Alcohols - Introduction, Nomenclature

The hydroxy derivatives of aliphatic hydrocarbons are termed alcohols. They contain one or more hydroxyl (OH) groups.

Example:
Methyl Alcohol CH-3OH
Ehtyl alcohol C-2H-5OH also written as CH-3CH-2OH
Propyl alcohol C-3H-7OH also written as CH-3CH-2CH-2OH

They are classified according to the number of hydroxyl groups in the molecule.

i. Monohydric alcohol contains one -OH group.

ii. A dihydric alcohol contains two -OH groups in the molecule

ex. ethylene glycol

CH2OH
|
CH2OH

iii. A trihydric alcohol contains three -OH groups in its molecules

ex.glycerol

CH2OH
|
CH2OH
|
CH2OH

Monohydric alcohols are represented by the general formula R-OH or CnH2n+1OH.

Monohydric alcohols are further classified according to the carbon atom to which the hydroxyl group is attached.

1. Primary alcohols: -OH group is attached to primary carbon atom. They contain the group -CH2OH

eg.: CH3CH2OH, CH3CH2CH2OH

2. Secondry alcohol: -OH group is attached to a secondary carbon atom. It contains a divalent >CHOH group.

Ex. iso-propyl alcohol

3. Tertiary alcohol: -Oh group is attached to tertiary carbon atom. C-OH group is present


Alcohols – Nomenclature
Common system
Alcohols are named as alkyl alcohols. The alkyl group attached to the –OH group is named and the word alcohol is added to it.
Ex: Methyl alcohol
IUPAC nomenclature system
‘e’ in the parent chain is replaced by ‘ol’
Ex: Methanol
Rules follow for arriving at the names of alcohols

1. The longest continuous chain containing the carbon bonded to the OH group is selected as the parent chain.
2. The carbon atoms in the chain are numbered in such a way that the carbon atom carrying the hydroxyl group gets the lowest number.
3. The position of substituents is indicated by suitable numbers.
Ex: 2-Methyl propan-1-ol

The alcohols with two –OH groups are named as diols and alcohols with three –OH groups are named as triols.

Ex: Ethane – 1,2 – diol, propane -1,2,3-triol