5.1 Introduction

Alcohols are aliphatic organic compounds that contain hydroxyl (-OH) groups. They can be synthesized by a wide range of methods, and the hydroxyl group may be converted to other functional groups. The structure of an alcohol resembles the structure of water where one of the hydrogen atoms of water is replaced by alkyl group. Although both have sp 3 -hybridized oxygen atoms, the C-O-H bond angle in methanol (108.9°) is considerably larger than the H-O-H bond angle in water (104.5°) because the methyl group is much larger than a hydrogen atom (Figure 1)

Figure 1

5.2 Nomenclature

The common name of an alcohol is derived from the common name of the alkyl group and the word alcohol.

Name the longest carbon chain that contains the carbon atom bearing the -OH group. Remove the final -e from the alkane name and add the suffix -ol to give the root name.
Start numbering from the carbon which is nearest to the hydroxyl group, and use the appropriate number to indicate the position of the -OH group.

Name all the substituents and give their numbers, as you would do for an alkane or an alkene

Alcohols with two - OH groups are called diols or glycols. They are named like other alcohols except that the suffix - diol is used.

Most of the common alcohols, up to about 11 or 12 carbon atoms are liquids at room temperature. Ethyl alcohol and propane have similar molecular weights, yet their boiling points differ by about 120 °C. There is a large difference between the dipole moment of ethyl alcohol and propane. The polarized C-O and O-H bonds and the nonbonding electrons add to produce a dipole moment of 1.69 D in ethanol but the dipole moment of propane is only 0.08 D (Figure 2).

Hydrogen bonding is the major intermolecular attraction responsible for ethanol's high boiling point. The hydroxyl hydrogen of ethanol is strongly polarized forming a hydrogen bond with a pair of nonbonding electrons from the oxygen atom of another alcohol molecule.

Thus, several of the lower-molecular-weight alcohols are miscible with water and form hydrogen bond with water. Alcohols are much better solvents for polar substances. The water solubility decreases as the alkyl group becomes larger. Alcohols with one-, two-, or three-carbon alkyl groups are miscible with water. A four-carbon alkyl group is large enough to be immiscible.

Hydroboration-oxidation of alkenes gives anti -Markovnikov alcohols with syn -stereochemistry. For example, hydroboration-oxidation of 1-methylcyclopentene leads to syn -addition to give trans -2-methylcyclopentanol (Scheme 1).