question_answer

                Nucleophilic addition reaction will be most favoured in:                                                                

A)                  \[C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}\overset{\begin{smallmatrix}  O \\  || \end{smallmatrix}}{\mathop{C}}\,-C{{H}_{3}}\]                

B)                  \[{{(C{{H}_{3}})}_{2}}C=O\]

C)                  \[C{{H}_{3}}C{{H}_{2}}CHO\]                                    

D)                  \[C{{H}_{3}}CHO\]

Correct Answer: D

Solution :

                                  The carbonyl compounds undergo nucleophilic addition reaction, because oxygen is more electronegative than carbon. As such, it with­draws shared it electron pair towards itself and gets partial negative charge, therefore carbon get partial positive charge and becomes susceptible to nucleophilic attack.                 Aldehydes are more reactive than ketones towards nucleophiles. This can be explained on the basis of inductive effect as well as steric effect. The addition of nucleophiles is based upon the positive charge present on carbon atom of   group has the presence of at least one alkyl group (except formaldehyde) which has +I effect (electron donating effect) and which decreases the positive charge of carbon, thereby making the attack to nucleophile difficult. The nucleophilic attack becomes more difficult in ketones having minimum of two alkyl groups. Hence, by means of attachment of alkyl groups (due to + I effect) rate of nucleophilic addition decreases.                 Order of \[+I\] effect in alkyl group                                 order of nucleophilic  addition in given carbonyl compound is                 \[C{{H}_{3}}CHO>C{{H}_{3}}-C{{H}_{2}}-CHO>{{(C{{H}_{3}})}_{2}}CO>\]                 \[\begin{align}   & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,O \\  & C{{H}_{3}}-C{{H}_{2}}-C{{H}_{2}}-\overset{||}{\mathop{C}}\,-C{{H}_{3}} \\ \end{align}\]