Rearrangement On Vinyllic Carbocation

Acid catalyzed hydration of phenyl acetylene a terminal alkyne involves a vinylic carbocation intermediate.

Rearrangement on vinyllic carbocation. The mechanism of a 1 2 hydride shift and 1 2 alkyl shift. 1 2 hydride shifts are fairly common in alkyl cations and is fast in the nmr time scale. The smaller alkyl substituent tends to be the substituent that shifts. This stabilizes the carbocation.

A carbocation is electron deficient and with an incomplete octet and a positive charge on it. Vinyl cations undergo 1 2 hydride shifts to form an allyl stabilized cation. So this tertiary carbocation is more stable than the secondary ones. In the first mechanism step the alkyne is protonated by hydronium ion a strong acid to produce a resonance stabilized secondary vinylic carbocation shown in red.

Let s do another carbocation rearrangement problem. However in vinyl cations this rearrangement is uncommon even though the rearrangement product in thermodynamically stable. So this one s actually a little bit easier than the previous one. According to me a rearrangement would lead to an allylic carbocation which is more stable than vinylic hence the rearrangement should be favorable.

Protonation of the double bond results in a secondary carbocation step 1. Rearrangement to a 2º carbocation is favored by relief of small ring strain in the case of pinene and relief of steric congestion in the case of camphene. The general rule in alkyl shifts is. Why do vinylic carbocations generally not undergo hydride rearrangement from neighbouring sp 3 carbon to get more stability.

So here s our carbocation and the carbon with the plus one formal charge is directly bonded to two other carbons which makes this a secondary carbocation. The positive charge is stabilized by the addition of a nucleophile thus the formation of a new covalent bond takes place. Therefore the most common 1 2 alkyl shift is a 1 2 methyl shift. If a secondary carbocation is vicinal to a quaternary carbon a 1 2 alkyl shift should occur.

This carbocation is also a benzylic carbocation. Carbocation rearrangements are extremely common in organic chemistry reactions are are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural reorganizational shifts within the molecule. Addition of a nucleophile.