Notes on Isomerism | Grade 11 > Chemistry > Fundamental Principles of Organic Chemistry | KULLABS.COM

Isomerism

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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

Types of isomerism

There are two types of isomerism:

  1. Structural isomerism
  2. Stereo-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) - a

Types of structural isomerism-

  • Skeletal or chain isomerism:Such types of structural isomers are due to different chain or skeletal of carbon atoms. For example-

I) Pentane (C5H12) - It has three possible isomers. They are-

a

II) Hexane (C6H14) - It has 5 possible isomers. They are-

a

  • Positional isomerism:Such types of isomers, which have a same functional group but different position, are known as positional isomers. For example-

:a

  • Functional isomerism:Such type of structural isomers are due to different functional group, For example-

(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).

  • Metamerism:Such type of structural isomerism are due to the different alkyl group attached to the same functional group. It is also sometimes called positional isomerism. For example-

i) C2H5-O-C2H5 (Ethoxyethane) , CH3-O-C3H7(Methoxypropane)

ii) C2H5-CO-C2H5 (3- pentanone) , CH3-CO-C3H7 (2- pentanone).

  • Tautomerism:It is a special type of functional isomerism, where the two functional isomers exist in dynamic equilibrium with each other. These isomers are known as tautomers and the property itself is called as tautomerism. For example-

(i) Ethyl acetoacetate exists in two forms called keto and enol forms.

as

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

Reference:

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).



  • 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.
  • There are two types of isomerism
  •  Isomers having same molecular formula but different structures are known as structural isomerism. 
  • Skeletal isomers are such types of structural isomers are due to different chain or skeletal of carbon atoms.
  • Such types of isomers, which have a same functional group but different position, are known as positional isomers.
  • Such type of structural isomerism are due to the different alkyl group attached to the same functional group, are called metamerism.
  • Tautomerism is a special type of functional isomerism, where the two functional isomers exist in dynamic equilibrium with each other.
  • 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.
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