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Chapter

Cover Reaction Dynamics

Introduction  

This introductory chapter provides an overview of elementary reactions in the gas phase. Most elementary reactions can be categorized as either unimolecular or bimolecular. Another class of reactions is association reactions. Association reactions and chemical activation can be modelled using the same theories as those developed to rationalize unimolecular reactions. The chapter then looks at reaction kinetics and dynamics, considering thermal rate coefficients and simple collision theory. It also offers some insight into the significance of the reaction cross-sections. Finally, the chapter highlights the role of the reaction probability on collision in determining the magnitude of the reaction cross-section and, hence, the thermal rate coefficient.

Book

Cover Pericyclic Reactions
Pericyclic Reactions starts with a chapter on the nature of pericyclic reactions and considers how important they are. The following chapter looks at cycloaddition reactions. The text thereafter examines the Woodward–Hoffmann rules and molecular orbitals. There follows a chapter on electrocyclic reactions. Towards the end, the book moves on to sigmatropic rearrangements before turning to group transfer reactions in the final chapter.

Chapter

Cover Essentials of Inorganic Chemistry 2

Ligand substitution reactions  

This chapter addresses ligand substitution reactions. Following the nomenclature proposed by Langford and Gray, substitution reactions of inorganic reactions are classified as associative, interchange, and dissociative. The chapter explains that in an associative mechanism the entering ligand binds to the central atom before any significant bond weakening to the leaving group occurs. It also notes that, in the case of a dissociative mechanism, the ligand departs before the entering group forms a significant bond with the central atom and an intermediate with a lower coordination number is formed. The formation of such an intermediate also results in two peaks in the reaction profile, but the activation energy leading to the intermediate is larger than for its decomposition. Finally, the chapter looks at rates of substitution of metal complexes.

Chapter

Cover Atkins’ Physical Chemistry

The equilibrium constant  

This chapter explores equilibrium constants, which lie at the heart of chemistry and are a key point of contact between thermodynamics and laboratory chemistry. It develops the concept of chemical potential and shows how it is used to account for the equilibrium composition of a reaction mixture. The equilibrium composition corresponds to a minimum in the Gibbs energy. By locating this minimum, it is possible to establish the relation between the equilibrium constant and the standard Gibbs energy of reaction. The chapter differentiates between exergonic and endergonic reactions. The reaction Gibbs energy is the slope of the plot of Gibbs energy against extent of reaction. Meanwhile, the equilibrium constant is the value of the reaction quotient at equilibrium.

Chapter

Cover Chemical Structure and Reactivity

The rates of reactions  

This chapter reviews the rate at which a chemical reaction proceeds. The experimentally determined rate law gives the dependence of the rate on the concentrations of the reactants. Rate constants have a strong temperature dependence given by the Arrhenius law. However, there is an energy barrier to most reactions. Complex reactions proceed by a series of elementary steps, called a mechanism. The kinetics of complex reactions can sometimes be simplified using the pre equilibrium or steady-state approximations. In a multi-step reaction, one step can often be identified as the rate limiting (or rate-determining) step. The chapter then looks at sequential reactions, the reactions of complex mechanisms, and chain reactions.

Chapter

Cover Stereoselectivity in Organic Synthesis

Introduction  

This introductory chapter provides an overview of stereoselectivity in organic synthesis. Given the importance of the three-dimensional structure of organic compounds, and the fact that all organic compounds arise as a result of a previous reaction or reaction sequence, it is not surprising that an area of chemistry which encompasses organic reactions and stereochemistry is central to organic chemistry. This book focuses on the area of stereoselective organic reactions, highlighting their usefulness in organic synthesis where appropriate. For the most part, it is concerned with reactions which involve the formation of tetrahedral carbon atoms within a molecular framework. The chapter then looks at diastereoselective reactions, enantioselective reactions, and stereospecific and stereoselective reactions.

Chapter

Cover Structure and Reactivity in Organic Chemistry

Organic reaction mechanisms and reaction maps  

This chapter discusses organic reaction mechanisms and reaction maps. Collisions between molecules provide the energy for bimolecular and unimolecular thermal reactions to occur. In a bimolecular reaction, three translational and (normally) up to three rotational degrees of freedom are lost in the formation of the activated complex; since the total has to remain the same, a corresponding number of new vibrational degrees of freedom are formed. In a unimolecular reaction, a molecular collision leaves one of the molecules in a vibrationally excited state which either reacts or simply loses its excess energy in a subsequent collision. The chapter then looks at molecular vibrations and potential energy diagrams. It also considers parallel and perpendicular effects; step-wise reactions; and single-step multibond cycloaddition reactions.

