What characteristic peaks are seen for aldehydes in IR spectroscopy?

In IR spectroscopy, aldehydes typically exhibit characteristic peaks in the regions of 1720-1740 cm-1 and 2700-2800 cm-1.

Infrared (IR) spectroscopy is a powerful tool for the identification of functional groups in organic compounds, including aldehydes. Aldehydes are characterised by the presence of a carbonyl group (C=O) and a hydrogen atom attached to the same carbon atom. This unique structure gives rise to specific absorption peaks in an IR spectrum.

The most prominent peak for an aldehyde in an IR spectrum is typically found in the region of 1720-1740 cm-1. This peak corresponds to the stretching vibration of the carbonyl group. The exact position of this peak can vary slightly depending on the specific structure of the aldehyde and the presence of any other functional groups. However, it is generally found in this region and is a strong, sharp peak due to the high polarity of the carbonyl group.

In addition to the carbonyl peak, aldehydes also exhibit a characteristic pair of peaks in the region of 2700-2800 cm-1. These peaks are due to the C-H bond in the aldehyde group. They appear as a pair of peaks because the C-H bond in an aldehyde can vibrate in two different ways - symmetrically and asymmetrically. The symmetric vibration gives rise to one peak, while the asymmetric vibration gives rise to the other. These peaks are typically weaker than the carbonyl peak but are still a useful indicator of the presence of an aldehyde group.

It's important to remember that while these peaks are characteristic of aldehydes, they are not exclusive to them. Other functional groups can also exhibit peaks in these regions. Therefore, the identification of an aldehyde in an IR spectrum should always be confirmed with additional evidence, such as the presence of other characteristic peaks or the use of complementary analytical techniques.