Isomerism previous year questions with solutions are given here. Isomerism is most widely observed in organic chemistry and has a huge application in chemistry and medicine. Isomers are molecules that have the same molecular formula but have a different arrangement of the atoms in space. They do not necessarily share similar properties unless they also have the same functional groups in the case of isomerism in organic chemistry.
Structural isomerism is the type of isomerism where the atoms and the functional groups are joined together in different ways. Chain isomerism is observed when the carbon molecules in the chain of an organic molecule are relocated, which alters the chain of the organic compound. Organic molecules usually have functional groups attached at the terminals such as halogen atoms, hydroxyl atoms, etc. The relocation of these functional groups between two identical organic compounds gives rise to Positional Isomerism.
The important topics in isomerism are geometrical isomerism, stereoisomerism, chain isomerism, positional isomerism and functional isomerism. Students can expect one or two questions from this topic. This article helps students to get an idea about the questions from the topic of isomerism. Questions asked in JEE Main generally follow a pattern and hence going through important topics at the last moment always helps. BYJU’S provides accurate solutions prepared by our subject experts. Students can easily download the questions and solutions in PDF format.
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JEE Main Previous Year Solved Questions on Isomerism
1. Monocarboxylic acids are functional isomers of :
(1) Esters
(2) Amines
(3) Ethers
(4) Alcohols
Solution:
Monocarboxylic acids are functional isomers of esters because esters and monocarboxylic acid have the same general (-COOH). A carboxylic acid is an ester derived from a carboxylic acid.
Hence option (1) is the answer.
2. Identify the compound that exhibits tautomerism:-
(1) 2-Pentanone
(2) Phenol
(3) 2-Butene
(4) Lactic acid
Solution:
The isomers of a compound which differ only in the position of the protons and electrons are called tautomers. Tautomerism is a reaction that involves a simple proton transfer in an intramolecular fashion. 2 pentanone exhibits tautomerism.
Hence option (1) is the answer.
3. Dipole moment is shown by:-
(1) trans-2, 3-dichloro- 2-butene
(2) 1, 2-dichlorobenzene
(3) 1, 4-dichlorobenzene
(4) trans-1, 2-dinitroethene
Solution:
The two dipoles in 1, 2-dichlorobenzene are unsymmetric. So it possesses a dipole moment.
Hence option (2) is the answer.
4. The alkene that exhibits geometrical isomerism is:-
(1) 2–butene
(2) 2–methyl–2–butene
(3) Propene
(4) 2–methylpropene
Solution:
The compounds which have the same molecular formula but differ in the relative spatial arrangement of atoms or groups in space are known as geometrical isomers. This phenomenon is called geometrical isomerism. 2-butene exists as cis or trans. It exhibits geometric isomerism because of restricted rotation around the double bond.
Hence option (1) is the answer.
5. Out of the following, the alkene that exhibits optical isomerism is:
(1) 2-methyl-2-pentene
(2) 3-methyl-2-pentene
(3) 4-methyl-1-pentene
(4) 3-methyl-1-pentene
Solution:
Two compounds which contain the same number and kinds of atoms, and bonds and different spatial arrangements of the atoms, but have non-superimposable mirror images are called optical isomers. Optical isomers can occur when there is an asymmetric carbon atom. It forms a chiral centre of the molecule. 3-methyl-1-pentene shows optical isomerism.
Hence option (4) is the answer.
6. Which of the following does not show geometrical isomerism?
(1) 1, 2-dichloro -1 – pentene
(2) 1, 3 – dichloro – 2 – pentene
(3) 1, 1 – dichloro – 1 – pentene
(4) 1, 4 – dichloro – 2 – pentene
Solution:
1, 1- dichloro-1- pentene does not show geometrical isomerism because of the unsymmetrical alkene.
Hence option (3) is the answer.
7. A similarity between optical and geometrical isomerism is that
(1) each forms an equal number of isomers for a given compound
(2) If in a compound one is present then so is the other
(3) both are included in stereoisomerism
(4) they have no similarity
Solution:
Optical and geometrical isomerism is included in stereoisomerism.
Hence option (3) is the answer.
8. Racemic mixture is formed by mixing two
(1) isomeric compounds
(2) chiral compounds
(3) meso compounds
(4) optical isomers
Solution:
Racemic mixture is formed when d and l forms of optical isomers are mixed in equimolar proportion.
Hence option (4) is the answer.
9. Which of the following will have meso-isomer also?
(1) 2- chlorobutane
(2) 2- hydroxypropanoic acid
(3) 2,3- dichloropentane
(4) 2-3- dichlorobutane
Solution:
2-3- dichlorobutane has 2 chiral centres and a plane of symmetry. So it will have a meso isomer.
Hence option (4) is the answer.
10. Which of the following compounds is not chiral?
(1) 1-chloropentane
(2) 3-chloro-2-methyl pentane
(3) 1-chloro -2-methyl pentane
(4) 2-chloropentane
Solution:
A chiral molecule is a type of molecule that has a non-superimposable mirror image.
1-chloropentane is not a chiral compound.
Hence option (1) is the answer.
11. Of the five isomeric hexanes, the isomer which can give two monochlorinated compounds is
(1) n-hexane
(2) 2, 3-dimethylbutane
(3) 2, 2-dimethylbutane
(4) 2- methylpentane
Solution:
The isomeric hexane should have two different types of H atoms and four similar types of H atoms to give two monochlorinated products. In 2,3-dimethyl butane, H atoms at positions ‘a’ and ‘b’ are the same and ‘b’ are the same. Thus two positions are available for chlorination.
Hence option (2) is the answer.
12. Which types of isomerism is shown by 2,3-dichlorobutane?
(1) Diastereo
(2) Optical
(3) Geometric
(4) Structural
Solution:
In 2, 3 dichlorobutane has 2 chiral centres.
The 2nd and 3rd carbons are chiral. They are not superimposable. So it shows optical isomerism.
Hence option (2) is the answer.
13. The alkene that exhibits geometrical isomerism is
(1) propene
(2) 2-methylpropene
(3) 2-butene
(4) 2-methyl -2-butene
Solution:
2- butene exhibits geometrical isomerism.
Hence option (3) is the answer.
14. Which one of the following conformations of cyclohexane is chiral?
(1) Boat
(2) Twist boat
(3) Rigid
(4) Chair
Solution:
The twist boat conformation of cyclohexane is optically active since it does not have any plane of symmetry.
Hence option (2) is the answer.
15. The number of structural isomers for C6H14 is
(1) 3
(2) 4
(3) 5
(4) 6
Solution:
C6H14 has 5 structural isomers.
Hence option (3) is the answer.
Also Read:- Isomers and isomerism
Video Lessons
Isomerism in Organic Compounds
Isomerism: General Organic Chemistry
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