9701 MCQ Solution 40

Question 40: [Organic > Carbonyl Compounds]:

In the reaction between an aldehyde and HCN catalysed by NaCN, which statements about the reaction mechanism are true?

1 A new carbon-carbon bond is formed.

2 In the intermediate, the oxygen carried a negative charge.

3 The last stage involves the formation of a hydrogen-oxygen bond.

Reference: Past Exam Paper – November 2002 Paper 1 Q40

Solution 40:

Answer: A.

The CN^- ions attack the organic compound creating a new C-C bond, thus 1 is correct. In the intermediate the oxygen will carry a negative charge in order to attract the remaining H⁺ ion, thus 2 is also correct. Due to the formation of the O-H bond 3 must also be correct thus A is the valid option. 

For the sake of argument, to ensure that 3 is completely correct the O-H bond must be the last stage. This is proved by the fact that the mechanism of this reaction: nucleophilic addition. This means that first the nucleophile (CN^-) attacks then the remaining ions (H⁺) react last, hence completely proving 3.