Friedel-Crafts reaction, Wagner-Meerwein

The carbonium ion may also be formed from an alkene or alcohol. The carbonium ion formed from any of these starting materials is particularly prone to rearrangement reactions. These are called Wagner-Meerwein rearrangements, and severely limit the synthetic utility of this reaction to form simple alkyl substituted aromatic compounds. The tendency to rearrange may be reduced if the acyl derivative is used instead. This modification is called the Friedel-Crafts acylation reaction, and it has the further advantage that normally only monoacylation occurs, instead of the polyalkylation that happens using the simple Friedel-Crafts reaction. 

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

In Section 5.2.5, we discussed the Friedel-Crafts alkylation of benzene with 2-chloropentane. This reaction includes a Wagner-Meerwein reaction in conjunction with other elementary reactions. The Lewis acid catalyst A1C13 first converts the chloride into the 2-pentyl cation A (Figure 11.3). Cation A then rearranges into the isomeric 3-pentyl cation B, in part or perhaps to the extent that the equilibrium ratio is reached. The new carbenium ion B is not significantly more stable than the original one (A),... 

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