Q8(b). 1H NMR: furfural, arenes, ethanol, nitroacetophenones

Introduction. Chemical shift tells the electronic environment of a proton, and spin–spin coupling (n+1 rule, coupling constant J in Hz) tells how many neighbouring protons it has and how they are related. In aromatic rings Jₒᵣₜₕₒ (7–9 Hz) ≫ Jₘₑₜₐ (1–3 Hz) > Jₚₐᵣₐ (<1 Hz).

(i) Furfural (furan-2-carbaldehyde)

Four signals. The ring protons form an AMX system; each is a doublet of doublets.

• CHO: δ 9.65 (s; a small 0.8 Hz long-range coupling may be seen).
• H-5 (next to O): δ 7.70, dd, J_(4,5)≈1.7, J_(3,5)≈0.8 Hz.
• H-3 (next to CHO): δ 7.25, dd, J_(3,4)≈3.6, J_(3,5)≈0.8 Hz.
• H-4: δ 6.60, dd, J_(3,4)≈3.6, J_(4,5)≈1.7 Hz.

Figure: H-3, H-4, H-5 on the ring.

In the furan ring J_(3,4) (≈3.6 Hz) is much larger than J_(4,5) (≈1.7 Hz): the 4,5-bond lies next to the electronegative ring oxygen, which lowers the coupling. Both are much smaller than benzene ortho couplings, because the five-membered ring has wider external angles. H-5 is most deshielded among ring H (adjacent to O); H-3 is deshielded by the C=O.

(ii)(A) Anthracene and phenanthrene

Anthracene (D₂ₕ) has only three kinds of H: H-9,10 δ 8.43 (singlet, 2H), H-1,4,5,8 δ 8.00 (m, 4H), H-2,3,6,7 δ 7.45 (m, 4H). Phenanthrene (C₂ᵥ) has five kinds: the bay protons H-4,5 at δ 8.70 (doublet, 2H, strongly deshielded by the facing ring), H-1,8 δ 7.90 (d), H-9,10 δ 7.75 (s), and H-2,3,6,7 δ 7.55–7.65 (m). So the lowest-field signal is a 2H singlet for anthracene, but a 2H doublet for phenanthrene, and the integrals divide 2: 4: 4 against 2: 2: 2: 4.

(ii)(B) Pure ethanol and ethanol with a trace of acid

In very pure, dry ethanol the OH proton exchanges slowly (lifetime on O longer than about 1/J). It couples with the CH₂: OH is a triplet (J≈5 Hz), CH₂ (δ 3.7) is a doublet of quartets (eight lines), CH₃ (δ 1.2) a triplet. A trace of acid (or base) catalyses rapid proton exchange; the spin of the OH proton is averaged, coupling to OH disappears, the OH becomes a sharp singlet and CH₂ a simple quartet.

(iii) 2- and 3-Nitroacetophenone

Scheme: Both have four ring H, arranged differently.

The low-field singlet near δ 8.75 marks the 3-isomer. In the 2-isomer the acetyl group is twisted out of the ring plane by the ortho nitro group, so the ring protons and the CH₃ are less deshielded by the C=O.

Conclusion. (i) Furfural: three dd ring signals with J_(3,4)≈3.6, J_(4,5)≈1.7, J_(3,5)≈0.8 Hz, and CHO at δ 9.65. (ii) Anthracene has three signals with a low-field singlet; phenanthrene five, with a low-field bay doublet. Pure ethanol shows an OH triplet; with acid the OH is a singlet and CH₂ a quartet. (iii) Only 3-nitroacetophenone shows an isolated low-field singlet (H-2).

Application. Proton-exchange decoupling is used to identify OH and NH signals (D₂O shake); coupling patterns of substituted benzenes are used routinely to assign isomers in drug impurity profiling.