question_answer

Which of the following compounds is most reactive towards nucleophilic addition reactions?

A)  \[C{{H}_{3}}CHO\]                         

B)  \[C{{H}_{3}}COC{{H}_{3}}\]

C)  \[{{C}_{6}}{{H}_{5}}CHO\]                          

D)  \[{{C}_{6}}{{H}_{5}}COC{{H}_{3}}\]

Correct Answer: A

Solution :

                \[C{{H}_{3}}CHO\] is the most reactive towards nucleophilic addition reactions. \[{{C}_{6}}{{H}_{5}}CHO>{{C}_{6}}{{H}_{5}}COC{{H}_{3}}>{{C}_{6}}{{H}_{5}}CO{{C}_{6}}{{H}_{5}}\] In case of aliphatic aldehydes and ketones, greater the number of alkyi groups attached to the carbonyl group greater is the electron density on the carbonyl carbon and hence lower is its reactivity towards nucleophilic addition reactions. Further more as the number and the size of the alkyl groups increases, the attack of the nucleophile on the carbonyl group becomes more and more difficult due to steric hindrance. As the steric hindrance increases, the reactivity decreases accordingly. Aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehydes and ketones. It is due to the reason that electron donating resonance effect (+R effect) of the benzene ring increases the electron density on the carbonyl carbon thereby repelling the nucleophiles.