estimations based on various sources).
2
Chemical Aspects of Industrial Chemistry
2.1 Stability and Reactivity of Chemical Bonds
Chemical reactions proceed by the linking and/or cleaving of chemical bonds. If we take the molecule A-B, for example, the covalent bond between A and B can be broken homolytically or heterolytically. In the first case each atom A and B receives one unpaired electron to form radicals, in the second case both electrons of the chemical bond go with either A or B, forming charged species (Scheme 2.1.1).
Scheme 2.1.1 Different modes of breaking the covalent bond in molecule A–B, forming either radicals (a) or charged species (b, c).
Radicals and charged species play a very important role as reactive intermediates in various organic transformations. Even if they may be present only in small quantities and for a short time (and thus are difficult to measure analytically), they play a crucial role in the mechanism of the ongoing reaction.
Note that the reverse reactions to those shown in Scheme 2.1.1 play a very important role for the formation of new covalent bonds. In addition, radicals or charged species can attack neutral compounds to form different radicals and charged species involving new chemical bonds. Scheme 2.1.2 gives examples of some practical relevance in chemical technology. In transformation (a), a methyl radical attacks a chlorine molecule to form chloromethane and a chlorine radical. This reaction is one of the key steps in technical methane chlorination. In transformation (b), an isopropyl carbocation attacks water to form isopropanol with the release of a proton, the key mechanism in the technical production of isopropanol and all higher secondary and tertiary alcohols. In transformation (c), an anionic methanolate ion acts as starter for an anionic polymerization reaction – one possible starting step in technical anionic polymerization.
Scheme 2.1.2 Reactivity of radicals, carbocations, and carbanions exemplified for reactions of relevance in chemical technology: reaction of (a) the methyl radical with chlorine (key-step in methane chlorination); (b) the isopropylium ion with water (key-step in isopropanol synthesis from propene); (c) methanolate with an alkene to form a carboanion (key-step in the start-up of an anionic polymerization reaction).
Note that the reactivity of radicals, carbocations, or carbanions (the negative charged counterpart of carbocations), is not always the same but depends strongly on the surrounding neighboring groups with their specific electronic and steric effects. As the influence of electronic and steric factors on the reactivity of molecules is also of key importance for many transformations in chemical technologies we will devote the following sub-sections to introducing these phenomena. For a more detailed treatment of the reactivity of organic molecules – that is certainly indispensable for all research efforts into new chemical transformations – excellent textbooks in organic chemistry can be recommended (Sykes, 1988; March, 1992; Sykes, 1996; Walter and Francke, 1998; Fanghlsquänel, 2004).
2.1.1 Factors that Influence the Electronic Nature of Bonds and Atoms
All effects that influence the electron density in a specific part of a molecule strongly affect the chemical reactivity of that part. While – for example – electron-rich parts show hardly any reactivity against , electron-poor parts will easily react with this strong electron-donor.
The most important effects that influence the electron density of specific parts of a molecule are the inductive and conjugative effects.
Inductive effects on electron density originate from the fact that the electron pair in a covalent σ-bond that links two different atoms (e.g., carbon and oxygen in a C–O bond) are never shared evenly. The more electronegative atom (in our example oxygen) will always receive more electron density, which leaves the carbon with some lack of electron density. Thus, a carbon atom attached to an oxygen atom (or any other strongly electronegative atom) always shows increased reactivity against strong electron-donor reagents relative to a carbon attached to another carbon. If the C–O bond is incorporated in a larger molecule, the carbon attached to the oxygen will also influence its immediate neighborhood by its ability to compensate for part of its electron deficiency by taking electron density from the surrounding atoms. Most groups attached to a carbon atom exert an inductive effect that pulls electrons away from the carbon (so-called –I-effect) because most atoms are more electronegative than carbon. Important exceptions are alkyl groups and metal atoms such as lithium (i.e., in organolithium reagents) or magnesium (i.e., in Grignard reagents of the type RMgX). A carbon linked to these groups receives more electron density than usual (so-called +I-effect) and becomes an electron-rich reagent that searches for electron-deficient partners for reaction. Note that all inductive effects on electron density are based on the permanent polarization of bonds and, therefore, these effects are also expressed in the physicochemical properties of the molecules, for example, their dipole moment.
