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.

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 found a dashed line was often used to guide bond formation in Diels-Alder or rearrangement reactions. I have adopted this convention and used the term ‘pre-bonds’ for them. By adopting them, students have reported mechanisms with ‘pre-bonds’ were easier to understand.

Table of Contents and Index
The Table of Contents and Index show the depth of coverage of reaction types. There is also an extended index to enable students using textbooks organized by functional groups. The index correlates reactions by reactions of starting materials and reactions to yield products by functional group type.

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.

In answering student questions, I have found some students do not wish to see a reaction mechanism. However, if those students knew the reaction mechanism, they would not be asking the question. The questions reveal a lack of understanding that is common.

Example Based Teaching
For organic chemistry, I prefer examples of reactions rather than generic descriptions with conditions and exceptions. Many students may think that chemistry is very black and white, right or wrong. However, there is a much larger degree of uncertainty than many students realize. For example, there is much confusion over SN1, SN2, E1, and E2 reactions. Before I wrote the mechanisms for these reactions, I first reviewed several texts. In doing so, I was seeking to learn what common factors I could present. I did not think that I could clarify this confusion in some insightful paragraph. I did two things. First, 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 another 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 reaction concerted reactions 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. A concerted reaction asks bromine to acquire a positive charge and the concerted reaction makes no electronic demands from an alkene. The first step for many other alkene reactions is electron donation. In other bromine reactions, it acts as an electron acceptor. I believe a step-wise mechanism better illustrates this chemistry and thus should be preferred. I would ask that you write a stepwise reaction and thus agree with other stepwise mechanisms (although you may also write it as a concerted reaction if you wish). I plan to discuss this reaction further at another time.

The addition of HCl to an alkene is known to not be a 2 + 2 addition. In writing other electrophilic reactions or alkenes, I sought a uniform reaction mechanism. I did not wish to require the electrons of an alkene to react differently with other electrophiles. Accordingly, other 2 + 2 addition reactions are written as stepwise reactions and maintain this pattern. (Reactions of nucleophiles with alkenes as electrophiles are a different mechanism and occur in another chapter of the book.) A definition for a concerted reaction is any steps that occur faster than atom movements. Thus, if the bonds do not rotate in the intermediate, then it can be considered concerted. I did not feel that a slow electrophilic reaction followed by a fast electron ring formation step contradicted a concerted reaction.

For example, although non-concerted Diels-Alder reactions have been suggested, no mechanistic advantage is served by a stepwise alternative. A Guide to Organic Chemistry Mechanisms does contain an example of a Diels-Alder reaction in which a stepwise mechanism may help explain the outcome of the reaction. However, the same result can also be anticipated by the polarity of the reactants. The example shows you how to discover the polarity and match them to the major product.