Chapter

Cover Organic Chemistry

Organic Synthesis  

This chapter looks at the reactions used in organic synthesis. It defines organic synthesis as the directed preparation of a sought compound usually, but not invariably, with some structural complexity from simple, readily available compounds. The chapter also develops the concept of retrosynthesis further and examines how to identify routes for syntheses of target compounds by applying organic reactions. Organic synthesis is one of the ultimate goals of organic chemistry and provides limitless opportunities to challenge the scientific imagination. The chapter then proceeds to discuss the synthons and the corresponding reagents as well as the reaction selectivity. Finally, the chapter elaborates on linear and convergent strategies, and presents some examples of organic synthesis.

Chapter

Cover Atkins’ Physical Chemistry

The rates of chemical reactions  

This chapter discusses the definition of reaction rate and outlines the techniques for its measurement, which shows that reaction rates depend on the concentration of reactants. It points out how ‘rate constants’ are characteristic of the reaction, which can be expressed in terms of differential equations known as ‘rate laws’. It also highlights the rates of consumption of reactants and formation of products that make it possible to predict how quickly a reaction mixture approaches equilibrium and lead to detailed descriptions of the molecular events that transform reactants into products. The chapter looks at experiments that show that reaction rates depend on the concentration of reactants in characteristic ways that can be expressed in terms of differential equations. It considers the establishment of the stoichiometry of the reaction and identification of any side reactions as the first step in the kinetic analysis of reactions.

Chapter

Cover Atkins’ Physical Chemistry

The Arrhenius equation  

This chapter discusses how rate constants of most reactions increase with increasing temperature. It introduces the ‘Arrhenius equation’, which captures this temperature dependence by using two parameters that can be determined experimentally. It also reviews the exploration of the dependence of reaction rates on temperature that leads to the formulation of theories that reveal the details of the processes that occur when reactant molecules meet and undergo reaction. The chapter looks at the temperature dependence of the rate of a reaction that depends on the activation energy and the minimum energy needed for reaction to occur in an encounter between reactants. It emphasizes how chemical reactions usually go faster as the temperature is raised, which is almost always due to the increase of the rate constant with temperature.

Chapter

Cover Atkins’ Physical Chemistry

Reactions approaching equilibrium  

The chapter discusses how rate laws must take into account both the forward and reverse reactions and describe the approach to equilibrium in order to be complete. It notes that the results of the analysis are relations between the equilibrium constant of the overall process and the rate constants of the forward and reverse reactions. It also looks at how the analysis of the time dependence reveals the connection between rate constants and equilibrium constants. The chapter emphasizes that both forward and reverse reactions must be incorporated into a reaction scheme in order to account for the approach to equilibrium. It reviews kinetic studies that are made on reactions that are far from equilibrium and if the concentration of the products is low and the reverse reactions are unimportant.

Chapter

Cover Organic Chemistry

Diastereoselectivity  

This chapter addresses diastereoselectivity. It explains how to make compounds as single diastereoisomers. The chapter begins by differentiating between stereospecific and stereoselective reactions. In stereospecific reactions, the mechanism means that the stereochemistry of the starting material determines the stereochemistry of the product and there is no choice involved. In stereoselective reactions, one stereoisomer of product is formed predominantly because the reaction has a choice of pathways, and one pathway is more favourable than the other. The reactions that give single diastereoisomers—in other words, those that are diastereoselective—all involve the creation of a new, tetrahedral stereogenic centre at a carbon that was planar and trigonal. Trigonal carbons that are not stereogenic (or chiral) centres but can be made into them are called prochiral. The chapter then considers stereoselective reactions of acyclic alkenes.

Chapter

Cover Why chemical reactions happen

Leaving groups  

This chapter explains leaving groups. It discusses the energy profile in line with leaving groups by using a diagram to emphasize its points. The chapter notes bond strength and stability as factors which are important for a good leaving group. Leaving group ability is a good guide to working out which reactions will go and which will not. The chapter highlights how some reactions do not occur despite their predicted mechanisms. It shares how relative energies of the species are vital in predicting whether reactions will go readily or not. The chapter also shows how to reverse reactions that do not take place by using ketone reacting to acyl chlorides as an example. It cites neutral nitrogen as the best leaving group.

Chapter

Cover Organic Chemistry

Reactions of enolates with carbonyl compounds: the aldol and Claisen reactions  

This chapter highlights reactions of enolates with carbonyl compounds: the aldol and Claisen reactions. The simplest enolizable aldehyde is acetaldehyde. What happens if we add a small amount of base, say NaOH, to this aldehyde? Some of it will form the enolate ion. Each molecule of enolate is surrounded by molecules of the aldehyde that are not enolized and so still have the electrophilic carbonyl group intact. The enolate ion will attack one of these aldehydes to form an alkoxide ion, which will be protonated by the water molecule formed in the first step. The product is an aldehyde with a hydroxy (ol) group and it has the trivial name aldol. This reaction is so important because of the carbon–carbon bond formed when the nucleophilic enolate attacks the electrophilic aldehyde.

