Q5(e). IR distinction of two pairs of compounds

Introduction. IR distinguishes the pairs by (i) the type of hydrogen bonding, which shifts O–H and C=O bands, and (ii) the aldehydic C–H stretch and the C=O position.

(i) Salicylic acid and p-hydroxybenzoic acid

• In salicylic acid the phenolic OH and the C=O form an intramolecular six-membered H-bonded ring. Both ν_O-H and ν_C=O are lowered, and the bands do not shift on dilution (in CCl₄), because the bond is inside one molecule.
• In p-hydroxybenzoic acid the groups are too far apart; H-bonds are intermolecular. Their bands are also lowered in the solid or concentrated solution, but on dilution the H-bonds break and a sharp non-bonded O–H band appears near 3600 cm⁻¹, with the C=O moving up.

Figure: Top: salicylic acid, intramolecular H-bond (chelate). Bottom: p-hydroxybenzoic acid, only intermolecular H-bonds.

(ii) Propionaldehyde and acetone

• Propanal shows two weak C–H bands near 2820 and 2720 cm⁻¹ (aldehydic C–H, Fermi doublet) and C=O near 1720–1740 cm⁻¹ (aliphatic aldehyde).
• Acetone has no band at 2720 cm⁻¹; its ketone C=O lies slightly lower (aliphatic ketones about 1715 cm⁻¹).

Conclusion. (i) The salicylic acid bands (O–H and C=O, lowered by an intramolecular H-bond) do not change on dilution; those of p-hydroxybenzoic acid do (non-bonded O–H near 3600 cm⁻¹ appears). (ii) Only propanal shows the 2720/2820 cm⁻¹ aldehyde C–H doublet; its C=O is also a little higher than that of acetone.

Application. The dilution test for intra- versus intermolecular H-bonding is used to assign o-hydroxy carbonyl compounds such as methyl salicylate (oil of wintergreen) and o-nitrophenol.