JEE Chemistry Isomerism MCQs Set 05

Answer: (a) Functional isomers

Explanation:
1. Both are \( \ce{C3H6O2} \), one an ester and the other an acid
2. Different functional groups give functional 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: (c) 2-Butene

Explanation:
1. 2-Butene has two different groups on each doubly bonded carbon
2. 1-Butene, propene and benzene cannot show cis-trans 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) \( \ce{CH3OCH3} \)

Explanation:
1. Ethanol and dimethyl ether both have the formula \( \ce{C2H6O} \)
2. An alcohol and an ether are functional 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) Spatial arrangement of atoms

Explanation:
1. Geometrical isomers have the same connectivity
2. They differ in the spatial arrangement about a double bond or ring

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) \( \ce{NH2-NH2} \)

Explanation:
1. Conformations need a single bond with rotation and substituents on both atoms
2. Hydrazine has an N-N single bond with H atoms and lone pairs on each nitrogen

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) 2,3,4-Trimethylhexane

Explanation:
1. In 2,3,4-trimethylhexane C-3 and C-4 each have four different groups
2. The other three have no stereocentre

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) 4

Explanation:
1. For n dissimilar stereocentres there are 2^n optical isomers
2. With n = 2 this gives 4

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) 1

Explanation:
1. The carbon bears four different groups: H, OH, COOH and the ester group OCOC2H5
2. No other carbon is mentioned, so the compound has 1 chiral 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: (a) Conformation

Explanation:
1. Rotation about a single bond gives different spatial arrangements called conformations
2. The other types need bond breaking

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) An optically active mixture

Explanation:
1. The pure acid (say R) gives (R,R) and (R,S) esters
2. These are diastereomers, formed in unequal amounts, so the product is an optically active mixture

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) \( \ce{(CH3)2C=C(Cl)Br} \)

Explanation:
1. Geometrical isomerism needs two different groups on each doubly bonded carbon
2. In (b) one carbon carries 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: (a) A meso compound

Explanation:
1. A meso compound has internal symmetry
2. It has stereocentres but is achiral and optically inactive

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) \( \ce{CH3CHBrCl} \)

Explanation:
1. In \( \ce{CH3-CHBrCl} \) the carbon has H, Br, Cl and \( \ce{CH3} \), all different
2. The others have two identical atoms on 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: (a) Diethyl ketone and methyl propyl ketone are position isomers

Explanation:
1. Diethyl ketone and methyl propyl ketone are metamers (different alkyl groups on the C=O), not position isomers
2. Statements (b), (c) and (d) are correct

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) Functional isomers

Explanation:
1. Both are \( \ce{C2H6O} \)
2. One is an ether and the other an alcohol, so they are functional 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) 2,3-Dichlorobutane

Explanation:
1. A meso form needs two identical stereocentres that mirror each other
2. 2,3-Dichlorobutane has such a pair; the others have non-identical or single stereocentres

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) Gaseous densities at the same temperature and pressure

Explanation:
1. Both have molar mass 46 g/mol, so at the same T and P (ideal gas) the densities are equal
2. Ethanol has hydrogen bonding, so the first three differ

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) \( \ce{HC#C-CHCl-CH3} \)

Explanation:
1. The carbon in \( \ce{HC#C-CHCl-CH3} \) has H, Cl, \( \ce{CH3} \) and ethynyl, all different
2. In (a) two groups are identical and (c) and (d) 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) Tautomerism

Explanation:
1. Nitroethane has an alpha hydrogen and can form the aci (nitronic acid) form
2. The nitro and aci forms are tautomers

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) \( \ce{CH3CH2-CHCl-CH3} \)

Explanation:
1. Only 2-chlorobutane has a carbon bonded to four different groups
2. The other two 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: (b) 12

Explanation:
1. From but-1-ene: 1-chloro (cis, trans), 2-chloro, 3-chloro (R, S) and 4-chloro = 6
2. From but-2-ene: 1-chloro (cis, trans) and 2-chloro (cis, trans) = 4; from isobutene: 2
3. Total 12

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

Explanation:
1. The four monochloro products are 1-chloro-2-methylbutane (chiral), 2-chloro-2-methylbutane, 2-chloro-3-methylbutane (chiral) and 1-chloro-3-methylbutane
2. Two of them are chiral, so two enantiomeric pairs

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) Stereoisomers

Explanation:
1. The same connectivity with a different spatial arrangement defines stereoisomers
2. Optical isomers are only one kind of stereoisomer

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) \( \ce{(CH)2(COOH)2} \)

Explanation:
1. \( \ce{(CH)2(COOH)2} \) is butenedioic acid, \( \ce{HOOC-CH=CH-COOH} \)
2. It has different groups on each C=C carbon (maleic and fumaric acids)

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.