Conjugative effects on electron density are based on the high degree of polarizability of π-electrons in unsaturated and, especially, in conjugated systems (systems with alternating single and double bonds, such as butadiene). In contrast to inductive effects, conjugative or mesomeric effects influence the electron density distribution in a molecule over large distances in expanded conjugated systems. Moreover, the conjugative effects result in atoms of alternating and fluctuating polarization and electron density in these systems. It is of great practical relevance that the possibility of stabilizing a positive or negative charge in a π-electron system by conjugative or mesomeric effects leads directly to a large increase in stability of such species. For example, the much stronger acid character of phenol compared to methanol can be understood as a direct consequence of the mesomeric stabilization of the phenolate ion after proton transfer (Scheme 2.1.3). Of course a similar kind of stabilization is not possible with any saturated aliphatic alcohol. Conjugative electronic effects are also permanent and they influence strongly the physicochemical properties of molecules with unsaturated bonds and conjugated π-electron systems.
Scheme 2.1.3 Stabilization of the phenolate ion by conjugative (or mesomeric) effects – note the two different notations of the arrows, indicating a reversible chemical reaction (left-hand side) and different mesomeric structures of the phenolate ion (three arrows in brackets).
2.1.2 Steric Effects
Sometimes two molecules do not react even though they are expected to on the basis of the electronic nature of their reactive centers. In most cases, steric effects account for this reduced reactivity. To understand the nature of steric effects we simply have to consider that two molecules have to approach each other very closely to enable the formation of a new covalent bond. If the reactive centers of both molecules are surrounded by bulky, inflexible, or geometrically restricted groups, the repulsive interaction of these surrounding groups can prevent the two reactive centers from approaching in the required way. As a consequence, the two molecules do not react or if they do the formed bond is very instable and can be cleaved easily by heating or by reaction with a less sterically demanding other reagent. Scheme 2.1.4 shows the unusual reactivity of two trityl radicals, which originates from the fact that the simple recombination of two trityl radicals is sterically too demanding to take place.
Scheme 2.1.4 Reactivity of two trityl radicals – due to steric reasons the expected combination of two radicals is suppressed and instead a less sterically demanding dimerization takes place.
2.1.3 Classification of Reagents
Strong electron donating reagents, such as HO−, search for electron-deficient counterparts to lower their energy by forming a stable covalent bond. Therefore, these species are called nucleophiles or nucleophilic reagents. In an analogous manner there also exist reagents that themselves are very poor in electron density and, therefore, search for electron-rich counterparts for reactions. The latter are called electrophiles or electrophilic reagents. Table 2.1.1 gives an overview of technically important nucleophiles and electrophiles. Note that the electronic character of these species can be very much understood using the arguments discussed in Section 2.1.1. Note further that to establish an order of strength among different nucleophiles one can take their basicity as a rough first approximation. An important difference, however, is that the terms “electrophilicity” and “nucleophilicity” are derived from kinetic experiments (therefore aspects like steric arguments can play a very important role) while the terms “acidity” and “basicity” are derived from a thermodynamic evaluation of the acid–base equilibria.
Table 2.1.1 Technically important nucleophiles and electrophiles – atoms in bold refer to the atoms that transfer or accept electrons to the substrate according to their nucleophilic or electrophilic nature (R represents an alkyl or aryl group, X represents a halide).
Technically important nucleophilesTechnically important electrophilesH−, HO−, RO−, RS−, CN−, RCO2−, R′-O-R″, R′-S-R″, R′3-N, RMgX, RLiH+, H3O+, NO2+, NO+, R3C+, SO3, CO2, BF3, AlCl3, Br2, O32.2 General Classification of Reactions
Organic reactions can be grouped into four basic types of transformations that all play very important roles in chemical technology. These four types will be briefly presented here and each type will be exemplified using one technically relevant example:
substitution reactionsaddition reactionselimination reactionsrearrangement or isomerization reactions.Substitution reactions are characterized by the fact that a substrate reacts with a second molecule by incorporating the second molecule in its structure and by releasing a part of the substrate. Substitution reactions can take place as electrophilic (see Section 2.2.5 for details), nucleophilic (Section 2.2.3), or radical substitution reactions (Section 2.2.2) depending on the nature of the attacking reagent. Scheme 2.2.1 shows the electrophilic substitution of a hydrogen atom at benzene by the nitronium electrophile NO2+. This technically relevant reaction liberates a proton and forms nitrobenzene. It represents an important step in the synthesis of nitrobenzene, the key-intermediate for the production of aniline.
