Halogens in Organic Synthesis Some Classical Reactions

9 HALOGENS IN ORGANIC SYNTHESIS SOME CLASSICAL REACTIONS 

We are now in a good position to appreciate a variety of halogen-based reagents that are part of the modern organic chemist s repertoire. Before we do so, however, it may be useful to briefly recapitulate some of the classical applications of halogens in organic chemistry. 

As briefly discussed in Chapter 1 (Section 1.11), molecular bromine adds to carbon-carbon double bonds by way of an ionic mechanism. The first step of the addition results in a cyclic bromonium ion, which is then opened up by a bromide ion. Such a 

The following substitution reaction, which occurs on exposure of the substrate to sodium formate in formic acid, proceeds with full retention of configuration o  

Suggest a mechanistic rationale. (Hint Think of halonium ion intermediates)  

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