benzaldehyde does not have a hydroxyl group like benzoin. Therefore, benzoin will have a very broad, stong peak in the 3200-3500 cm-1 region (O-H stretching region) and benzaldehyde will not. Both compounds contain carbonyl groups and their C=O stretch will be between 1680-1720cm-1, maybe lower since they are both conjugated. However, benaldehyde will have two additional C-H stretch peaks (from the H off of the carbonyl) between 2850-2800 and 2750-2700 cm-1.