Q7(d). 1H NMR of ethyl (E)-but-2-enoate
Assign the observed chemical shift values of 1.22, 1.88, 4.13, 5.81 and 6.95 for the following compound:
Answer in brief
1.22 OCH₂CH₃; 1.88 CH₃-C=; 4.13 OCH₂; 5.81 H-α; 6.95 H-β. The β-H is most deshielded by conjugation with C=O, and the large J (15–18 Hz range) shows the trans alkene.
Key points
- 4.13 against 1.22: the ethyl group of the ester; OCH₂ is next to electronegative O and appears at 4.13 (quartet), its CH₃ at 1.22 (triplet).
- 6.95 against 5.81: in an α,β-unsaturated carbonyl compound the π electrons are pulled towards C=O (C=C-C=O <→ ⁺C-C=C-O⁻), leaving a partial positive charge on the β-carbon. H-3 (β) is therefore far more deshielded (6.95) than…
- 1.88: the CH₃ on C-3 is allylic; it is coupled to H-3 (about 7 Hz) and, weakly, to H-2 (four-bond allylic coupling).
- The large vinylic coupling, J_(H2,H3) in the 15–18 Hz range, confirms the trans (E) geometry (cis vinylic couplings are about 10 Hz).
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. The compound is ethyl (E)-but-2-enoate (ethyl crotonate): an α,β-unsaturated ester with H and H trans. Five kinds of proton give five signals. Shifts follow the electronic environment; multiplicities follow the n+1 rule.
Figure: Proton sets: a OCH₂CH₃, b OCH₂, c allylic CH₃, d H-2 (α), e H-3 (β).
| δ | H | multiplicity | reason |
| 1.22 | a, OCH₂CH₃ (3H) | triplet (J≈7 Hz) | ordinary CH₃, next to CH₂ |
| 1.88 | c, CH₃-CH= (3H) | doublet of doublets (7 Hz and a small allylic J) | allylic CH₃, slightly deshielded by C=C |
| 4.13 | b, OCH₂ (2H) | quartet (J≈7 Hz) | on the ester oxygen: strongly deshielded |
| 5.81 | d, H-2, =CH-CO (1H) | doublet of quartets (large trans J, small allylic J) | vinylic, α to C=O |
| 6.95 | e, H-3, CH₃-CH= (1H) | doublet of quartets (large trans J, 7 Hz) | vinylic, β to C=O: most deshielded |
- 4.13 against 1.22: the ethyl group of the ester; OCH₂ is next to electronegative O and appears at 4.13 (quartet), its CH₃ at 1.22 (triplet).
- 6.95 against 5.81: in an α,β-unsaturated carbonyl compound the π electrons are pulled towards C=O (C=C-C=O <→ ⁺C-C=C-O⁻), leaving a partial positive charge on the β-carbon. H-3 (β) is therefore far more deshielded (6.95) than H-2 (α, 5.81). H-3 is also split by the CH₃ (quartet).
- 1.88: the CH₃ on C-3 is allylic; it is coupled to H-3 (about 7 Hz) and, weakly, to H-2 (four-bond allylic coupling).
- The large vinylic coupling, J_(H2,H3) in the 15–18 Hz range, confirms the trans (E) geometry (cis vinylic couplings are about 10 Hz).
Conclusion. 1.22 OCH₂CH₃; 1.88 CH₃-C=; 4.13 OCH₂; 5.81 H-α; 6.95 H-β. The β-H is most deshielded by conjugation with C=O, and the large J (15–18 Hz range) shows the trans alkene.
Application. The α/β vinyl-proton pattern and the size of J are used routinely to assign E/Z geometry of enones and acrylates, for example in checking the products of Wittig and Knoevenagel reactions.
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All parts of Question 7
- 7(a) Pinacol rearrangement; Wolff–Kishner beside a ketal15 marks
- 7(b) Electrocyclic reactions: hexatriene, pentadiene, and ethene15 marks
- 7(c) Synthetic rubbers: Buna-S and neoprene10 marks
- 7(d) 1H NMR of ethyl (E)-but-2-enoate10 marks