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.
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| Monohydric alcohols |
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| Polyhydric Alcohols |
PHYSICAL PROPERTIES OF ALCOHOLS
1. Boiling points : Hydrogen bonding is the major inter-molecular
attraction responsible for high boiling 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.
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| Hydrogen bonding between molecules of alcohols |
For polyhydroxy alcohols, since the number of –OH groups has increased
hence extent of hydrogen bonding also increases which increases the
boiling point.
Polyhydroxy alcohols have higher solubility due to extensive hydrogen
bonding.
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.