You are employed at Phenomenal Phenols, Inc., and have been asked by your Supervisor, O. H. Gruppa, to identify a compound A that was isolated from natural sources. Compound A, mp 129�130 �C, is soluble in NaOH solution and in hot water. The IR spectrum of A shows prominent absorptions at 3300�3600 cm�1 (broad) and 1680 cm�1; the EI mass spectrum of A has prominent peaks at m/z = 166 and 107, and the CI mass spectrum has an M 1 peak at m/z =167. The NMR spectrum of A is given below. Titration determined that A has two acidic groups with pKa values of 4.7 and 10.4. The UV spectrum of A is virtually unchanged as the pH of a solution of A is raised from 2 to 7, but the ?max shifts to much higher wavelength when A is dissolved in 0.1 M NaOH solution.
The Correct Answer and Explanation is:
Based on the data provided, compound A is gallic acid (3,4,5-trihydroxybenzoic acid).
✅ Explanation (Approx. 300+ Words)
To identify compound A, we analyze the various pieces of spectral and chemical information:
🔬 1. Melting Point and Solubility:
- Melting point: 129–130 °C — consistent with gallic acid.
- Solubility: Soluble in NaOH and hot water → this indicates acidic functional groups, likely phenolic and carboxylic groups.
📉 2. IR Spectrum:
- 3300–3600 cm⁻¹ (broad): Indicates O–H stretching, typical of hydroxyl groups (especially phenols).
- 1680 cm⁻¹: Suggests C=O stretching, typical of a carboxylic acid.
💥 3. Mass Spectrometry:
- EI-MS:
- m/z = 166 → Likely the molecular ion (M⁺).
- m/z = 107 → A common fragment ion, possibly due to loss of COOH group.
- CI-MS:
- m/z = 167 → M + 1 peak (supports MW of 166 g/mol).
This mass is consistent with gallic acid (C₇H₆O₅):
- Molar mass: 170 g/mol (but often loses water or COOH in fragmentation).
- Exact mass from formula C₇H₆O₅ = 170.12 g/mol, with possible loss of small neutral molecules in EI-MS (explaining m/z = 166).
📊 4. NMR Spectrum:
Although the NMR spectrum isn’t provided in full detail here, gallic acid typically shows:
- Aromatic protons as a singlet due to symmetric substitution (only 2 protons on benzene ring).
- Broad signals from hydroxyl protons (OH groups).
- Downfield peak (~12 ppm) from COOH proton.
⚗️ 5. Acidity (pKa values):
- pKa = 4.7 → consistent with carboxylic acid group.
- pKa = 10.4 → typical for phenol OH.
🌈 6. UV-Visible Spectrum:
✅ Conclusion:
All the evidence — mass (m/z 166), IR (O–H, C=O), solubility, UV shift in base, and acidity profile — points strongly to gallic acid as compound A.