Chemical Education Today edited by
Edward J. Walsh Allegheny College Meadville, PA 16335
Organic Chemistry Joseph M. Hornback. Brooks/Cole: Pacific Grove, CA, 1998. ISBN 0 5343 5254 5. $94.95.
The publisher’s advertisements tout this text as “efficiently organized by mechanisms so students can group and learn reactions more easily” and a text that “greatly enhances their ability to remember reactions”. These characterizations are true with the emphasis on “remember reactions”. However, an organic chemistry course is more than just presenting a series of reactions to be remembered. I generally think of the introductory organic chemistry course as an interplay of three aspects: structure, reactions, and synthesis of organic molecules. This text uses reaction mechanisms as the organizing principle, introduces structure where it is necessary to support the reactions to be studied, and considers synthesis after the mechanisms of appropriate reactions have been discussed. As a first edition, this text is technically very good. There are very few typographical or diagram errors, and in general colors are used to advantage to help readers understand diagrams and tables. Each chapter includes several tables, diagrams, examples, and problems that should help students follow the development of material. However, many of the tables and diagrams are located on pages following text discussions, making it difficult to use the diagram or table to best advantage. The diagrams are quite well annotated and easy to follow on their own, but the text discussions can be confusing when the reader has to keep flipping pages to refer to the diagram. Also, the yellow color used in the text is difficult to see, especially in diagrams with a tan background. Use of this color significantly reduces the effectiveness of several tables and diagrams. “Elaborations” that are spread throughout the text and discuss applications of the material to relevant medical, biological, industrial, or experimental situations are likely to be of general interest to students. Early use of curved arrows to interconvert resonance structures helps students become accustomed to “thinking with arrows”. The consistent use of resonance structures throughout the text helps reinforce the need to show resonance structures whenever resonance arguments are invoked. Chapter summaries generally have two elements: an enumeration of the topics including lists of reactions, and a list of “After completing this chapter you should be able to:” operations. The latter list should be more useful to students in developing an understanding of the material. In fact, students should read each summary of operations before reading the chapter to help them understand the author’s goals for the chapter. Interestingly, the preface seems to address the instructor rather than the student. It mentions students in the third person and explains the text’s approach in a way that is understandable only to someone familiar with alternative approaches. The tone of the preface suggests assumptions about the abilities of readers to appreciate the logic of the discipline. This sense of assumptions is reinforced throughout the text.
The chapter introductions are brief and truly informative only to those already familiar with the subject matter. Although the organization of the text follows a logical sequence of increasing reaction complexity in terms of the number of steps in the reaction mechanisms and provides structural background as needed to support discussions of reactions, the logic is not explicitly emphasized in the introductions or discussions. The logic may be apparent to someone familiar with organic reaction mechanisms, but is not likely to be obvious to students as they work their way through the material. Thus, instructors should be sure to emphasize the logic of the developing material. The reaction chapters (7, 8, 10, 14–16, 18–20) all have a similar format. The reaction and mechanism are presented quite clearly using curved arrows, but there is no explanation of how we know that the reaction operates by the provided mechanism. The mechanism is then used to predict the effects of structural changes on the rates of reactions, and finally, a variety of examples is enumerated. Most discussions result in the formulation of a set of rules that help predict the outcome for a particular combination of reactants. Sets of rules can be memorized and be helpful in getting answers. But the text explanations of how and why each mechanism operates are likely to be quite mysterious to students. Although most of these explanations are understandable to someone familiar with the material, they tend to be uneven, and most students will need considerable assistance from an instructor to appreciate their significance and how they were devised. Four representative examples are provided below. In the comparison of rates of electrophilic aromatic substitution vs addition to alkenes (pp 838–839), relative stabilities of both the reactants and transition states are systematically considered to predict relative free energies of activation. However, in the discussion of substituent effects on SN2 reactions (pp 263–264), only steric effects are considered and they are explained as hindrance of back-side approach of the nucleophile, with no mention of the stabilities of reactants or transition states. A few pages later (pp 274–275), the discussion of effects of substituents on the rates of SN1 reactions considers only hyperconjugation effects on the stability of the intermediate carbocation, indicating that “Any change that makes the carbocation more stable will also make the transition state more stable, resulting in a faster reaction.” The stability of the transition state is considered with no mention of reactant stabilities. Later, the discussions of inductive and resonance effects on the rate of substitutions at the carbonyl group (p 695) include the statement “Electronwithdrawing groups make the carbonyl carbon more electrophilic and increase the rate”, suggesting that rate can be judged from the structure of the reactant without considering transition state stability. Although each explanation correctly identifies the most important factor, the last three consider only one aspect of the reaction. So only the first discussion considers all of the factors that should be explored; the others do not indicate how the specific factor discussed was selected or how others were deemed less important. Thus, the student is likely to be
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Chemical Education Today
Book & Media Reviews left confused when asked to analyze an unfamiliar reaction. The discussions of IR include a large number of nicely explained example spectra using color coordination between structural features and spectral peaks. However, in the explanation of IR and the absorption frequencies of individual bonds (p 505), the Hooke’s law equation is introduced without reference to the spring model for bonds, so that it seems to come from nowhere. The explanation of NMR is reasonably good, with the exception of a rather confusing method for predicting chemical shifts of protons near more than one functional group. Discussions of organic syntheses are likely to overwhelm students. Chapter 9 explores simple syntheses using SN1, SN2, E1, and E2 reactions. It is organized according to the functional group synthesized and ends with a brief introduction to retrosynthetic analysis. The approach of working back from the target molecule to reasonable starting materials is introduced; however, retro arrows are not used. The syntheses are short, but the number of reactions is quite large and there is not much emphasis on how to recognize potential synthetic reactions from their products. In Chapter 16, enolate reactions are added to the synthetic tools, the formalism of retro arrows is established, and there are a few illustrations of retrosynthetic analyses. Again, there is no systematic discussion of the logic for recognizing potential synthetic steps from the products of the step, and the number of potential reactions is really unmanageable for students who are just learning the process. Chapter 21 adds 13 pages of reactions (from the summary list, pp 1021–1033), with only a short discussion of the retroanalysis of two of the reactions. The three synthetic chapters are excellent references for quite specific information on a wide variety of reactions. These chapters contain enough material for a reasonable onesemester synthetic organic course. Consequently, just about any desired reaction should be included, but most instructors will need to be quite selective in choosing a subset of the synthetic reactions for their introductory courses and the discussion of retrosynthetic analysis will need considerable reinforcement. Special topics chapters include Industrial Organic, Synthetic Polymers, Carbohydrates, Amino Acids, Peptides and Proteins, Nucleotides and Nucleic Acids, and Natural Products. This text has many positive characteristics, and the negative aspects relating to explanations are shared by most other texts on the market. Thus, instructors who prefer to organize their courses along mechanistic lines should give this text careful consideration. R. Daniel Libby Chemistry Department Moravian College Bethlehem, PA 18018
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