What is the significance of the disappearance of the peak around 1700 cm-1 in an IR spectrum?

The disappearance of a peak around 1700 cm-1 in an IR spectrum is significant because it typically corresponds to the carbonyl (C=O) stretch vibration, which is characteristic of carbonyl-containing functional groups such as ketones, aldehydes, and carboxylic acids. When this peak is no longer present, it suggests that a reaction has occurred where the carbonyl compound has been converted into another functional group that does not feature a carbonyl bond.

In the context of converting benzophenone, which contains a carbonyl group, the loss of the peak at approximately 1700 cm-1 would indicate that benzophenone has undergone a transformation, perhaps to a compound lacking a carbonyl group, such as a hydroxyl-containing compound or even an ether, depending on the reaction conditions. This change is crucial for confirming the progress of a reaction and the successful conversion of starting materials to products in organic synthesis.