Metamerism and Ring Chain Isomerism - JEE PDF Download

This type of isomerism is due to the unequal distribution of carbon atoms on either side of the multivalent functional group (i.e. –O-,-S-,-NH-, –CO- etc.) in the molecule of compounds belonging to the same class. For example, methyl propyl ether and diethyl ether both have same molecular  formula
(a) CH₃CH₂COCH₂CH₃ is a metamer of CH₃COCH₂CH₂CH₃ 
        Pentan-3-one                                                        Pentan-2-one
These two are also related as position isomers as the position of  in the two isomers is different.

(b) CH₃-O-CH₂CH₂CH₃ is a metamer of CH₃CH₂O-CH₂CH₃
Methoxy propane                                                      Ethoxy ethane
(Methyl n-propyl ether)                                              (Diethyl ether)

(c)  CH₃CH₂-NH-CH₂CH₃          is a metamer of     CH₃-NH-CH₂CH₂CH₃ 
         Diethyl amine                                                                        (Methyl n-propyl amine)

Ring-Chain Isomerism

Compounds having same molecular formula but possessing open chain and cyclic structures are called ring-chain isomers and the phenomenon is called ring-chain isomerism.

Degree of Unsaturation

Deficiency of two hydrogen atoms in a molecule is a result of either a pi-bond or a ring in the structure of that molecule. The sum of pi-bonds and rings in the structure of a compound collectively is called degree of unsaturation or double bond equivalents in that compound. The most general type of formula for any organic species is (C_aH_bN_cO_d). 
If the compound contains other atoms also, the tetravalent atoms are replaced by carbon, monovalent atoms are replaced by hydrogen, divalent atoms are replaced by oxygen and trivalent atoms are replaced by nitrogen. Then all oxygen and all nitrogen atoms are removed from the formula. However, for the removal of each N atom, one H atom is removed from the molecular formula. As a result of all these operations, we will get a hydrocarbon. Now this concluded hydrocarbon is compared with saturated alkane to determine the degree of unsaturation or double bond equivalents.