Isomerism Theory PDF | Structural, Geometrical & Optical Isomerism Explained

The name was given by Berzilius. Two or more than two organic compounds having the same molecular
formula and molecular weight but different physical and chemical properties are called isomers and the
phenomenon is called isomerism.

ISOMERISM


Structural Isomerism Stereoisomerism


Chain Position Ring Chain Functional Metamerism Tautomerism
Configurational Conformational

Geometrical Optical

Structral Isomerism
1. Chain Isomerism (CI) : The compounds which have same molecular formula, same functional group, same
position of functional group or multiple bond or substituent but different arrangement of carbon chain (different
parent name of compound) shows chain isomerism.

Ex. CH3 CH2 CH2 CH3 CH3 CH CH3
CH3
1–Butane (4C) 2–Methyl propane (3C)
Ex. CH3 CH2 CH CH2 CH2 C CH3
CH3
Butene (4C) 2–Methyl-1-propene (3C)

Ex. CH3 CH2 CH2 CH2 OH CH3 CH CH2 OH
CH2
1–Butanol (4C) 2–Methyl–1–propanol (3C)
2. Position Isomerism (PI) : The compounds which have same molecular formula, same functional group,
same parent carbon chain but different position of functional group or multiple bond or substituents, shows
position isomerism.
Ex. CH2 CH CH2 CH3 CH3 CH CH CH3
But–1–ene But–2–ene
Ex. CH3 CH2 CH2 CH2 OH CH3 CH2 CH CH3
OH
1–Butanol 2–Butanol
Ex. CH3 CH2 CH2 CH2 Cl CH3 CH2 CH CH3
Cl
1–Chlorobutane 2–Chlorobutane
Example of CI and PI :

(i) C4H10 have two isomers : Both butane and isobutane are chain isomers.
Ex. CH CH CH CH CH3 CH CH3
3 2 2 3

CH3
Butane Isobutane

,(ii) C5H12 have three isomers : All of three structures are chain isomers because only carbon chain (parent)
is different.
CH3
CH3 CH2 CH CH3 CH3 C CH3
CH3 CH2 CH2 CH2 CH3
CH3 CH3
Pentane 2–Methyl butane 2 , 2 –
Dimethylpropane
(iii) C6H14 has 5 isomers

(a) CH3CH2CH2CH2CH2CH3 (b) CH3 CH2 CH2 CH CH3
CH3
Hexane 2–Methyl pentane

(c) CH3 CH2 CH CH2 CH3 (d) CH3 CH CH CH3

CH3 CH3 CH3
3–Methyl pentane 2,3–Dimethyl butane
CH3
(e) H3C C CH2 CH3  a–b, b–d, a–c, c–d —→ Chain Isomers

CH3
2,2–Dimethyl butane  b–c, d–e —→ Position Isomers
(iv) C7H16 has 9 isomers

1. CH3 CH2 CH2 CH2 CH2 CH2 CH3 Heptane
2. CH3 CH2 CH2 CH2 CH CH3 2–Methylhexane
CH3
3. CH3 CH2 CH2 CH CH2 CH3 3–Methylhexane
CH3
CH3

4. CH3 CH2 CH2 C CH3
2,2–Dimethyl pentane
CH3
CH3
5. CH3 CH2 C CH2 CH3 3,3-Dimethylpentane
CH3
CH3

6. CH3 CH CH CH2 CH3 2,3–Dimethylpentane
CH3
CH3 CH3
7. CH3 CH CH2 CH CH3 2,4–Dimethylpentane
CH2 CH3

8. CH3 CH2 CH CH2 CH3 3–Ethylpentane
CH3
9. CH3 CH C CH3 2,2,3–Trimethylbutane
CH3 CH3

,(v) C3H6Cl2 has 4 isomers : Position of chlorine atom is different in all the structure, so these are position
Isomers.
Cl Cl

