Butane-1,3-diol is a dihydric alcohol because it contains two hydroxyl (–OH) groups. However, it is not a glycol because its –OH groups are attached to carbon atoms that are not adjacent (they are on carbon 1 and carbon 3). In contrast, glycols are diols in which the two –OH groups are present on neighboring carbon atoms (1,2-diols or vicinal diols), such as ethane-1,2-diol, propane-1,2-diol, and butane-2,3-diol. Therefore, butane-1,3-diol is a dihydric alcohol but not classified as a glycol.
The alcohol which is a dihydric but not a glycol:
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