The increasing order of the rate of HCN addition to compounds A -D is

 

(A) HCHO      (B) CH₃COCH₃       (C) PhCOCH₃   (D) PhCOPh

 

1) C<D<B<A       2) A<B<C<D        3) D<B<C<A      4) D<C<B<A

Hello.

For reactivity of corbonyl compounds, just keep one factor in mind while attempting such questions :::::::

STEARIC HINDRANCE. 

 

This is the most important factor while attempting such questions regarding carbonyl compounds( ketones and aldehydes).

Therefore, the correct option is (4).

Now see, stearic hindrance in (a) is least coz the groups attached to the =CO group are the two hydrogens, which are the lightest of all substituents. Hence, the nucleophile will have no problem while attacking the C of =CO.

 

similarly, the order of bulkiness is :-

    -H > -CH₃ > -Ph

 

Therefore the corect order is 4) D<C<B<A.

 

 

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