Literature IR spectrum of salicylic acid – copy image of the IR (KBr disc) from SDBS and attach here – (For all IR spectra: labelling of diagnostic peaks, written structure, and figure title explaining conditions of IR (i.e. KBr) and for what compound.)
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IR Spectrum of Salicylic Acid (KBr disc)
Figure Title: IR Spectrum of Salicylic Acid Using a KBr Disc, Highlighting Diagnostic Peaks
Written Structure: Salicylic acid (C₇H₆O₃) consists of a benzene ring with two functional groups: a hydroxyl (-OH) group and a carboxylic acid (-COOH) group in the ortho position.
Key Diagnostic Peaks in the IR Spectrum:
- O-H Stretching (Carboxylic Acid): Broad absorption band around 2500-3300 cm⁻¹ due to hydrogen-bonded O-H stretching.
- O-H Stretching (Phenolic): Sharp peak near 3200-3600 cm⁻¹ due to the phenolic hydroxyl group.
- C=O Stretching: Strong absorption near 1700 cm⁻¹ attributed to the carboxylic acid group.
- C=C Stretching (Aromatic Ring): Peaks in the region of 1500-1600 cm⁻¹ due to aromatic ring vibrations.
- C-O Stretching: Medium intensity peaks around 1200-1300 cm⁻¹ due to the C-O bond in both the carboxylic and phenolic groups.
- Out-of-Plane C-H Bending: Peaks near 750-800 cm⁻¹ characteristic of an ortho-substituted benzene ring.
Explanation
Infrared (IR) spectroscopy identifies functional groups by their vibrational modes. In salicylic acid:
- The broad O-H stretching bands reflect hydrogen bonding in the carboxylic acid and phenolic groups.
- The sharp C=O stretch indicates the carbonyl group’s strong dipole moment.
- Aromatic C=C stretching vibrations confirm the presence of a benzene ring.
- C-O stretching and bending vibrations reflect contributions from hydroxyl and carboxylic functionalities.
The KBr disc method involves dispersing a fine powder of the sample in potassium bromide and pressing it into a pellet. This ensures minimal interference and provides clear spectral readings.
By analyzing these peaks, the IR spectrum confirms the compound’s functional groups and overall structure.
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