Scheme 2.2.1 Electrophilic substitution of a hydrogen atom at benzene to form nitrobenzene.
Addition reactions proceed typically at unsaturated bonds such as C=C, C=O, C=N, CN or carbon–carbon triple bonds. A molecule is added to the substrate and the product forms without release of any another molecule. With all substrates becoming part of the product, the atom economy of addition reactions is very favorable. Because today's chemical technology is largely based on unsaturated base chemicals obtained in the steam cracker process (e.g., ethylene, propylene, butenes, benzene, see Chapter 6.6), addition reactions are of the highest relevance in the whole petrochemistry. Scheme 2.2.2 shows as one important example, namely, the addition of hydrogen to benzene to form cyclohexane, a key intermediate in the production of, for example, adipinic acid or caprolactam (nylon).
Scheme 2.2.2 Addition of hydrogen to benzene to form cyclohexane.
Elimination reactions can be regarded as the reverse of addition reactions. One substrate is converted into at least two molecules, with dehydrogenation, dehydration, dehalogenation, and dehydrohalogenation reactions being of highest technical relevance. Scheme 2.2.3 shows, as an example of a technically relevant elimination reaction, the dehydrochlorination of dichloroethane, a key step in the production of vinyl chloride.
Scheme 2.2.3 Elimination of HCl from dichloroethane to form vinyl chloride.
Rearrangement or isomerization reactions proceed typically at carbocations or other electron-deficient positions of a molecule. In rearrangement reactions the substrate stabilizes itself by rearranging its structure without changing the number and type of its atoms. Thus, rearrangement reactions proceed without addition/release of molecules other than substrate and product. Rearrangement reactions of technical importance are the isomerization of linear alkanes to branched alkanes (important to increase the quality of fuels) and the rearrangement of cyclohexanone oxime to ε-caprolactam (Scheme 2.2.4).
Scheme 2.2.4 Rearrangement of cyclohexanone oxime to form ε-caprolactam.
The following sub-sections highlight important mechanistic aspects of organic reactions. They focus on the question of how a certain organic transformation proceeds and how it can be influenced beneficially, for example, by the use and choice of a catalyst, the choice of solvent, or reaction parameters. Of course, a certain understanding of the type of reaction mechanism is also very helpful in choosing the right kinetic model for kinetic investigations in the context of process development studies.
2.2.1 Acid–Base Catalyzed Reactions
Acid-catalyzed reactions are characterized by the fact that either a proton (in the case of Brønsted acid catalysis) or a strongly electron-deficient catalyst (in case of Lewis acids) interacts with the substrate, typically by the intermediate formation of a carbenium ion. The latter is highly activated and undergoes transformations, for example, in the form of substitution, addition, or rearrangement reactions. After reaction the acid catalyst is liberated from the product. Technically important examples of all three types of transformation are (i) Lewis acid (here typically AlCl3) catalyzed electrophilic substitution to form ethylbenzene from benzene (the key-step in styrene production), (ii) Brønsted acid catalyzed addition of water to ethene to form ethanol, and (iii) isomerization of n-hexane to iso-hexane catalyzed by strong Brønsted acids to improve the quality of fuel for Otto engines.
In the case of base-catalyzed reactions the substrate comes into contact with either HO− or any other highly electron-rich catalyst (e.g., alcoholates, strongly basic amines, metal alkyls). Again, the substrate is activated, typically via the intermediate formation of carbanion species. A technically important example of base catalysis is the transesterification of natural oils to fatty acid methyl esters (FAME, better known as “biodiesel”), a process typically catalyzed by methanolate salts.
2.2.2 Reactions via Free Radicals
As mentioned in Chapter 2.1, the formation of radicals requires the homolytic cleavage of a covalent bond. Energetically such homolytic cleavage is particularly favorable in gas-phase reactions and for liquid-phase reactions in nonpolar solvents. In polar solvents, however, the energy contribution from the solvation of ionic species formed in heterolytic cleavage reverses the picture and heterolytic cleavage becomes more favorable.
