L4 - Alcohols and Phenols
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What is a Primary, Secondary, and Tertiary Alcohol?
| By convention, when we write the molecular formula or the condensed structural formula (rather than the structural formula) for alcohols, we write the –OH group at the end, for example, C2H5OH or CH3CH2OH for ethanol. The position of the –OH group can vary, however, and make alcohols quite different, in terms of their chemical and physical properties. For example, the 1 in propan-1-ol indicates that the hydroxyl group is bonded to a carbon atom at the end of the carbon chain, that is, CH3CH2CH2OH(l) rather than CH3CHOHCH3(l). Structural models of alcohols with four or more carbon atoms suggest that three structural types of alcohols exist. |
| Primary Alcohol | Secondary Alcohol | Tertiary Alcohol |
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| in which the carbon atom carrying the –OH group is bonded to one other carbon atom, as in CH3CH2CH2CH2OH(l), butan-1-ol | in which the carbon atom carrying the –OH group is bonded to two other carbon atoms, as in CH3CHOHCH2CH3(l), butan-2-ol | in which the carbon atom carrying the –OH group is bonded to three other carbon atoms, as in (CH3)3COH(l), 2-methylpropan-2-ol |
When naming alcohols with more than two carbon atoms, we indicate the position of the hydroxyl group. For example, there are two isomers of propanol, C3H7OH: propan-1-ol is used as a solvent for lacquers and waxes, as a brake fluid, and in the manufacture of propanoic acid; propan-2-ol, or isopropanol, is sold as rubbing alcohol and is used to manufacture oils, gums, and acetone. (The prefix iso- indicates that the hydroxyl group is bonded to the central carbon atom.) Both isomers of propanol are toxic to humans if taken internally.
