Alcohols

Alcohols are hydroxy‐substituted alkanes, alkenes, or alkynes in which the substitution occurs on a saturated carbon.

The general formula for alcohols is R&bond;OH, where the R group can represent the alkyl, alkenyl, or alkynal groups. In the case of substitution on alkenes and alkynes, only saturated carbons may be substituted. For example, the following compounds are all alcohols:





If the hydroxyl group were substituted for a hydrogen on an unsaturated carbon, an alcohol would not form. For example, substituting the hydroxyl group for a terminal hydrogen of 1‐propene gives an unstable enol that tautomerizes to a ketone.




Nomenclature

You can use both the common and IUPAC systems to name alcohols. In the common system, you name an alcohol by listing the alkyl group and adding the word alcohol. Following are some examples of alcohols and their common names:





In the IUPAC system, use the following series of rules to name alcohols:

1. Pick out the longest continuous chain to which the hydroxyl group is directly attached. The parent name of the alcohol comes from the alkane name for the same chain length. Drop the ‐e ending and add ‐ol.

2. Number the parent chain so that the carbon bearing the hydroxyl group has the lowest possible number. Place the number in front of the parent name.

3. Locate and name substituents other than the hydroxyl group.

The following examples show how you apply these rules:





You may classify alcohols as primary (1°), secondary (2°), or tertiary (3°), based on the class of carbon to which the hydroxyl group (&bond;OH) is directly bonded. For example, 1‐propanol is a 1° alcohol, 2‐propanol is a 2° alcohol, and 2‐methyl‐2‐propanol is a 3° alcohol.

Physical properties

Alcohols contain both a polar —OH group and a nonpolar alkyl group. As a result of this composition, alcohols that have small alkyl chains tend to be water soluble. As alkyl chain length increases, water solubility decreases.

Through the OH group, alcohols are capable of forming hydrogen bonds to themselves, other alcohols, neutral molecules, and anions. This bond formation leads to abnormally high boiling points compared to other organic molecules of similar carbon chain length.