Q19-9P - Organic Chemistry (Mcmurry)

Question

Cyclohexanone forms a cyanohydrin in good yield but 2,2,6-trimethylcyclohexanonedoes not. Explain.

Step-by-Step Solution

Answer

2,2,6-trimethylcyclohexanone does not form cyanohydrin in good yield due to the presence of steric hindrance in it.

Step 1:Nucleophile addition of HCN

Nucleophilic addition reaction on “Aldehydes and unhindered ketones” with HCN results in the formation of cyanohydrins with the chemical formula (RCH(OH)C $\equiv$N).

 

Nucleophile addition of hydrogen cyanide (HCN) on carbonyl carbon is a “reversible and base-catalyzed” chemical reaction.

Step 2: Hindered Ketone

The nucleophile addition of cyanide to the carbonyl group of ketone 2,2,6trimethylcyclohexanone is the reversible reaction; cyanohydrin formation is an equilibrium process.

2,2,6 trimethylcyclohexanone is sterically hindered by the presence of three methyl groups, making it difficult to attack the upcoming nucleophile cyanide $\left({\text{CN}}^{\text{-}}\right)$ on the carbonyl carbon; therefore, the equilibrium of the reaction lies toward the side of unreacted ketone more than the product cyanohydrin.

The equilibrium favoured toward the unreacted ketone is shown as follows:

Formation of cyanohydrin not favoured