Here's an example explaining Wagner-Meerwein Rearrangements
2,2-dimethyl-1-bromopropane has very low SN2 reactivity , due to stearic hindrance. Under conditions that favor SN1 reactivity, such as solution in wet formic acid, neopentyl bromide reacts at roughly the same rate as ethyl bromide. Both of these compounds are 1º-alkyl halides, and for an SN1 reaction the rate determining step requires ionization to a 1º-carbocation. As noted in the carbocation stability order shown below, such carbocations are relatively unstable and are formed slowly.
Carbocation
Stability | CH3(+) | < | CH3CH2(+) | < | (CH3)2CH(+) | ? | CH2=CH-CH2(+) | < | C6H5CH2(+) | ? | (CH3)3C(+) | |
The product from ethyl bromide is ethanol, the simple and direct substitution product, but neopentyl bromide yields 2-methyl-2-butanol instead of the expected neopentyl alcohol. A change in the way the five carbon atoms in this product are bonded to each other has clearly taken place. Once formed, the ethyl cation can only be transformed by a substitution or elimination process. In the case of the neopentyl cation, however, the initially formed 1º-carbocation may be converted to a more stable 3º-carbocation by the 1,2-shift of an adjacent methyl group with its bonding electrons. A mechanism demonstrating such a rearrangement is shown below, and it explains the overall structural changes very nicely.
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Increasing the stability of carbocation intermediates is not the only factor that leads to molecular rearrangement. If angle strain , torsional strain or steric crowding in the reactant structure may is relieved by an alkyl or aryl shift to a carbocation site, such a rearrangement is commonly observed. The following examples illustrate rearrangements induced by the strain in a small ring. Although a 3º-carbocation is initially formed, the angle and torsional strain of the four-membered ring is reduced by a methylene group shift resulting in ring expansion to a 2º-carbocation
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