Chemistry Classification of Alcohols, Phenols and Ethers

Topics Covered :

● Introduction
● Classification of Alcohols and Phenols
● Classification of Ethers

Introduction :

`=>` 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.

Classification :

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

Classification of Alcohols and Phenols :

`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 :

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.