Book

Cover Electrode Dynamics
Electrode Dynamics provides an introduction to the field of electrode dynamics. The word electrochemistry commonly instils fear in students. This text aims to distil this fear with a gentle introduction to the kinetics of electron transfer reactions, and explores the potential applications of electrochemistry methodology. The early chapters provide a general introduction to the factors which control the rate of an electrode reaction. The later chapters deal with a variety of electrochemical applications including the study of surface processes, reaction mechanisms, electrosynthesis and the combination of electrochemistry with complementary techniques such as spectroscopy.

Chapter

Cover Stereoelectronic Effects

Rearrangements and fragmentations  

This chapter addresses two groups of reactions of synthetic importance and mechanistic interest, where the key interactions are through two or three bonds: fragmentations and rearrangements. Fragmentation reactions are observed when a strong electron donor interacts with a good leaving group three carbons away. The simplest case is the acid-catalysed fragmentation of a 1,3-diol to an alkene and a ketone or aldehyde. The addition of a nucleophile to a carbonyl group is a simple way to generate a donor oxyanion, and where the addition is sufficiently selective, clean reactions are observed. Meanwhile, rearrangement reactions may be observed even in some conformationally mobile systems, where it must compete with epoxide formation. Although various sorts of rearrangements are known, the most common and important are 1,2-shifts.

Chapter

Cover Elements of Physical Chemistry

Integrated rate laws  

This chapter focuses on how the composition of a reaction mixture can be predicted by integrating a rate law. An integrated rate law is an expression that gives the concentration of a species as a function of the time. They are called integrated rate laws because a rate law is a differential equation, and solving such an equation involves the mathematical technique of integration. Integrated rate laws have two principal uses. One is to predict the concentration of a species at any time after the start of the reaction. Another is to help find the rate constant and order of the reaction. The chapter then looks at the integrated rate laws for a variety of simple reaction types, including zeroth-order reactions, first-order reactions, and second-order reactions. It also considers the concept of half-life.

Chapter

Cover Essentials of Inorganic Chemistry 2

Redox reactions  

This chapter analyses redox reactions. It explains that the oxidation–reduction reactions of transition metal complexes proceed via two primary mechanistic pathways: outer and inner sphere reactions. An outer sphere mechanism occurs when an electron is transferred between two coordination complexes in solution, both of the coordination spheres remain intact, and no ligand is transferred between them. The chapter notes that it is only possible to establish such a mechanism unambiguously when both of the coordination complexes are substitutionally inert. Finally, the chapter describes the inner sphere mechanism: the electron transfer is mediated by an ambidentate ligand which is capable of bridging the two metal centres in the transition state and is transferred from one complex to the second in the course of the reaction.

Chapter

Cover Organic Stereochemistry

Stereospecific and stereoselective reactions  

This chapter displays a number of ways in which the terms stereospecific and stereoselective have been used by some chemists. It contends that a reaction is stereospecific if two stereoisomeric reactants give distinct stereoisomeric products. In contrast, a reaction is stereoselective if stereoisomeric products are formed in unequal amounts irrespective of any stereoisomerism in the reactant(s). The chapter uses stereospecific to describe reactions in which the stereochemical outcome is determined by the orbitals of the functional group(s) necessarily involved in the reaction. The chapter then shifts to focus on the polar reactions for carbon. It demonstrates its classification, namely: substitutions, additions, and eliminations to better understand its context in organic molecules. Towards the end, the chapter describes the concepts of radical and pericyclic reactions.

Chapter

Cover Organic Chemistry

Pericyclic Reactions: Cycloadditions, Electrocyclic Reactions, and Sigmatropic Rearrangements  

This chapter covers pericyclic reactions and their its types. In these so-called pericyclic reactions, all the bond forming and bond breaking occurs together with a nonpolar, concerted cyclic redistribution of valence electrons via aromatic transition structures. The chapter then presents and examines the three main groups of pericyclic reactions: cycloadditions, electrocyclic reactions, and sigmatropic rearrangements: the last two are both isomerization reactions. It then examines the Diels-Alder reactions and 1,3-dipolar cycloadditions, and reviews other cycloadditions and related reactions. Next, the chapter notes that pericyclic reactions are little influenced by changes in the nature of the solvent and they are not generally subject to catalysis. It then analyzes the molecular orbital considerations of pericyclic reactions.