`=>` Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by `color{red}(–OH)` group.

`=>` These classes of compounds find wide applications in industry as well as in day-to-day life.

● Ordinary spirit used for polishing wooden furniture is chiefly a compound containing hydroxyl group, ethanol.

● The sugar we eat, the cotton used for fabrics, the paper we use for writing, are all made up of compounds containing `color{red}(–OH)` groups.

`=>` An alcohol contains one or more hydroxyl `color{red}(OH)` group(s) directly attached to carbon atom(s), of an aliphatic system `color{red}(CH_3OH)` while a phenol contains `color{red}(–OH)` group(s) directly attached to carbon atom(s) of an aromatic system `color{red}(C_6H_5OH)`.

`=>` The subsitution of a hydrogen atom in a hydrocarbon by an alkoxy or aryloxy group (`color{red}(R–O //Ar–O)`) yields another class of compounds known as ‘ethers’, for example, `color{red}(CH_3OCH_3)` (dimethyl ether).

`=>` You may also visualise ethers as compounds formed by substituting the hydrogen atom of hydroxyl group of an alcohol or phenol by an alkyl or aryl group.

The classification of compounds makes their study systematic and hence simpler.

`text(Mono, Di, Tri or Polyhydric Compounds :)` Alcohols and phenols may be classified as mono–, di–, tri- or polyhydric compounds depending on whether they contain one, two, three or many hydroxyl groups respectively in their structures as given below : See fig.1.

`=>` Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached.

(i) `color{green}("Compounds containing" C_(sp^3) - OH "bond ")` : In this class of alcohols, the `color{red}(–OH)` group is attached to an `color{red}(sp^3)` hybridised carbon atom of an alkyl group. They are further classified as follows :

● `color{green}("Primary, Secondary and Tertiary Alcohols" )` : In these three types of alcohols, the `color{red}(–OH)` group is attached to primary, secondary and tertiary carbon atom, respectively as shown in fig.2.

● `color{green}("Allylic Alcohols ")` : In these alcohols, the `color{red}(—OH)` group is attached to a `color{red}(sp^3)` hybridised carbon next to the carbon-carbon double bond, that is to an allylic carbon. For example : See fig.3.

● `color{green}("Benzylic Alcohols ")` : In these alcohols, the `color{red}(—OH)` group is attached to a `color{red}(sp^3)`—hybridised carbon atom next to an aromatic ring. For example : See fig.4.

(ii) `color{green}("Compounds containing" C_(sp^2)− OH "bond" )` : These alcohols contain `color{red}(—OH)` group bonded to a carbon-carbon double bond i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols.

Vinylic alcohol : `color{red}(CH_2 = CH – OH)`

Phenols : See fig.5.

Ethers are classified as :

(i) `color{green}("Simple or Symmetrical ")` : If the alkyl or aryl groups attached to the oxygen atom are the same. Example : Diethyl ether, `color{red}(C_2H_5OC_2H_5)`.

(ii) `color{green}("Mixed or Unsymmetrical ")` : If the two groups are different. Example : `color{red}(C_2H_5OCH_3)` and `color{red}(C_2H_5OC_6H_5)` are unsymmetrical ethers.