Concerted reactions
May 2016
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).
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.