Radical reactions are of greatest importance in chemical technologies. The combustion of hydrocarbons – surely the most important organic reaction in volumetric terms – involves the formation of radicals in the same way as most oxidation reactions for the production of chemicals (e.g., oxidation of cyclohexane to cyclohexanol). Other very important radical reactions include thermal cracking of hydrocarbons [e.g., in the steam cracker process (Chapter 6.6) or in the delayed coker process], radical substitution reactions (e.g., alkane chlorination or alkane sulfoxidation), and radical polymerization reactions [for the production of, for example, polystyrene, poly(vinyl chloride), or polymethacrylate)]. The latter reactions involve formally the addition of a radical to the monomer alkene followed by chain propagation to form the polymer.
All radical reactions start with the initial formation of radicals in the reaction mixture. This decisive step can proceed either photochemically (as, for example, in the technical sulfoxidation and sulfochlorination processes) or thermally (as in all technical oxidation and cracking reactions as well as in most radical polymerizations). A third important type of radical formation proceeds via redox reaction with a one-electron transfer either using metal salts (e.g., Fe2+/Fe3+ or Cu+/Cu2+) or via electrolysis. Scheme 2.2.5 gives examples of technical relevance for all three radical formation mechanisms.
Scheme 2.2.5 Different ways of forming radicals exemplified for reactions of technical importance: (a) photochemical cleavage of chlorine to form a chlorine radical as applied in the sulfochlorination of alkanes; (b) thermal cleavage of bis-α-azoiso-butyronitrile as applied in the radical polymerization of styrene; (c) formation of OH radicals from hydrogen peroxide as applied in radical oxidation reactions.
Radicals are very reactive due to their unpaired electron. Once formed, they typically react very quickly with organic molecules in addition, substitution, or rearrangement reactions. If radicals react with neutral molecules, new radicals form and the reaction can quickly propagate as low energy barriers are characteristic for this kind of radical reactions. Note that, as a consequence of their high reactivity, radicals react in most cases in a less selective manner than carbocations or carbanions.
A radical reaction or radical chain propagation (such as in alkene polymerization) is terminated by either the reaction of two radicals or by disproportionation of the radical into alkane and alkene (Scheme 2.2.6). The latter reaction plays the dominant role in petrochemical cracking processes. Alternatively, a radical reaction can be stopped by adding to the reaction mixture substances that react very easily with radicals by forming very stable radicals themselves so that the propagation reaction is terminated. Examples of such radical scavenger molecules are phenols, quinones, and diphenylamines.
Scheme 2.2.6 Chain termination in radical reactions by either (a) reaction of two radicals, exemplified for poly(vinyl chloride) synthesis, Ra = initial radical, or (b) disproportionation, exemplified for the reaction of two butyl radicals relevant in steam cracking.
2.2.3 Nucleophilic Substitution Reactions
In a nucleophilic substitution, one substituent of a saturated carbon atom is exchanged with another substituent. A typical example is the reaction of a haloalkane R–X with the hydroxide ion HO− to form the respective alcohol:
Kinetic studies of numereous nucleophilic substitution reactions have demonstrated that there exist two borderline cases. In the first case, also referred to as a SN2 reaction, the reaction rate is proportional to the concentration of both R–X and HO− [Eq.(2.2.1)], in the second case, called a SN1 reaction, the reaction rate is only dependent on the concentration of R–X [Eq. (2.2.2)]:
(2.2.1)
(2.2.2)
A more detailed mechanistic analysis reveals that in the case of an SN2 reaction both R–C and are involved in the rate-determining step (formation of the transition state), while in a SN1 reaction heterolytic cleavage of the C–X bond is the rate-determining step, and, thus, only the concentration of R–X influences the kinetics. Scheme 2.2.7 displays the two different borderline cases and their rate-determining steps.
Scheme 2.2.7 Mechanistic borderline cases in nucleophilic substitution reactions: (a) SN2 reaction with formation of the transition state being the rate-determining step; (b) SN1 reaction with heterolytic cleavage of the C–Cl bond being the rate-determining step.
