Vanillin NUJOL 2.5 2.6 2.7 2.8 2.9 3 100 CHO 3171.4 1509.6 859.6 1665.7 1266.6 733.5 1588.0 1154.9 633.2 OCH
OH MICRONS 3.5 4.5 5.5 6 7 8 9 10 11 12 13 NICOLET 20SX FT-IR 14 15 16 17 18 19 21 22 +0.0 90 80 70 60 50 TRANSMITTANCE 40 30 20 10 0 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 WAVENUMBERS 1200 1000 800 600 Figure 13.1 IR spectrum of vanillin ABSORBANCE 2.0 450
The Correct Answer and Explanation is:
Based on the provided image, the correct answer is an analysis and assignment of the major absorption bands in the infrared (IR) spectrum of vanillin, which confirms its molecular structure.
The IR spectrum of vanillin (4-hydroxy-3-methoxybenzaldehyde) displays several characteristic absorption bands that correspond to its specific functional groups. The analysis of these peaks provides a spectroscopic confirmation of the molecule’s structure.
The most prominent feature in the high frequency region is the very broad and strong absorption centered at 3171.4 cm⁻¹. This peak is characteristic of the hydroxyl (O-H) group’s stretching vibration. Its significant breadth is due to extensive intermolecular hydrogen bonding, which is expected for a phenolic hydroxyl group in a solid sample.
In the double bond region, the most intense and sharp peak appears at 1665.7 cm⁻¹. This absorption is definitively assigned to the carbonyl (C=O) stretching vibration of the aldehyde group. The frequency is lower than that of a simple aliphatic aldehyde because of conjugation with the adjacent aromatic ring, which weakens the C=O bond. Following this, two strong peaks at 1588.0 cm⁻¹ and 1509.6 cm⁻¹ are attributed to the C=C stretching vibrations within the aromatic benzene ring.
The fingerprint region, below 1500 cm⁻¹, contains complex but highly informative peaks. A very strong band at 1266.6 cm⁻¹ is due to the C-O stretching vibrations. This likely represents a combination of the phenolic C-O stretch and the asymmetric C-O-C stretch of the methoxy (ether) group. Another significant peak is observed at 1154.9 cm⁻¹. Finally, the bands in the 900-650 cm⁻¹ range relate to C-H out-of-plane bending. The peak at 859.6 cm⁻¹, for instance, is consistent with the substitution pattern on the aromatic ring. The presence of strong Nujol (mineral oil) peaks from the sample preparation method can be seen around 2900 cm⁻¹, but they do not interfere with the key functional group assignments for vanillin.
