JEE Chemistry Isomerism MCQs Set 02

Answer: (b) \( \ce{(CH3)3C-OH} \)

Explanation:
1. Diethyl ether is \( \ce{C4H10O} \)
2. \( \ce{(CH3)3C-OH} \) (tert-butyl alcohol) is also \( \ce{C4H10O} \), while the other options have 3 or 5 carbons

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) Molecular formula

Explanation:
1. Isomers are compounds with the same molecular formula but different structures
2. Their physical and chemical properties generally differ

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) Staggered and chair

Explanation:
1. Staggered ethane has the least torsional strain
2. The chair form of cyclohexane is free of angle and torsional strain

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) \( \ce{CH3-CH2-CH(NH2)-CH3} \)

Explanation:
1. Only \( \ce{CH3CH2-CH(NH2)-CH3} \) has a carbon bonded to four different groups (H, \( \ce{NH2} \), \( \ce{CH3} \), \( \ce{C2H5} \))
2. The others have no 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: (c) \( \ce{C6H5CH2CH2CH3} \)

Explanation:
1. (a), (b) and (d) each have a carbon with four different groups
2. Propylbenzene has no stereocentre, so it is achiral

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) 3

Explanation:
1. 1,1-Dibromoethene and the cis and trans forms of 1,2-dibromoethene
2. Three isomers in all

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) 2-Chlorobutane

Explanation:
1. 2-Chlorobutane has a carbon with H, Cl, \( \ce{CH3} \) and \( \ce{C2H5} \)
2. The others have no chiral carbon

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) Isomers

Explanation:
1. This is the definition of isomers
2. Isotopes differ in neutrons, isobars have equal mass number

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) \( \ce{CH3-CHCl-CH2-CH3} \)

Explanation:
1. Only 2-chlorobutane has a carbon with four different groups
2. The others have a symmetrical or terminal 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) A ball

Explanation:
1. A sphere has a plane of symmetry and is superimposable on its mirror image
2. The letters P and F and a hand are chiral

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) Resolution

Explanation:
1. The racemic mixture is converted into diastereomers (or separated by another method)
2. This separation of enantiomers is resolution

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

Explanation:
1. Lactic acid has one stereocentre
2. Two enantiomers (d and l) exist

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) \( \ce{BrCH2-CH(OH)-CH3} \)

Explanation:
1. In \( \ce{BrCH2-CH(OH)-CH3} \) the carbon with OH has H, OH, \( \ce{CH3} \) and \( \ce{CH2Br} \), all different
2. The other molecules have no such 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: (a) Chain isomers

Explanation:
1. Both are \( \ce{C4H10} \) but have different carbon skeletons
2. So they are chain 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) \( \ce{CH3CH2CH=CH2} \) and \( \ce{CH3CH=CHCH3} \)

Explanation:
1. 1-Butene and 2-butene differ only in the position of the double bond
2. (a) and (d) are chain isomers and (c) is functional isomerism

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) 2-Butene

Explanation:
1. 2-Butene has restricted rotation about C=C and two different groups on each doubly bonded carbon
2. 2-Butyne is linear and the other two have no C=C

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) Four different groups are linked to a carbon atom

Explanation:
1. A carbon bonded to four different groups is chiral
2. Its mirror image is not superimposable

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. Each doubly bonded carbon of 2-butene carries two different groups
2. In the others one carbon has two identical groups

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) 4

Explanation:
1. Chain isomerism first appears at \( \ce{C4H10} \) (butane and isobutane)
2. Methane, ethane and propane have only one structure

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) Metamerism

Explanation:
1. Metamerism needs a polyvalent atom such as O, S or N between two alkyl groups
2. Alkenes can show chain, position and geometrical isomerism

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) 4

Explanation:
1. Pentan-1-ol, 2-methylbutan-1-ol, 3-methylbutan-1-ol and 2,2-dimethylpropan-1-ol
2. There are four primary alcohols

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) 4

Explanation:
1. Butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol
2. Four alcohols

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) \( \ce{ClCH=CHCl} \)

Explanation:
1. Each doubly bonded carbon of 1,2-dichloroethene has two different groups (H and Cl)
2. So cis and trans forms exist

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.