The infrared spectra of benzoin and benzaldehyde are given in this experiment. Interpret the principal peaks in the spectra.
Infrared spectrum of benzoin, KBr.
Infrared spectrum of benzaldehyde (neat).
The Correct Answer and Explanation is:
To interpret the principal peaks in the IR spectra of benzoin and benzaldehyde, we must identify the characteristic functional groups in each compound and match them with their corresponding IR absorption bands.
Benzoin (C₁₄H₁₂O₂)
Benzoin contains the following functional groups:
- Hydroxyl group (–OH)
- Carbonyl group (C=O) from a ketone
- Aromatic rings
Key IR peaks:
- Broad O–H stretch: ~3200–3550 cm⁻¹
– A broad peak due to hydrogen bonding of the hydroxyl group. - C=O stretch (ketone): ~1710–1725 cm⁻¹
– Sharp and strong, typical of a conjugated ketone. - C–H stretch (aromatic): ~3000–3100 cm⁻¹
– Multiple peaks due to aromatic C–H bonds. - C=C stretch (aromatic ring): ~1450–1600 cm⁻¹
– Medium to strong peaks indicating the benzene rings.
Benzaldehyde (C₇H₆O)
Benzaldehyde contains:
- Aldehyde group (–CHO)
- Aromatic ring
Key IR peaks:
- Aldehyde C–H stretch: ~2720 and 2820 cm⁻¹ (two weak-medium peaks)
– Characteristic “Fermi doublet” unique to aldehydes. - C=O stretch (aldehyde): ~1725–1740 cm⁻¹
– Sharp, strong absorption. - Aromatic C–H stretch: ~3000–3100 cm⁻¹
– Similar to benzoin. - C=C stretch (aromatic ring): ~1450–1600 cm⁻¹
– Indicates presence of the benzene ring.
Summary Explanation
Infrared (IR) spectroscopy is a useful tool for identifying functional groups in organic compounds. Each functional group has a unique set of absorption bands, allowing us to differentiate between compounds such as benzoin and benzaldehyde.
Benzoin, an α-hydroxy ketone, shows a broad O–H stretch around 3200–3550 cm⁻¹ due to hydrogen bonding, a clear indicator of the hydroxyl group. Additionally, its C=O stretch near 1715 cm⁻¹ signifies the ketone group, particularly slightly shifted due to conjugation with the aromatic rings. The aromatic C–H stretching near 3000–3100 cm⁻¹ and C=C stretching around 1450–1600 cm⁻¹ confirm the presence of two phenyl rings.
In contrast, benzaldehyde lacks the –OH group and instead contains an aldehyde. The aldehyde C–H stretch appears as a distinctive doublet near 2720 and 2820 cm⁻¹, known as a Fermi resonance—this feature is absent in ketones like benzoin. The C=O stretch appears slightly higher than benzoin, around 1725–1740 cm⁻¹, consistent with an unconjugated aldehyde carbonyl. As with benzoin, the aromatic ring causes peaks in the 1450–1600 cm⁻¹ region and C–H stretching above 3000 cm⁻¹.
These spectral differences, particularly the O–H and aldehyde C–H signals, are critical for distinguishing between these structurally similar compounds using IR spectroscopy.
