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What is the difference between functional group isomerism and metamerism?

Compounds having the same molecular formula but different

number of carbon atoms ( alkyl groups) on either side of  functional

group ( i.e., -O-,-S-, -NH-, -C(=O)-) are called metamers and  the

phenomenon is called metamerism. Metamerism occurs amongst the  members

of the same homologus family.For example,

CH3CH2-O-CH2CH3 is metamer of CH3-O-CH2CH2CH3 OR CH3-O-CH(CH3)2

CH3CH2COCH2CH3 is metamer of CH3COCH2CH2CH3 OR CH3COCH(CH3)2

It may be noted that metamers may also be position isomers,

e.g., pentan-2-one and pnetan-3-one may be regarded as position isomers

 as well as metamers.

A group is functional when it is already present in a chemical compound, and this group also determines, wholly or partly, the distinct properties of the compound. Thus, when you consider the structure or composition of ethanol (CH2CH2OH), the hydroxyl group (-OH) is entirely a functional group. The chemical properties of ethanol mostly revolve around the potential of the -OH group in reacting with sodium metal, PCl5, conc.H2SO4 or acetic acid, as is the case with any other alcohol having the same -OH group.

The same -OH will be called a substituent when it becomes a prospective replacement for another group that is already present in a molecule by chemical interaction. Thus, when ethyl chloride (CH3CH2Cl) is reacted with alcoholic KOH, the -OH group of the latter becomes a substituent, as it is expected to replace the chlorine atom which is already present in the molecule.

In position isomerism a functional group changes position on the chain. In the diagram, pentan-2-ol has become pentan-3-ol. Many aromatic isomers exist because substituents can be positioned on different parts of the benzene ring. Only one isomer of phenol or hydroxybenzene exists but cresol or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. Xylenol has one hydroxyl group and two methyl groups and a total of 6 isomers exist.