Question

How are PRIMARY, SECONDARY, TERTIARY and AROMATIC ALCOHOLS distinguished in the IR spectrum and how are they similar?

Answer 1

Differences: It is difficult to differentiate between peaks in the region 1000-1270 cm-1 due to interference by similar bands from strong CH₃ bending peaks among the alcohol molecules. But, there would be strong peaks of the C-O asymmetric stretching. For primary alcohols this stretching will appear in the range of 1075 - 1000 cm-1, for secondary it will appear at 1150 - 1075 cm-1 and for tertiary it appears between 1210 - 1100 cm-1. For aromatic alcohols there would be additional stretching at 1680 cm-1 and peak at 3000-3100 cm-1 for C=C bond and =C-H respectively. Additionally, aromatic alcohols are known to have a broader O-H band than alcohols.

Similarity: Alcohols and Phenols both have unique IR spectrum and show strong and broad stretching bands between 3400 and 3300 cm⁻¹.