Rearrangement Reactions of Alkyl and Alkenyl Halides

Numerous examples of the Favorskii (Faworsky) rearrangement, including some with labeled carbon (see Problem 7.13), are known and the reaction will be encountered again. 

Problem 7.13. When 2-chlorocyclohexanone-2- C is treated with sodium methoxide in methanol, labeled methyl cyclopentanecarboxylate results. Careful degradation of the carboxylic acid ester (Chapter 9) showed that the location of the position of the labeled carbon was restricted to three carbon atoms, (a) On which three carbons is the label found (b) What is the distribution of the label (i.e., what percent of the label is found at each carbon)  

Rearrangements, it will be recalled, involve nonrandom structural interconversions in which the same number of nuclei is present in the substrate and the product but their connectivity has changed. For example, in addition to undergoing SnI and 

Sn2 reactions, allylic halides can react with rearrangement of the carbon-carbon double bond. 

Many of the rearrangements of alkyl chlorides were first observed in the exploration of the chemistry of the naturally occurring group of hydrocarbons called terpenes (see Chapter 6, Section 1, Oxidation, for an introduction to this family of CIO hydrocarbons). Although similar rearrangements were known before the twentieth century began, their more detailed examination was not undertaken until about 1920.  

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