Discussion
The 1H-NMR spectral data of a compound X is as follows: δ (ppm): 11.6 (s, 1H, exchangeable with D2O), 7.60 (d, 1H, J = 7.2 Hz), 7.20 (t, 1H, J = 7.2 Hz), 6.90 (d, 1H, J = 7.2 Hz), 3.70 (s, 3H), 3.58 (s, 3H). Based on the spectral data, choose the appropriate structure for compound X.
- A.
- B.
- C.
- D.
Answer: B
The compound is 2,3-dimethoxybenzoic acid, option (B).
The 11.6 ppm singlet that exchanges with D2O is the carboxylic acid proton. The two 3H singlets at 3.70 and 3.58 ppm are two different OCH3 groups. The three aromatic signals (doublet, triplet, doublet, all with J = 7.2 Hz) are three adjacent ring protons: a triplet needs a proton with two neighbours, so the ring is 1,2,3-trisubstituted. Only (A) and (B) have three adjacent ring hydrogens.
In (A), 2,6-dimethoxybenzoic acid, the two OCH3 groups are equivalent by symmetry and would give a single 6H singlet. Options (C) and (D) do not have three adjacent ring hydrogens, so neither can show a doublet, triplet, doublet pattern. Only (B) gives two distinct OCH3 singlets and a d, t, d pattern.
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