What peaks should you see for the IR spectrum of methyl benzoate? Characterize the spectrum.
Answer:
You should see the following peaks.
The structure of methyl benzoate is
It has a monosubstituted benzene ring, an ester group, and a methyl group.
1. C-O stretching
The strongest peak should be the ester C=O stretch.
This normally appears at \(1750-1735 \mathrm{~cm}^{-1}\) but conjugation with the ring shifts the peak to \(1730-1715 \mathrm{~cm}^{-1}\)
There should also be one medium and two strong bands in the region from \(1300-1000 \mathrm{~cm}^{-1}\)
The two strong bands arise from the symmetric and antisymmetric \(\text { C-O }\) stretches of the ester \(\text { C-OO }\) group, and the weaker band is the ether \(\text { CO }\) stretch of the \(\mathrm{OCH}_{3}\) group.
2. C-C Stretching
The benzene ring has characteristic medium-strength \(\text { C-C }\) stretching bands in the \(1600-1585 \mathrm{~cm}^{-1}\) and \(1500-1400 \mathrm{~cm}^{-1}\) regions.
3. C-H bending
The monosubstituted phenyl group has characteristic strong bending vibrations at
\(750-700 \mathrm{~cm}^{-1}\) and \(710-690 \mathrm{~cm}^{-1}\)
The methyl group usually shows a weak band at \(1380 \mathrm{~cm}^{-1}\) and a medium band at \(1260 \mathrm{~cm}^{-1}\)
4. C-H Stretching
The aromatic \(\mathrm{C}-\mathrm{H}\) stretch is usually weak and occurs at \(3100-3000 \mathrm{~cm}^{-1}\)
The aliphatic \(\mathrm{CH}_{3}\) group shows stronger symmetric and antisymmetric stretches at \(2960 \mathrm{~cm}^{-1}\) and \(2870 \mathrm{~cm}^{-1}\)
Here’s the actual IR spectrum of methyl benzoate.