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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.
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