Several important factors influence whether a given nucleophilic substitution reaction proceeds more according to the SN1 or the SN2 mechanism:
Choice of solvent: The higher the dielectric permittivity ε of the solvent the better is the solvation of the ion pair in the SN1 mechanism, leading to increasing probability of an SN1 mechanism. Note that nucleophilic substitution according to the SN1 mechanism is typically not found in gas-phase reactions.Groups surrounding the carbon atom at which the substitution reaction proceeds: The higher the steric demand of the remaining substituents at the carbon atom the more difficult it is to realize the transition state required by the SN2 route. Note that the SN2 mechanism requires an attack of the nucleophile from the opposite site of the leaving, weaker nucleophile. In addition, the groups surrounding the carbon atom also affect the probability of SN1 versus SN2 mechanism by means of their electronic influence. If the groups attached to the carbon atom exert an electron donating inductive effect, such as, for example, in the case of a carbon atom attached to three methyl groups, then the carbenium ion formed in the SN1 reaction is greatly stabilized and this mechanistic path becomes more probable.Nature of the incoming group: For obvious reasons the strength of the incoming nucleophile does not influence the rate of a SN1 reaction, the incoming nucleophile is not involved in the rate determinung step. For SN2 reactions the reaction rate increases with increasing nucleophic character of the incoming group.Nature of the leaving group: The nature of the leaving group influences both the rate of SN1 and SN2 reactions. The relative tendency of a leaving group X to leave the molecule R–X is influenced by the strength and polarization of the R–X bond as well as by the stability and solvation of the leaving group X.2.2.4 Reactions via Carbocations
Carbocations are formed by several reactions. One example has been discussed already in the context of the SN1 reaction (Scheme 2.2.8a). Other important options include the addition of protons to double bonds, for example, the addition of a Brønsted acid to an alkene or ketone (Scheme 2.2.8b and c, respectively). The addition of a Lewis acid to a carbonyl group can also lead to a type of carbocation, an effect that is exploited in all kinds of technical Friedel–Crafts acylation reactions (Scheme 2.2.8d). Finally, in high-temperature refinery processes, the formation of carbocations from alkanes is of highest relevance. Here acidic catalysts are usually applied that abstract a hydride from the alkane to form hydrogen and a carbocation at the alkane substrate (Scheme 2.2.8e).
Scheme 2.2.8 Important routes for carbocation formation: (a) heterolytic bond cleavage as observed in SN1 reactions; (b) addition of acids to alkenes – key step in alcohol formation from alkenes; (c) addition of protons to carbonyl bonds; (d) addition of Lewis acids to carbonyl bonds; (e) hydride abstraction.
The stability of carbocations increases for alkyl cations with the number of alkyl groups that surround the positive charge and thereby stabilize it by their inductive effects. Thus, a methyl carbocation CH3+ is the most unstable and reactive one while the tert-butyl cation [(CH3)3C]+ is the most stable and least reactive. This stability order is also the reason why carbocations frequently undergo isomerization and rearrangement reactions after formation, a reactivity that is very important for all isomerization reactions in refineries (here branched hydrocarbons are highly desired due to their higher octane number – see Chapters 6.9 and 6.10).
Carbocations can – once formed – undergo in principle the following transformations:
combination with a nucleophileabstraction of a protonaddition to an unsaturated bondrearrangement.Scheme 2.2.9 demonstrates these different options for a C6-carbocation that carries its positive charge at carbon number 3 (C3). While the reaction with the nucleophile HO− leads to 3-hexanol, abstraction of a proton will produce 3-hexene. Addition of ethylene or any alkene would result in an addition reaction forming a new, very reactive carbocation. As a consequence, cationic polymerization would result from the addition of this alkene. Finally, the cation tends to rearrange itself to a more stable carbocation, for example, the 2-methylpentyl cation if no other reactant is around for reaction and the conditions are appropriate. The technical relevance of these different options is obvious for alcohol production from alkenes, for catalytic cracking (where significant amounts of alkenes are formed under specific, applied reaction conditions by proton abstraction from carbocations), for cationic polymerization processes, and for fuel reforming.
Scheme 2.2.9 Reactivity of carbocations exemplified for a hexyl ion carrying the cationic charge at carbon number 3 (C3).
