Two or more compounds having the same molecular formula but different physical or chemical properties are called isomers.
Functional isomers are compounds having same molecular formula or general formula but different functional group and different physical and chemical properties.
e.g., : Alcohols and ethers $\left( C _{n} H _{2 n}+2 O \right)$
Aldehydes and ketones $\left( C _{n} H _{2 n} O \right)$
Acids and esters $\left( C _{n} H _{2 n} O _{2}\right)$
Alcohol is formed when a saturated carbon atom bonds to a hydroxyl (-OH) group. It is an organic compound that contains a hydroxyl functional group attached to a carbon atom.
Phenol is formed when the -OH group replaces the hydrogen atom in benzene. It is an organic compound in which a hydroxyl group directly attaches to an aromatic hydrocarbon.
Ether is formed when oxygen atom bonds to two alkyl or aryl groups. It is an organic compound that has an oxygen atom that is connected to two aryl and alkyl groups.
