Question: What major IR absorptions are present above 1500 ${\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$for each compound?

The IR absorption frequency will vary according to the functional group present in the compound. Each functional group has its particular range of frequency.

The IR frequency is dependent upon the functional group present in the molecule.

a. C$\mathrm{sp}$-H at 3300${\mathrm{cm}}^{-1}$C${\mathrm{sp}}^3$-H at 2850–3000${\mathrm{cm}}^{-1}$C–C triple bond at 2250${\mathrm{cm}}^{-1}$

Triple bonded carbons have the higher IR absorption

b. O–H at 3200–360 ${\mathrm{cm}}^{-1}$C${\mathrm{sp}}^{}$–H at 2850–3000${\mathrm{cm}}^{-1}$

Hydroxyl bond has the higher IR absorption

c. C${\mathrm{sp}}^3$ –H at 2850–3000${\mathrm{cm}}^{-1}$C=O at 1700${\mathrm{cm}}^{-1}$

Carbonyl bond has the higher IR absorption

d. O–H at > 3000${\mathrm{cm}}^{-1}$ C${\mathrm{sp}}^2$–H at 3000–3150${\mathrm{cm}}^{-1}$C=O at approx. 1700${\mathrm{cm}}^{-1}$phenyl group at 1600, 1500${\mathrm{cm}}^{-1}$.The OH of the RCOOH is even broader than the OH of alcohol (3500–2500${\mathrm{cm}}^{-1}$).

The carboxylic group shows higher IR absorption