Wagner Meerwein Rearrangement with Mechanism and it’s Application

Reactions involving the change in the carbon skeleton through the rearrangement of the carbocation intermediate are collectively known as Wagner-Meerwein rearrangement.

When neopentyl bromide is hydrolysed under SN 1 condition it Is found that instead of the expected neopentyl alcohol (Me3CCH20H), 2-methyl-butan-2-ol and 2-methyl but-2-ene are formed

Similarty, neopentyl alcohol on dehydration gives 2-methyl-but-2-ene and 2-methyl-but-1-ene.

Mechanism Protonation of the hydroxyl group of the alcohol followed by the loss of water molecule affords a 1°carbocation. The 1° carbocation then rearranges to the relatively more stable 3° carbocation by 1, 2-methyl shift before the product is formed. The greater stability of the 3° carbocation provides the necessary force for the migration of the methyl group from the adjacent carbon. The rearranged carbocation 1hen stabilizes itself by the loss of a ~-hydrogen to form olefin. Zaitsev’s rule governs as to which of the two ~-hydrogens will be eliminated.

(Hydrolysis of neopentyl halide similarly gives first a rearranged 3° carbocation which can either accept a nucleophile from the solvent to give alcohol or eliminate a proton to give olefin; see eq. 1.)

Rearrangement does not occur if the initially formed carbocation can stabilize by other factors. Thus. 3, 3-dimethyl-2-bromo butane (neopentyt type) undergoes~ 1 hydrolysis with rearrangement.

while its phenyl analogue gives product without rearrangement

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