UPSC Civil Services (Mains), Chemistry optional · 2021 · Paper II
Q1(d). Two aldehydes towards a nucleophile in the presence of BF₃·OEt₂
Question
Discuss the reactivity of the following compounds towards a nucleophile in the presence of BF₃·OEt₂: (i) p-trifluoromethylbenzaldehyde (ii) p-tolualdehyde.
Answer in brief
BF₃ makes both aldehydes more electrophilic; the complexed p-CF₃ aldehyde is the stronger electrophile, so with enough BF₃ to complex both, (i) reacts faster than (ii).
Key points
- BF₃ accepts an oxygen lone pair: the carbonyl carbon becomes more electrophilic.
- (i) CF₃ withdraws electrons (−I): larger δ+ on carbon; faster nucleophilic attack.
- (ii) CH₃ donates (+I, hyperconjugation): smaller δ+; slower attack.
- Same donation makes oxygen of (ii) more basic: binds BF₃ more firmly.
- Excess BF₃: both fully complexed, (i) > (ii); little BF₃: gap narrows.
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.
CoversLewis acid catalysisBF3 etherateHammett sigmanucleophilic addition to carbonylp-tolualdehydeLUMO lowering
All parts of Question 1
- 1(a) Origin of the CN group in RCOO⁻ + BrCN10 marks
- 1(b) Benzyltrimethylammonium salt with sodium amide at low and at high temperature10 marks
- 1(c) 2-Cyclohexylpropan-2-ol with BF₃·OEt₂: products A and B10 marks
- 1(d) Two aldehydes towards a nucleophile in the presence of BF₃·OEt₂10 marks
- 1(e) Two transformations by electrocyclic relays10 marks