Question

On treatment with HBr, vinylcyclohexane undergoes addition and rearrangement to yield 1-bromo-1-ethylcyclohexane. Using curved arrows, propose a mechanism to account for this result.

Short answer

Protonate the vinyl group, rearrange carbocation, then bromine attacks to form 1-bromo-1-ethylcyclohexane.

Step-by-step solution

Protonation of Vinyl Group

Vinylcyclohexane contains a double bond between the vinyl group and the cyclohexane ring. In the presence of HBr, the double bond can be protonated to form a more stable carbocation. Use a curved arrow to show the electron pair from the double bond attacking the hydrogen in HBr, generating a more stable alkyl carbocation at the secondary carbon of the vinyl group.

Carbocation Formation and Rearrangement

The secondary carbocation formed in Step 1 is subject to rearrangement. A 1,2-hydride shift can occur, where a hydrogen atom (with its bonding electrons) from an adjacent carbon atom migrates to the carbocation. This shift leads to the formation of a more stable tertiary carbocation on the cyclohexane ring.

Nucleophilic Attack by Bromine

With the tertiary carbocation now formed on the cyclohexane ring, the bromide ion from the dissociated HBr can attack this carbocation site. Use a curved arrow to demonstrate the lone pair on the bromide ion attacking the positively charged carbon center, completing the nucleophilic substitution and forming 1-bromo-1-ethylcyclohexane.

Key concepts

Vinylcyclohexane

Vinylcyclohexane is an interesting compound with both a vinyl group and a cyclohexane ring. The vinyl group consists of a carbon-carbon double bond directly attached to a carbon that is part of a ring. This unique structure offers exciting opportunities in chemical reactions due to the presence of the double bond. In organic chemistry, double bonds are reactive sites. When paired with reagents like HBr, they can undergo specific transformations. In the case of vinylcyclohexane, HBr adds to the double bond, triggering further rearrangements. Understanding the nature of vinylcyclohexane is crucial for predicting reactions like carbocation rearrangement, where the compound undergoes shifts to form more stable intermediates. The double bond's reactivity is central to these processes and contributes to the complexity and utility of such organic molecules.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of chemical reaction where a nucleophile, an electron-rich species, attacks an electron-poor target. This results in the replacement of a leaving group by the nucleophile. In the mechanism involving vinylcyclohexane, once the more stable tertiary carbocation is formed, the bromide ion, which is now a nucleophile, comes into play.

• The bromide ion attacks the positively charged carbon center of the carbocation.
• This attack completes the nucleophilic substitution, replacing the hydrogen previously associated with the Br.

This step ensures that the final product, 1-bromo-1-ethylcyclohexane, is generated, marking the completion of this substitution process. Understanding this concept helps in visualizing the movement of electrons and how new bonds are formed.

Hydride Shift

A hydride shift is a fascinating event in organic reactions, involving the migration of a hydrogen atom along with its bonding electron pair. This phenomenon is a type of rearrangement often seen in carbocations, contributing to their stability. In the reaction with vinylcyclohexane, after the initial carbocation is formed, a 1,2-hydride shift takes place:

• A hydrogen atom, along with its electrons, moves from an adjacent carbon to the positively charged carbon center of the initial secondary carbocation.
• This shift creates a more stable tertiary carbocation, situated on the cyclohexane ring instead of the vinyl group.

This rearrangement is crucial because the formation of a more stable carbocation increases the likelihood of a successful nucleophilic attack, thus steering the reaction towards completion. Understanding hydride shifts is essential for mastering complex rearrangements in organic chemistry.