Compounds having same molecular formula but different physical and chemical properties due to different structures are known as isomers and the property itself is known as isomerism.
(Greek, Isos= equal, meros= parts)
For example -
Ethanol - C2H5OH, molecular formula - C2H6O
Methoxymethane - CH3-O-CH3, molecular formula - C2H6O
There are two types of isomerism:
1) Structural isomerism
Such type of isomers is due to different structures. It means isomers having same molecular formula but different structures are known as structural isomerism. For example -
Butane (C4H10) - CH3-CH2-CH2-CH3
2-Methyl propane (C4H10) -
Types of structural isomerism-
I) Pentane (C5H12) - It has three possible isomers. They are-
II) Hexane (C6H14) - It has 5 possible isomers. They are-
(I) Possible isomers of molecular formula C2H6O are - C2H5OH (Ethanol) and CH3-O-CH3 (methoxymethane).
Functional group is -OH (Alcohol)
(II) Possible isomers of molecular formulaC2H4O2 are - CH3COOH ( ethanoic acid) and HCOOCH3 (methyl methanoate).
The functional group is -COOH (carboxylic acid) and -COOR (ester group).
(III) Possible isomers of molecular formula C3H6O are- CH3-CO-CH3(propanone) and CH3-CH2-CHO (propanal),
(IV) Possible isomers of molecular formula C3H8O are- CH3-CH2-CH2-OH (propanol) and CH3-O-CH2CH3 (methoxy ethane).
i) C2H5-O-C2H5 (Ethoxyethane) , CH3-O-C3H7(Methoxypropane)
ii) C2H5-CO-C2H5 (3- pentanone) , CH3-CO-C3H7 (2- pentanone).
(i) Ethyl acetoacetate exists in two forms called keto and enol forms.
2) Stereo- isomers
When isomerism is caused by the different arrangements of atoms or groups in three-dimensional space, the phenomenon is called stereoisomerism.
The stereoisomers have the same structural formula but differ in the arrangement of atoms or group of atoms in a three-dimensional configuration.
It is of two types-
a) Geometrical or cis-trans isomerism
b) Optical isomerism
Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.
Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).