A Guide to Organic Chemistry Mechanisms

Objective of the book

The purpose of this book is to teach students how to learn organic chemistry. Because reactions and reaction mechanisms are the most difficult challenge, the focus of this book is entirely on reaction mechanisms.

Curved Arrows

From experiments I performed in my class, I learned that many students became confused in their attempts to learn reaction mechanisms. A particular difficulty was the use of the curved arrow convention, especially when a new bond is formed from a covalent bond. For example, in a Markovnikov addition reaction, the student must know the carbocation stabilities before comprehending the meaning of the curved arrow. The curved arrows do not tell the student what the product must be. I have adopted the dashed line convention to make reactions easily understood.

Table of Contents and Index

The Table of Contents and Index show the depth of coverage of reaction types. There is also a functional group index.

Reaction Mechanisms

A reaction mechanism is an attempt to explain the electron movements that lead from the starting materials to the products. In order to predict the products of a reaction, knowing how reactions take place is key. The mechanisms are written to give a logical explanation that will ultimately result in the product.

Example Based Teaching

For organic chemistry, I prefer examples of reactions rather than generic descriptions with conditions and exceptions. It is impossible to generalize all SN1, SN2, E1, and E2 reactions nor clarify them with some insightful paragraph. Before I wrote the mechanisms for these reactions, I first reviewed several texts. I sought common factors for each. I looked for examples that produced clear-cut results and that all textbooks would agree upon. That is, if you learned this reaction, you could be certain that it was correct. Secondly, I also looked for examples in which by-products were clearly identified. This gives an indication of other electron movements that might be competing with the described reaction. It was my objective that as you learn what the optimal conditions, you can begin to recognize the limitations to those reactions and expect to encounter other mechanisms or by-products.

Concerted Reactions

Concerted reactions are one of the largest differences in the mechanisms in A Guide to Organic Chemistry Mechanisms and other books. I have made a philosophic choice to not write reactions as concerted unless there was overwhelming evidence in its favor, or if no mechanistic understanding was served in a stepwise equivalent.

The reaction of an alkene with bromine is generally written as a concerted reaction involving three pairs of electrons and bromine ending up with a positive charge. I however disagree with this single-step concerted mechanism. Bromine is an electron acceptor (for example, Br- + Br2 Br3-) and alkenes are electron donors in many reactions. I believe a step-wise mechanism better illustrates this chemistry. However, you may write it as a concerted reaction if you wish. If you see it written as a stepwise reaction, this may help you understand the impetus for the reaction and agree with other mechanisms.

You needn’t take adhering to my mechanisms too seriously. It is more important for you to know how to reach the final products. I’ve suggested what occurs to me as the most logical explanation for a reaction. If your textbook or your professor suggests another, you should feel free to follow their advice. In such cases, my mechanism may be a helpful aid to recalling the formation of a product.