- The IR spectrum of benzoic acid is included on page 3. Assign three specific
absorbance bands (from the peak list below the spectrum) to the corresponding bond
vibrations. Select three distinct bond vibrations, in other words, do not list three
absorbance bands that are all due to arene C-H stretching. Your answers should be
submitted in the following format (a different compound, acetophenone, is used as an
example):
IR Spectrum of Acetophenone
- 1691 cm-1, ketone C=O stretch
The Correct Answer and Explanation is :
For benzoic acid, here is an example of how you could assign three absorbance bands to their corresponding bond vibrations:
IR Spectrum of Benzoic Acid
- 1700 cm⁻¹, C=O stretch (carboxylic acid)
The strong absorption band at approximately 1700 cm⁻¹ corresponds to the stretching vibration of the carbonyl group (C=O) in the carboxyl group (-COOH) of benzoic acid. The C=O stretch is typically found in the range of 1700–1750 cm⁻¹, but can shift slightly depending on the conjugation or environment. In benzoic acid, the carboxyl group experiences a broadening of the C=O stretching absorption due to hydrogen bonding. - 1600 cm⁻¹, aromatic C=C stretch
The absorption at around 1600 cm⁻¹ is associated with the stretching vibrations of the aromatic ring’s C=C bonds. Aromatic compounds generally show a series of peaks in the range of 1500–1600 cm⁻¹ due to the in-plane stretching of the carbon-carbon double bonds within the benzene ring. The peak at 1600 cm⁻¹ in benzoic acid corresponds to this type of vibration in the aromatic system. - 2500–3300 cm⁻¹, O-H stretch (carboxylic acid)
The broad absorption band in the region of 2500–3300 cm⁻¹ is characteristic of the O-H stretching vibration of the hydroxyl group in the carboxylic acid (-COOH). This stretch is typically broad because of hydrogen bonding, which causes a broad and strong peak in this region. In benzoic acid, the O-H stretch is especially prominent, indicating strong hydrogen bonding in the carboxyl group.
Explanation:
- C=O Stretch (1700 cm⁻¹): The carbonyl stretch is a very important feature of carboxylic acids, and the specific positioning of this stretch can help differentiate carboxylic acids from other carbonyl-containing compounds, such as ketones or aldehydes.
- Aromatic C=C Stretch (1600 cm⁻¹): The absorption of C=C stretches from the aromatic ring is a signature of aromatic compounds. In benzoic acid, this helps identify the presence of the benzene ring structure.
- O-H Stretch (2500–3300 cm⁻¹): The broad, strong O-H stretch at a higher frequency is crucial in identifying carboxylic acids due to the ability of the hydroxyl group to engage in hydrogen bonding, which broadens the peak and lowers the vibrational frequency slightly.
These distinct bond vibrations correspond to key functional groups in benzoic acid and can be used to confirm its identity when analyzing its IR spectrum.