Q1(c). The reaction that is not a condensation, and its mechanism
Out of the following list of reactants along with the reagents, identify the reaction which is not a condensation reaction. Propose a suitable mechanism for the products formed in this reaction: (i) Acetaldehyde is reacted with dilute KOH; (ii) Benzaldehyde is reacted with acetic anhydride in presence of sodium acetate; (iii) Benzaldehyde is reacted with malonic ester in presence of a base; (iv) Benzaldehyde is reacted with concentrated KOH.
Answer in brief
(iv): benzaldehyde with concentrated KOH is the Cannizzaro reaction, a disproportionation giving benzyl alcohol and potassium benzoate; no new C–C bond forms.
Key points
- PhCHO has no α-hydrogen: no enolate, so no aldol-type condensation.
- OH⁻ adds to C=O: tetrahedral adduct (fast, reversible).
- Adduct delivers hydride to a second PhCHO: slow step.
- One molecule is oxidised (benzoate), one reduced (benzyl alcohol).
- Proof: in D₂O, PhCH₂OH has no C–D: H passes carbon to carbon.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversCannizzaro reactionaldol condensationPerkin condensationKnoevenagel condensationhydride transferdisproportionation
All parts of Question 1
- 1(a) Ionisation of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅–SO₂10 marks
- 1(b) (4-Nitrophenyl)oxirane with methoxide in methanol10 marks
- 1(c) The reaction that is not a condensation, and its mechanism10 marks
- 1(d) Acid-catalysed rearrangement of two phenyl-substituted 1,2-diols10 marks
- 1(e) Heating cyclopentadiene10 marks