JEE Chemistry Isomerism MCQs Set 04

Answer: (a) Enantiomers

Explanation:
1. They are non-superimposable mirror images
2. Such pairs are enantiomers

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) Ethane

Explanation:
1. Ethane has the smallest torsional barrier (about 12 kJ/mol)
2. Hexachloroethane is hindered by bulky Cl, and ethylene and acetylene have pi bonds

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) Maleic and fumaric acid

Explanation:
1. Maleic acid is the cis and fumaric acid the trans form of butenedioic acid
2. The other pairs are different compounds

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) sec-Butylamine

Explanation:
1. sec-Butylamine \( \ce{CH3CH2-CH(NH2)-CH3} \) has a carbon with four different groups
2. The others have none

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) Neopentane and isopentane

Explanation:
1. Both are \( \ce{C5H12} \) with different carbon skeletons
2. The other pairs are position isomers, position isomers and metamers

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: (a) An asymmetric carbon atom

Explanation:
1. A carbon with four different groups makes the molecule chiral
2. Symmetry elements make a molecule achiral

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) 2-Methylbutane and neopentane

Explanation:
1. Both are \( \ce{C5H12} \) with different carbon skeletons
2. The other pairs are position 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: (b) Optical isomers

Explanation:
1. 2-Bromopropanoic acid has one stereocentre, so both enantiomers form
2. The pair is optical 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: (a) Propene

Explanation:
1. Propene has two H atoms on one C=C carbon
2. Its two groups on that carbon are identical, so no cis-trans form exists

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) 2-Methylpropane

Explanation:
1. 2-Methylpropane gives 1-chloro-2-methylpropane and 2-chloro-2-methylpropane
2. n-Pentane gives 3, benzene 1 and 2,4-dimethylpentane 3

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: (a) Central carbon atom

Explanation:
1. The central carbon holds four different groups in a tetrahedral arrangement
2. It is the stereocentre

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) Restricted rotation about a double bond

Explanation:
1. The pi bond prevents rotation about the C=C bond
2. The groups are locked on the same or opposite sides

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: (d) Diastereoisomers

Explanation:
1. Diastereoisomers are stereoisomers that are not mirror images
2. Enantiomers are mirror images

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) Chain

Explanation:
1. n-Butanol and isobutanol are \( \ce{C4H9OH} \) with different carbon chains
2. This is chain 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: (a) \( \ce{CH3CH=CHCH3} \)

Explanation:
1. 2-Butene has two different groups on each doubly bonded carbon
2. In the others a doubly bonded carbon has two identical methyl groups

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) 3

Explanation:
1. But-1-ene, but-2-ene and 2-methylpropene
2. Three open-chain structures (but-2-ene's cis and trans forms are stereoisomers, not structural)

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) 16

Explanation:
1. Open-chain glucose has four stereocentres
2. The number of stereoisomers is 2^4 = 16

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. Substituting at C-3 gives \( \ce{CH3CH2-CH(X)-C4H9} \), which has four different groups
2. At C-1 and C-7 the carbon keeps three identical H, and at C-4 the two chains are the same, so the product is achiral

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) 9

Explanation:
1. Heptane, two methylhexanes, four dimethylpentanes, 3-ethylpentane and 2,2,3-trimethylbutane
2. In total 1 + 2 + 4 + 1 + 1 = 9

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: (a) \( \ce{CH3-CHCl-COOH} \)

Explanation:
1. \( \ce{CH3-CHCl-COOH} \) has a carbon bonded to H, Cl, \( \ce{CH3} \) and COOH
2. The others have no such carbon

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) Both are included in stereoisomerism

Explanation:
1. In both types the atoms are connected in the same order but arranged differently in space
2. Both come under stereoisomerism

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) Plane polarised light

Explanation:
1. A Nicol prism allows vibrations in only one plane to pass
2. The emerging beam is plane-polarised light

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) 1-Chloro-2-methylbutane

Explanation:
1. In \( \ce{ClCH2-CH(CH3)-CH2CH3} \) C-2 has H, \( \ce{CH3} \), \( \ce{C2H5} \) and \( \ce{CH2Cl} \)
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.