Q68E - Organic Chemistry (Mcmurry)

Question

We saw in section 17-4 that ketones react with ${\mathbf{\text{NaBH}}}_{\mathbf{\text{4}}}$ to yield alcohols. We’ll also see in section 22-3 that ketones react with ${\mathbf{\text{Br}}}_{\mathbf{\text{2}}}$ to yield $\mathit{\alpha }$-bromo ketones. Perhaps surprisingly, treatment with ${\mathbf{\text{NaBH}}}_{\mathbf{\text{4}}}$ of the $\mathit{\alpha }$ -bromo ketone from acetophenone yields an epoxide rather than a bromo alcohol. Show the structure of the epoxide, and explain its formation.

     Acetophenone                                An $\mathit{\alpha }$-bromo ketone

Step-by-Step Solution

Answer

                                          Epoxide

Step 1: Epoxides

Epoxides are cyclic organic compounds containing one oxygen atom in the carbon ring. Epoxides are formed by the nucleophilic substitution reaction of carbonyl compounds.

Step 2: Explanation

The mechanism for the formation of epoxide is shown below.

Acetophenone                                 An $\alpha$-bromo ketone                    Epoxide

The hydride ion from the Sodium boro hydride undergoes a reaction to the carbonyl center to give an oxide ion which then forms epoxide by leaving the Bromide ion from the compound.