Polar Addition and Elimination Reactions

Addition and elimination processes are the reverse of one another. There is a close relationship between the two reactions, and in many systems the reaction can occur in either sense. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

The addition and elimination reactions can proceed by similar mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as in the elimination reaction. The reversible acid-catalyzed reaction of alkenes with water represents such a system. 

The initial discussion will focus on addition reactions. The discussion here is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chapter 1, Section 1.10). 

Several limiting generalized mechanisms can be described for polar additions. 

Another familiar pair of reactions is hydrohalogenation and dehydrohalogenation. 

The addition of hydrogen halides to alkenes has been studied from a mechanistic point of view for many years. One of the first aspects of the mechanism to be established was its regioselectivity, that is, the direction of addition. A reaction is described as regioselective if an unsymmetrical alkene gives a predominance of one of the two possible addition products the term regiospecific is used if only one product is formed.  

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Polar Addition and Elimination Reactions non-stereospecific or syn Br Bra - anti Br+ Br- ... 

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