This book is written at the level of an advanced, first-year general chemistry course. It might be used as a supplement for an honors course, for example. Concepts are emphasized, mathematics minimized. The style is informal. Material is presented crisply and clearly. On occasion, unusual topics are included, thereby enhancing the utility and interest in the approach. Chapters are relatively short, mostly 15-25 pages in length. Although four major reaction types constitute the body of the text, many others of interest are considered. The book is abundantly illustrated. Excellent computer-readable figures (in gif and tif formats) and an appendix (in Adobe Acrobat PDF format) are available on the Oxford Web site. ¹ The authors treat background material in the early chapters. Most chapters end with a logical tie-in to the next.

Entropy and the Second Law of Thermodynamics are introduced in Chapter 2. The concepts of randomness and disorder in everyday life, such as changes in the physical state of water, are used to familiarize students with the concept of entropy change. The discussion evolves from that of the entropy change of the universe to the change in system Gibbs free energy, which is more readily determined. The authors expend considerable effort in relating Gibbs free energy changes to underlying changes in both enthalpy and entropy. Consideration of the energy changes arising from the formation and breaking of chemical bonds leads to a lengthy discussion of ionic bonding in Chapter 3 and to the background behind covalent bonding in Chapters 4-6. Highlights of these chapters include use of the Born-Landé equation, which embodies nearest-neighbor repulsions for ionic solids in addition to the usual Madelung constant, and a lengthy development of orbital energies and photoelectron spectroscopy. The authors rely on a plethora of visual representations of wave functions and their properties obtained from HyperChem calculations in place of explicit mathematical formalism. In Chapter 5, LCAO—MO theory for diatomic molecules is developed. The consequences of the center of inversion symmetry for homonuclear molecules, a topic many general chemistry texts omit, are discussed as well. The orbital energies of AOs presented in Chapter 4 are used qualitatively to explain the change in the order of filling MOs that occurs between N₂ and O₂ and the polarity of bonds in heteronuclear diatomics. The statement in Chapter 5 that no He₂ molecules have ever been observed is incorrect. In fact, very weakly bound van der Waals molecules of He₂ are found in pulsed high pressure helium expansions into the gas phase at extremely low temperatures. In Chapter 6, the authors present a lucid discussion of hybridization, choosing to hybridize orbitals in terminal atoms as well, a matter of individual preference.

In Chapter 7, the MO approach is used extensively in discussions of the formation of H₂ from H⁺ and H^–; the formation of lithium borohydride from LiH and BH₃; nucleophilic addition to carbonyls and carbon–carbon double bonds; nucleophilic substitution; and nucleophilic attack on acyl chlorides. Highlights include an excellent discussion of HOMO–LUMO interactions, and the use of curly arrows showing the movement of electron pairs. Consideration of a final reversible reaction—ethanoic acid reacting with methanol to form the ester methyl ethanoate and water—leads to an excellent discussion of equilibrium in terms of Gibbs free energy in Chapter 8. Of special interest is the topic of driving unfavorable reactions, two relevant examples being formation of peptide bonds coupled to the favorable hydrolysis of ATP, and the formation of sugars by photosynthesis. The topics of Chapter 9 comprise the factors controlling the rates of reaction. Two misconceptions could arise from a standard discussion done well here. One concerns the activation energy for an elementary reaction, which is usually close to but not equal to the barrier height. It is legitimate to make this approximation for convenience, but it should be so noted. The statement on p 149, “the apparent activation energy for a reaction is the energy difference between the reactants and the highest energy transition state on the reaction pathway,” is not strictly correct.² This concept is not generally appreciated. The statement is true for the specific examples given, however.

In Chapter 10, bonding in extended conjugated systems and resonance are introduced with the horse + donkey = ass analogy (it is likely the hybrid animal mule is intended instead of ass, however). For butadiene, the π-MOs are given with wave function coefficients, then the authors consider propenal, allyl anion, carboxylate anion, and amides. A nice treatment of the conjugative effect in terms of an increase in energy of the LUMO, and the inductive effect to explain reactivity of amides, acyl chlorides, and esters is given. Informative surface plots of MOs are used here to illustrate the principal ideas. Substitution and elimination reactions are treated in more detail in Chapter 11. Negative charge stabilization by delocalization and factors stabilizing positive charges in cations are discussed. Presented here are the reasons for carbenium ion preferences, for example, CH₃⁺ (highly reactive) being less favorable than (CH₃)₃C⁺. Acylium ion (H₃C—C…O:⁺ σ conjugation) properties are also explored. An increase in the number of alkyl groups is found to increase the ion’s stability. The competition between behavior in a nucleophilic mode or as a base leading to an elimination reaction is explored. Markovnikov’s rule is restated in terms of the stability of the cation formed.

Two important influences on reactivity are explored in Chapters 12 (Solvent Effects) and 13 (Leaving Groups). Solvent properties are tied to relative permittivity as a guide for polar solvents. Topics include hydrogen bonds, protic and aprotic solvents, hydration numbers, and the solubility of ionic compounds in terms of ΔG. Both enthalpy and entropy effects are discussed; entropy is usually the deciding factor for ions. The authors make great use of the Born equation (Gibbs energy of solvation) and Hess’s law cycles. The treatment of leaving groups and likely reaction pathways is short but extremely informative, especially for physical chemists. Leaving group ability is compared to acid strength (pK_aof the conjugate acid). The finding that the order of leaving group ability from carbon does not always exactly match the order of acidity of the conjugate acids is described. The leaving group’s bond strength to carbon does not always parallel its bond strength to hydrogen.

The final chapter includes a lengthy discussion of competing reactions, here bringing together concepts previously developed. Topics considered are reactions of carbonyls with hydroxide (formation of enolates, isotopic labeling, and energy profiles for alternative reactions); reactions of enolates (manipulation of the reaction by changing the alkylating agent or the solvent); unsymmetrical enolates (thermodynamic and kinetic products); and substitution versus elimination revisited (alteration of the ratio between the two types, and comparison and analysis of energy profiles).

So, why do chemical reactions happen? According to the authors, answers to four basic questions are needed: “Why does this particular chemical reaction increase the entropy of the universe? Why is a reaction exothermic? Why is one arrangement of atoms lower in energy than another? How can we make the product we want?” This book successfully addresses these questions, providing comprehensive answers at a level appropriate for advanced students and scholars in chemistry.

Notes

1. See the Oxford University Press Companion Web site for Why Chemical Reactions Happen (accessed Nov 2003).
2. See, for example, Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper & Row: New York, 1987; pp 283-285.