2.2.5 Electrophilic Substitution Reactions at Aromatic Compounds
Aromatic compounds are characterized by their π-electron systems, which create a high electron density above and below the planar six-membered ring of carbon atoms. Consequently, aromatic compounds are easily attacked by electrophiles and the reconstitution of the energetically favored aromatic character leads to replacement of one substituent at the carbon ring with the attacking electrophile. In total, an electrophilic substitution reaction takes place. By the same argument, nucleophilic substitution reactions at aromatic rings are much more difficult, but are possible if strong nucleophiles and activated aromatic substrates (e.g. nitrobenzol or pyridine) are used (see Sykes, 1988; March, 1992 for details). In the following paragraphs we focus solely on the technically very relevant electrophilic substitution reactions.
Scheme 2.2.10 displays the general mechanism of an electrophilc substitution reaction for the important example of nitrobenzene synthesis from nitric acid/sulfuric acid and benzene. This reaction is a key step in the industrial synthesis of aniline, which is obtained subsequently by nitrobenzene hydrogenation.
Scheme 2.2.10 Nitration of benzene using HNO3/H2SO4 – an example of an electrophilic substitution reaction of technical relevance.
While in the case of the nitration reaction the attacking electrophile NO2+ is generated from the HNO3/H2SO4 mixture, in other electrophilic substitution reactions a Lewis-acid catalyst plays a very important role in generating the reactive electrophile. Examples are the Lewis-acid catalyzed chlorination or bromination of aromatic compounds (typical catalysts: FeCl3 or FeBr3) and Friedel–Crafts alkylation with alkyl halide or alkenes (typical catalyst: AlCl3). In each case, interaction of the Lewis acid with the approaching non-aromatic substrate leads to a large increase in the electrophilicity of the attacking reagent. Another technically important example of a non-catalyzed electrophilic substitution reaction is the sulfonation of benzene and other aromatic compounds. The reaction proceeds quickly in mixtures of SO3 and sulfuric acid, in which SO3 acts as a strong electrophile.
A question of high practical relevance for all electrophilic substitution reactions is the influence of an already existing substituent Y on the aromatic ring on the reactivity and regioselectivity of a second substitution reaction with the electrophile X. Comparing substitution reactions with X for different aromatic starting materials (with Y = H for benzene as the reference), two distinctive patterns can be distinguished:
The second substitution is comparably fast or even faster than in the case of benzene and the second substituent is found preferably at the 2-(ortho) or 4-(para) position of the product (with respect to the substituent present in the aromatic substrate defining the 1-position).The second substitution is slower than in the case of benzene and the second substituent is preferably found at the 3-(meta) position.To explain these patterns, electronic influences are most relevant. In addition, steric factors play a certain role for substitution at the 2-(ortho) position. Substituents Y with a free electron pair on the atom that is to be attached to the aromatic ring (e.g. OCOR, NHCOR, OR, OH, NH2, NR2) provide this electron pair for conjugative stabilization of the cationic transition state formed after attack of the electrophile. This leads to an acceleration of the reaction (lowering of the energetic level of the transition state) and to preferable electrophilic substitution at ortho- and para-positions. For these positions, stabilization involving the free electron pair of Y is more favorable than for the meta-position.
In contrast, substituents Y with no free electron pair at the atom attached to the aromatic ring (e.g. R3N+, Cl3C, NO2, CHO, COOH) provide no electron pair for conjugative stabilization of the cationic transition state. Without this conjugative stabilization only the stronger electron-withdrawing effect of this group of higher electronegativity affects the reaction mechanism. These substituents exert an electron-withdrawing effect and thus the electrophilic substitution is slowed down compared to the reaction with benzene. However, the ability to stabilize the positive charge of the transition state is greater for substitution at the meta-position than for the other two positions, leading to a preferred nucleophilic substitution at this position.
So far, our discussion has always referred to kinetic arguments. It has to be considered, however, that most reaction systems that undergo electrophilic substitution reaction can also promote intra- or even intermolecular exchange of substituents. In the case of intermolecular exchange, this results in an isomerization reaction of the different regioisomers with the relative thermodynamic stabilities of the different isomers as the driving force. As a consequence we can obtain in short-term experiments kinetic product mixtures (depending on the above-mentioned arguments) and if we apply longer reaction times these kinetic mixture transform into the thermodynamic mixture of regioisomers. Detailed knowledge of the substitution and isomerization kinetics as well as of the temperature-dependent equilibria allows us to adjust the obtained product mixture to a certain degree to meet market needs.
