Characterize the IR spectrum of methyl benzoate?
Answer:
First, let’s get the structure down. Methyl, indicating the alkyl group coming off the bottom-right carboxyl oxygen of a benzyl ester group.
Within this molecule, we have the following relevant bonds:
C=C (aromatic)
C-H (aromatic)
C=O (ester)
C-H (alkanes)
Notice how a double bond will not easily bend, so we don’t really talk about those bending, only stretching.
Within the bonds we’ve just listed, we have the following resonant frequencies that characteristically coincide with the most prominent vibrational activities of each functional group (Techniques in Organic Chemistry; Mohrig, Hammond, and Schatz):
C=C (aromatic)
\(\text { 1620 1440 cm }^{-1}\) medium to weak strength (stretch)
C − H (aromatic)
\(3100 \sim 3000 \mathrm{~cm}^{-1}\) medium to weak strength (stretch)
\(900 \sim 680 \mathrm{~cm}^{-1}\) strong strength (bend)
C=O (ester) — conjugated
\(1730 \sim 1715 \mathrm{~cm}^{-1}\) strong strength (stretch)
C – O (ester)
\(1300 \sim 1000 \mathrm{~cm}^{-1}\) strong strength (stretch)
C – H (alkanes)
\(2990 \sim 2850 \mathrm{~cm}^{-1}\) medium to strong strength (stretch)
\(1480 \sim 1430,1395 \sim 1340 \mathrm{~cm}^{-1}\) medium to weak strength (bend)
You can see the spectrum here:
Specifically:
- The aromatic C=C stretch is the thin weak peak you see near 1600
- The aromatic C-H bend is the thin strong peak you see near 700 and the stretch is the thin, well-defined, weak peaks right above 3000
- The ester C=O conjugated stretch is the strong, slightly wide peak right above 1700
- The ester C-O stretch is the strong, slightly wide peak you see right below 1300
- The alkane C-H bends are the thin medium/weak peaks you see near 1500~1300 (including the “sidearm” off of the ester peak) while the single thin weak peak near 3000 is the stretch.
BONUS POINTS: can you spot the \(\mathrm{CO}_{2}\) peak in your spectrum? Here’s a hint: it’s near \(2400 \mathrm{~cm}^{-1}\)