Question

Question: Draw a stepwise mechanism for the following reaction

Short answer

Answer

Mechanism of reaction

Step-by-step solution

Reaction of alcohols with hydrogen bromide

When alcohols are reacted with HBr, a nucleophilic substitution reaction takes place to produce alkyl bromide.

The reaction involves the removal of hydroxyl group as water and consecutive addition of bromide forming alkyl bromide.

The reaction follows the S_N1 mechanism. The reaction proceeds in two steps- formation of carbocation and attack of the nucleophile.

Therefore, the reactivity of alcohols is in the order:

Tertiary > seconday > primary

Resonance in carbocation

The carbocation from primary alcohols is highly unstable.

If the carbocation formed in the first step has a double bond in conjugation with the positive charge, it can undergo electron delocalization to attain stability.

This delocalization is called resonance and produces more than one structure for the carbocation intermediate.

Nucleophilic attacks occur on both the canonical structures of the carbocation, forming multiple products.

Reaction of but-2-enol with HBr

The reaction follows the S_N1 mechanism. It involves two steps

The first step is the proton abstraction by alcohol, followed by removal of water to form the carbocation intermediate.

The carbocation formed in this step can undergo resonance. Electron delocalization occurs between a positive charge and double bond to form two canonical structures.

In the second step, the nucleophile is added to the carbocation intermediate. The addition of a nucleophile to the two canonical structures of intermediate produces two products that are positional isomers.

The reaction mechanism can be schematically given as follows:

Mechanism of reaction between alcohol and HBr