2.2.6 Electrophilic Addition Reactions
The technically most important electrophilic addition reactions proceed at alkenes and alkynes. The reactive π-electrons of these compounds are attacked by electrophiles, resulting in the formation of a positively charged reaction intermediate. Stabilization of this positive charge plays a very important role in the regioselectivity of electrophilic addition reactions. This is demonstrated in Scheme 2.2.11 for the addition of HBr to propene, a reaction that produces almost uniquely the product 2-bromopropane and almost no 1-bromopropane as a consequence of the inductive stabilization of the secondary propyl cation compared to the cation with the charge at C1. This selctivity, where the proton becomes attached to the carbon with fewer alkyl substituents, is known as Markovnikov's rule.
Scheme 2.2.11 Electrophilic addition of HBr to propene – due to the higher stability of the secondary carbenium ion there is hardly any 1-bromopropane found in the product.
Besides the addition of halides and hydrogen-halide acids to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels–Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures).
2.2.7 Nucleophilic Addition Reactions
Nucleophilic addition reactions are mainly of technical interest in the context of further reactions at C=O groups present in aldehydes or ketones. The electronic nature of a carbonyl group is characterized by the greater electronegativity of the oxygen atom compared to the carbon atom. Thus, the carbon atom is the preferred place of nucleophilic attack, that is, of reaction with an electron-rich reagent. Scheme 2.2.12 gives as an example the technically important cyanohydrin reaction. Other important nucleophilic additions are the reaction of carbonyl compounds with alcohols and water, bisulfite and metal hydrides.
Scheme 2.2.12 Nucleophilic addition of CN
− to acetone in the cyanohydrin synthesis.2.2.8 Asymmetric Synthesis
A compound posessing a carbon atom that is surrounded by four different substituents exists in two stereoisomers that are like image and mirror image and are, therefore, not superimposable. Such a compound is said to be “chiral” and both stereoisomers are called “enantiomers.” Figure 2.2.1 shows the two enantiomers of 2-butanol. The central carbon is also called “asymmetric” and a synthesis that produces selectively one stereoisomer is therefore called an asymmetric synthesis.
Figure 2.2.1 The two enantiomers of 2-butanol.
The two enantiomers of a chiral compound have the same chemical and physicochemical properties in an achiral environment. Nevertheless, asymmetric synthesis is a very important field in preparative organic synthesis and fine chemicals production because nature is full of chiral receptors, catalysts, and reactants. Thus, the different enantiomers of chiral products typically exhibit very different performance and properties when applied as agrochemical, fragrance, or pharmaceutical in the chiral biological environment.
For compounds with more than one asymmetric carbon atom there exist enantiomers and diasteriomers. In detail, a compound with n asymmetric carbon atoms can be formed in 2n different configurations. Some of these behave like image and mirror image – these are pairs of enantiomers. However, there are also pairs of stereoisomers that are not mirror images of each other. These are called diastereomers. Note that diastereomers differ in their physicochemical and chemical properties even in an achiral environment.
Synthesis of a chiral compound from an achiral compound requires a prochiral substrate that is selectively transformed into one of the possible stereoisomers. Important prochiral substrates are, for example, alkenes with two different substituents at one of the two C-atoms forming the double bond. Electrophilic addition of a substitutent different from the three existing ones (the two different ones above and the double bond) creates a fourth different substituent and, thus, an asymmetric carbon atom. Another class of important prochiral substrates is carbonyl compounds, which form asymmetric compounds in nucleophilic addition reactions. As exemplified in Scheme 2.2.13, prochiral compounds are characterized by a plane of symmetry that divides the molecule into two enantiotopic halves that behave like mirror images. The side from which the fourth substituent is introduced determines which enantiomer is formed. In cases where the prochiral molecule already contains a center of chirality, the plane of symmetry in the prochiral molecules creates two diastereotopic halves. By introducing the additional substituent diasteromers are formed.
Scheme 2.2.13 Nucleophilic addition to a prochiral carbonyl compound to form a new asymmetric C-atom with two enantiomers.
