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Question: Given below is the list of compounds: A. \[HCHO\] B. \[C{H_3}CHOC{H_3}\] C. \[PhCOC{H_3}\] D...

Given below is the list of compounds:
A. HCHOHCHO
B. CH3CHOCH3C{H_3}CHOC{H_3}
C. PhCOCH3PhCOC{H_3}
D. PhCOPhPhCOPh
The increasing order of the rate of HCNHCN addition to compounds, A,B,C and D is.
1. A<B<C<DA < B < C < D
2. D<B<C<AD < B < C < A
3. D<C<B<AD < C < B < A
4. C<D<B<AC < D < B < A

Explanation

Solution

The more the compound is reactive towards nucleophilic addition reaction, the more is the rate of HCNHCN addition to the compound. Also greater the electron deficiency of the C atom of carbonyl group, higher the rate of reaction.

Complete step by step answer:
HCN is basically hydrogen ion (H+{H^ + }) + cyanide ion (CN−C{N^ - }). We know that cyanide ion behaves as a good nucleophile. Thus, the addition of HCNHCN is a nucleophilic addition reaction.
The aldehydes have smaller hydrogen atoms attached to the carbonyl carbon which makes it easier for a nucleophile to attack on that specific compound and thus they are reactive towards nucleophilic addition reactions. On the other hand, ketones have bulkier alkyl groups on both the sides of the carbonyl compound, which makes it difficult for the nucleophile to attach because of the increased steric hindrance. In this way, due to +I effect (will increase stability due to more electron density on carbonyl carbon because of the presence of electron donating groups like methyl group), ketones are less reactive towards nucleophilic addition reactions than aldehydes.
Thus, the most reactive of all the given compounds is formaldehyde i.e.   HCHO\;HCHO, as it has 2 hydrogen atoms attached to the carbonyl carbon which makes it feasible for CN−C{N^ - } ions to attack.
It is clear that the more is the +R effect, the stable is the compound and the reactivity will be lower. Phenyl group shows more +R effect than the simpler methyl group, which does not show +R effect at all. The second compound i.e. CH3CHOCH3C{H_3}CHOC{H_3} have two methyl groups attach to it, which are smaller in size as compared to the phenyl groups, thus, this compound would be second most reactive compound amongst all the given compounds.
The fourth compound i.e. PhCOPhPhCOPh is the most stable and least reactive of all due to increased +I effect and increased steric hindrance by two phenyl groups because as the size of the alkyl group increases, their reactivity decreases.

Hence, the increasing order of the rate of HCNHCN addition to compounds would be 3.D<C<B<AD < C < B < A.

Note: All the alkyl groups show +I effect and all the aryl groups example benzene ring will not show any +I effect, because benzene ring does not act as electron donating species. Thus, the less reactivity of the third and fourth compound is due to the +R effect and steric hindrance, there is no +I effect present there.