1. H3C CH2 CH Cl 2. H2C CH2 CH2 Cl

1,1–Dichloropropane 1,3–Dichloropropane
Cl Cl
3. H3C C CH3 4. H2C CH CH3
Cl Cl
2,2–Dichloropropane 1,2–Dichloropropane
(vi) C5H11Cl has 8 isomers
(vii) C8H10 has 4 aromotic isomers

CH3 CH2 CH3
CH3
(o,m,p)


(viii) C 6H 4X 2 ——→ 3 Aromatic isomers
C6 H4 XY ——→ 3 Aromatic isomers
C 6H 3X 3 ——→ 3 Aromatic isomers
C6 H3 XYZ ——→ 10 Aromatic isomers
Question : Structures CH3—CH2—CH CH2 and CH3 C CH2 are :–
CH3
Ans. Chain Isomers

Molecular formula No. of Isomers Molecular formula No. of Isomers
C 4 H 10 2 C 8 H 18 18
C 5 H 12 3 C 9 H 20 35
C 6 H 14 5 C 10 H 2 2 75
C 7 H 16 9

3. Ring chain isomerism (RCI) : Same molecular formula but different mode of linking (open chain or closed
chain) of carbon atoms.
CH3 CH CH2
[open chain]
C3H6
CH2
[closed chain or ring]

They have same molecular formula so they are Ring chain isomers.


(a)
Question : Relate a,b and c:– H3C C CH
(b)
C3H4 CH2 C CH2

(c) CH



Ans. a–b ——→ Functional Isomers
a–c , b–c——→ Ring-chain Isomers, Functional Isomers

, Special points :  Alkenes with cycloalkane and alkynes (Alkadienes) with cycloalkenes show Ring-chain Isomerism.
 Ring-chain Isomers are also Functional Isomers but priority is given to Ring-chain Isomers.
Question : CH3—CH CH—CH3 and CH2 CH2 are :–

CH2 CH2
(1) Ring-chain Isomers (2) Chain Isomers (3) Functional Isomers (4) Position Isomers
Ans. (1)
OR
(1) Functional Isomers (2) Position Isomers (3) Chain Isomers (4) Metamerism

Ans. (1)
OR
(1) Functional Isomers (2) Position Isomers (3) Ring-chain Isomers (4) 1 and 3 Both

Ans. (4)
Question : Relate structures a,b,c and d. (a) CH3—CH2—C CH (b) CH2 C CH—CH3
CH3
(c) (d)

a, b ——→ Functional Isomers
a, c. ——→ Ring-chain Isomers and Functional Isomers
b, c ——→ Ring-chain Isomers and Functional Isomers
a, d. ——→ Ring-chain Isomers and Functional Isomers
c, d. ——→ Chain Isomers
b, d. ——→ Ring-chain Isomers and Functional Isomers

5 4 3 2 4 3 2
Question : CH3 CH2 CH2 CH2 and CH3 CH2 CH CH3 are ?
1CN 1CN
Pentanenitrile 2–Methyl butanenitrile
Ans. Molecular formula same, Functional group same, position of Functional group same but different parent carbon
atom chain so both are Chain isomers
Question : How many minimum carbons required for Chain isomerism and Position isomerism in alkanes ?
Ans. 4, 6
Question : How many minimum carbons required for Chain isomerism and Position isomerism in alkenes ?
Ans. 4, 4
Question : How many minimum carbons required for Chain isomerism and Position isomerism in alkynes ?
Ans. 5, 4
4. Functional Isomerism :- Same molecular formula but different functional groups.
Following compounds show Functional isomerism, as they have same molecular formula and different functional
group.
(i) Alcohol and ether ——→ CH 3 —CH 2—OH and CH 3 —O—CH 3
(i) Aldehydes and ketones ——→ CH3 CH2 C H and CH3 C CH3

O O
(iii) Acids and ester ——→ CH3 C OH and H C O CH3

O O
(iv) Cyanide and isocyanide ——→ CH3—CH2—CH2—CN and CH3—CH2—CH2—NC