AAMC FL Practice Exam

Which of the following wavenumbers indicates a characteristic carbonyl peak in IR spectroscopy?

1. 1700 cm-1

2. 3300 cm-1

3. 3000 cm-1

4. 1580-1610 cm-1

The correct answer is: 1700 cm-1

The characteristic carbonyl peak in infrared (IR) spectroscopy typically appears around 1700 cm-1. This region corresponds to the stretch of the C=O bond, which is a very strong and prominent absorption feature in the IR spectrum. Carbonyl groups can be found in various functional groups such as aldehydes, ketones, carboxylic acids, and esters, all of which exhibit a strong peak in this vicinity due to the high polarity of the C=O bond, making it especially susceptible to IR radiation. The reason this wavenumber is a clear indicator of a carbonyl group relates to the nature of the bond involved and the vibrational modes that these molecules undergo when they absorb infrared light. This absorption causes a significant and identifiable peak that can be used to confirm the presence of a carbonyl functional group. In contrast, the other wavenumbers listed correspond to different types of bonds or functional groups. The range around 3300 cm-1 is often associated with the stretching of O-H bonds, indicating alcohols or phenols. The region around 3000 cm-1 generally corresponds to C-H stretching vibrations found in alkanes and alkenes, and 1580-1610 cm-1