Indian Forest Service (Mains), Chemistry optional · 2023 · Paper II
Q1(h). Substituent effects in Cannizzaro reaction
Question
Rank the para-substituted benzaldehydes I Me, II NMe₂, III OMe and IV NO₂ in decreasing Cannizzaro reactivity.
Answer in brief
The usual qualitative order is IV > I > III > II.
Key points
- The reaction begins with hydroxide addition to the aldehyde. Electron withdrawal increases carbonyl electrophilicity and favours the tetrahedral donor species.
- Para NO₂ withdraws strongly through inductive and resonance effects and therefore gives the most reactive member in this comparison.
- Methyl is a weaker donor than methoxy or dimethylamino. OMe and especially NMe₂ donate by resonance, reducing aldehyde electrophilicity.
- All four lack an enolizable alpha H; the ranking is qualitative under comparable strongly basic conditions.
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 reactionsubstituted benzaldehydessubstituent effectscarbonyl electrophilicityhydride transfer
All parts of Question 1
- 1(a) Aromaticity of heterocycles5 marks
- 1(b) Evidence for a tetrahedral intermediate5 marks
- 1(c) Benzyne capture by tetracyclone5 marks
- 1(d) Para-alkyl effects on benzyl solvolysis5 marks
- 1(e) Dehydrobromination through an ion pair5 marks
- 1(f) Why nucleophilic shifts are common5 marks
- 1(g) A double oxy-Cope rearrangement5 marks
- 1(h) Substituent effects in Cannizzaro reaction5 marks