Question

The labels on a shipment of the five chemicals A –E below were washed off by accident when the delivery truck went through a truckwash with the doors open. A clever student took infrared spectra of each of the resulting unknowns. Unknown #1 had a strong, broad IR absorption at 3400–3600 cm–1, and no absorptions between 1600 and 2900 cm–1. Unknown #2 had a strong absorption at 1680 cm–1 and no absorptions above 3100 cm–1. What are the structures of unknowns 1 and 2?

Answer 1

Unknown 1: -OH group at 3400-3600

Unknown 2: Ketone and alkene group

Explanation:

Unknown 1:

This compound has a strong IR absorption at 3400 - 3600cm-¹; this indicates that it is an alcoholic group (OH)

No absorption between 1600 and 2900cm-¹: this rule out the presence of ketone (C = O)

Hence the unknown compound is OH group

Unknown 2:

This has strong IR absorption at 1680cm-¹: this indicates the presence of (C=O) group.

No absorptions above 3100cm-¹: this indicates the presence of C = C - H

So therefore, unknown 2 is most likely to be Ketone and alkene group