Show the IR, Mass and NMR spectrum of trans-cinnamic acid with brief explanations.
The Correct Answer and Explanation is:
Spectral Analysis of Trans-Cinnamic Acid
1. Infrared (IR) Spectrum of Trans-Cinnamic Acid
Key IR Absorptions:
- Broad O–H stretch (carboxylic acid): ~2500–3300 cm⁻¹
- C=O stretch (carboxylic acid): ~1700 cm⁻¹
- Aromatic C=C stretch: ~1600, 1500 cm⁻¹
- C–H stretch (sp², alkene and aromatic): ~3020–3100 cm⁻¹
- C–H stretch (sp³, alkyl): ~2850–2960 cm⁻¹
The IR spectrum prominently displays a very broad and intense O–H stretch due to hydrogen bonding of the carboxylic acid. The strong, sharp C=O stretch near 1700 cm⁻¹ confirms the presence of a carbonyl group. The aromatic and alkene C=C bonds produce medium intensity bands near 1600 and 1500 cm⁻¹.
2. Mass Spectrum of Trans-Cinnamic Acid
Molecular formula: C₉H₈O₂
Molecular weight: 148 g/mol
Key peaks:
- m/z 148 – Molecular ion peak [M⁺]
- m/z 103 – Base peak, likely from loss of COOH (–45)
- m/z 77 – Benzyl fragment (C₆H₅⁺)
The molecular ion at m/z 148 confirms the intact molecular weight. The base peak at m/z 103 results from fragmentation of the carboxylic acid group. The phenyl ring cleavage gives the stable benzyl cation at m/z 77.
3. Proton NMR (¹H NMR) Spectrum of Trans-Cinnamic Acid (in CDCl₃)
Key signals:
- δ 12.0 ppm – 1H, broad singlet (carboxylic acid proton, –COOH)
- δ 7.2–7.5 ppm – 5H, multiplet (aromatic protons, phenyl ring)
- δ 6.3–7.0 ppm – 2H, doublets (trans-alkene protons, –CH=CH–)
The acidic proton of the carboxylic group appears far downfield due to deshielding. The aromatic protons give a multiplet centered near 7.3 ppm. The vinyl protons on the alkene show trans coupling (J ≈ 16 Hz), giving two doublets with a chemical shift difference due to their differing electronic environments.
Conclusion
The combined IR, mass, and NMR spectra provide clear evidence for the functional groups present in trans-cinnamic acid, including the phenyl ring, carboxylic acid group, and the trans-configured alkene. Each technique contributes complementary structural information for compound confirmation.
