Types of alcohols

Alcohols are organic compounds containing hydroxyl (–OH) groups. The word alcohol is one of the oldest chemical terms, derived from the early Arabic al-Kuhl which meant “the powder” and later the “essence”. Ethyl alcohol is found in alcoholic beverages, cosmetics and drug preparation. Methyl alcohol (wood alcohol) is used as fuel and solvent. Alcohols are synthesized by a variety of methods and the hydroxyl group may be converted to most other functional groups. Therefore alcohols are versatile synthetic intermediates.

 Structure and Classification of Alcohols

The alcohol family is classified according to the type of carbinol carbon atom; the one bonded to the –OH group.

Alcohols are also classified according to the number of hydroxyl groups present in their molecules. Monohydric alcohols contain one hydroxyl group, dihydric two and trihydric three .

However if alcohol contains two or more hydroxyl groups , it is generally called as a polyhydric alcohol.

Monohydric alcohols

Polyhydric Alcohols

PHYSICAL PROPERTIES OF ALCOHOLS

1. Boiling points : Hydrogen bonding is the major inter-molecular attraction responsible for high boiling of alcohols.

Hydrogen bonding between molecules of alcohols

Boiling point decreases with increase in branching as the surface area on branching decreases which results in decrease of Vander-Waal forces which decreases boiling point.

For polyhydroxy alcohols, since the number of –OH groups has increased hence extent of hydrogen bonding also increases which increases the boiling point.

2.Solubility

Alcohols form hydrogen bonds with water and several of the lower-molecular weight alcohols are miscible with water. The water solubility decreases as the alkyl group (hydrophobic group) becomes larger. Alcohols with one, two or three carbon alkyl groups are miscible with water. A four carbon alkyl group is large enough that some isomers are not miscible, yet t-butyl alcohol with a compact spherical shape is miscible. Phenol is unusually soluble for a six-carbon alcohol because of its compact shape and the particularly strong hydrogen bonds formed between phenolic –OH groups and water molecules. Branching in alkyl part makes the alkyl part more compact and spherical. This reduces the area of hydrophobic alkyl part and decreases the solubility of alcohols as the extent of hydrogen bonding decreases.

Polyhydroxy alcohols have higher solubility due to extensive hydrogen bonding.