JEE Chemistry Isomerism MCQs Set 03

Answer: (a) 4

Explanation:
1. Each of the two double bonds has different groups on its carbons
2. So each can be cis or trans, giving 2 x 2 = 4 isomers

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (c) Optical activity

Explanation:
1. Optical activity is the ability to rotate plane-polarised light
2. The other terms describe different phenomena

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) \( \ce{CH3CHOHCOOH} \)

Explanation:
1. Only lactic acid has a carbon bonded to four different groups
2. The others have no stereocentre

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (b) 2

Explanation:
1. The compound \( \ce{CH3CH2-C(OH)(CH3)-COOH} \) has one stereocentre (C-2)
2. So there are two stereoisomers

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (c) Optical isomerism, geometrical isomerism

Explanation:
1. Stereoisomers have the same connectivity but different spatial arrangement
2. Optical and geometrical isomerism are both stereoisomerism (position isomerism and metamerism are structural)

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (c) Metamers

Explanation:
1. Both are \( \ce{C4H10O} \) ethers with different alkyl groups on oxygen
2. Such isomers are metamers

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (d) Polarimetry

Explanation:
1. Enantiomers have identical IR spectra, mass spectra and melting points
2. They differ only in the direction in which they rotate plane-polarised light, measured by a polarimeter

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (a) Functional

Explanation:
1. \( \ce{R-CN} \) and \( \ce{R-NC} \) have the same formula but different functional groups
2. The relation is functional isomerism

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (d) Butanone

Explanation:
1. Diethyl ether is \( \ce{C4H10O} \), as are the first three options
2. Butanone is \( \ce{C4H8O} \)

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) Functional isomerism

Explanation:
1. Both are \( \ce{C3H8O} \), one an ether and the other an alcohol
2. Different functional groups mean functional isomerism

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (a) Acetone and propionaldehyde

Explanation:
1. Acetone and propanal are both \( \ce{C3H6O} \) but one is a ketone and the other an aldehyde
2. The others are metamers, chain isomers and position isomers

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) 3

Explanation:
1. The isomers are 1,2,3-, 1,2,4- and 1,3,5-trimethylbenzene
2. Three in all

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (d) Diastereoisomers

Explanation:
1. Stereoisomers that are not mirror images of each other are diastereoisomers
2. Enantiomers are mirror images

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (b) 5

Explanation:
1. Benzylamine, N-methylaniline and o-, m- and p-toluidine
2. Five isomers

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) \( \ce{C2H5-CH(CH3)-C3H7} \)

Explanation:
1. 3-Methylhexane has a carbon with H, \( \ce{CH3} \), \( \ce{C2H5} \) and \( \ce{C3H7} \)
2. None of the other options has a stereocentre

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (d) 2-Methylpropene

Explanation:
1. In 2-methylpropene one doubly bonded carbon has two identical methyl groups
2. The other three have different groups on each C=C carbon

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) Conformers

Explanation:
1. Conformers differ by rotation about single bonds only
2. The other isomers need bond breaking

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (d) 4

Explanation:
1. The compound has two different stereocentres, so 2^2 = 4 stereoisomers
2. These form two pairs of enantiomers, i.e. four optically active forms

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (a) \( \ce{DCH2CH2CH2Cl} \)

Explanation:
1. In (a) no carbon has four different groups
2. In (b), (c) and (d) one carbon carries four different groups (H, D and two different carbon chains or Cl)

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (d) Geometrical isomers

Explanation:
1. They differ in the arrangement of groups about the double bond
2. This is geometrical (cis-trans) isomerism

Teacher's Note:
1. Enantiomers differ only in optical rotation; diastereomers differ in all physical properties.
2. Meso compounds have stereocentres but are optically inactive because of internal symmetry.

Answer: (d) \( \ce{CHClBrI} \)

Explanation:
1. \( \ce{CHClBrI} \) has a carbon bonded to H, Cl, Br and I
2. All four groups are different

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.

Answer: (b) Functional

Explanation:
1. \( \ce{R-CN} \) and \( \ce{R-NC} \) have different functional groups
2. They are functional isomers

Teacher's Note:
1. Write out the structures of all isomers systematically (skeleton first, then position of groups) when counting.
2. Remember that the number of stereoisomers is 2^n only when the stereocentres are all different.

Answer: (b) 2-Butanol

Explanation:
1. 2-Butanol has a carbon with H, OH, \( \ce{CH3} \) and \( \ce{C2H5} \)
2. The others have no stereocentre (and 3-butanol is not a valid name)

Teacher's Note:
1. Look for a carbon with four different groups to spot chirality, and for two different groups on each doubly bonded carbon to spot geometrical isomerism.
2. Check that the isomers have the same molecular formula before naming the type.