Question

Write down functional isomers of a carbonyl compound with molecular formula \[\ce{C3H6O}\]. Which isomer will react faster with \[\ce{HCN}\] and why? Explain the mechanism of the reaction also. Will the reaction lead to the completion with the conversion of whole reactant into product at reaction conditions? If a strong acid is added to the reaction mixture what will be the effect on concentration of the product and why?

Answer 1

\[\ce{C3H6O}\] will have following isomer.

\[\ce{\underset{Proponal (I)}{CH3CH2CHO}}\]

\[\begin{array}{cc}
\phantom{}\ce{O}\\
\phantom{}||\\
\ce{\underset{Propan-2-one (II)}{CH3 - C - CH3}}
\end{array}\]

Compound I will react faster with HCN due to less steric hindrance and greater positive charge on carbon atom of carbonyl group. Two methyl groups increase electron density on carbonyl carbon in compounds II hence the rate of nucleophilic attack is less.

Mechanism of the reaction:

\[\ce{HCN + OH- -> CN- + H2O}\]

The reaction will not lead to completion since it is a reversible reaction, hence equilibrium is established.
If a strong acid is added to reaction mixture, the addition is inhibited because the formation of \[\ce{CN}\]” ions form \[\ce{HCN}\] is prevented.