Compare the IR spectrum of benzoin ) with the IR spectrum of benzil . Explain how the changes in the important peaks are related to the differences between the two
structures.
To compare the IR spectra of benzoin and benzil and understand the changes in the important peaks, we first need to understand the structures of both compounds.
1. Benzoin (C14H12O2):
Benzoin has a hydroxyl group (-OH) attached to one of the aromatic rings. It consists of two benzene rings connected by a carbonyl group (C=O) in the middle.
2. Benzil (C14H10O2):
Benzil is a diketone, which means it contains two carbonyl groups (C=O) attached to different aromatic rings.
Now, let's analyze the important peaks in the IR spectra of both compounds:
1. Benzoin IR spectrum:
- The peak around 3500-3200 cm-1 corresponds to the O-H stretching vibration of the hydroxyl group (-OH).
- The peak around 1715 cm-1 represents the C=O stretching vibration of the carbonyl group.
- The peaks around 1600-1500 cm-1 are due to the aromatic C=C stretching vibrations in the benzene rings.
2. Benzil IR spectrum:
- Since benzil has two carbonyl groups, there will be two distinct peaks around 1715 cm-1, corresponding to the C=O stretching vibrations of each carbonyl group.
- The aromatic C=C stretching vibrations in the benzene rings will still appear around 1600-1500 cm-1, similar to benzoin.
Now, let's compare the changes in the important peaks and their relationship to the structural differences between benzoin and benzil:
1. The disappearance of the O-H peak:
Benzil lacks the hydroxyl group present in benzoin, so there is no O-H stretching vibration.
2. Increase in the number of C=O peaks:
In benzil, there are two carbonyl groups, whereas in benzoin, there is only one. Therefore, benzil will have two distinct peaks corresponding to the C=O stretching vibrations of each carbonyl group.
3. Similarity in the aromatic C=C peaks:
Both benzoin and benzil have benzene rings, so the aromatic C=C stretching vibrations will appear in a similar region for both compounds.
In summary, the IR spectra of benzoin and benzil differ primarily due to the presence or absence of the hydroxyl group and the number of carbonyl groups. The disappearance of the O-H peak and the increase in the number of C=O peaks in benzil compared to benzoin reflect the structural differences between the two compounds. However, the aromatic C=C peaks remain similar in both spectra due to the presence of benzene rings in both compounds.