chemistry : class12 Isomeric Variation : Physical properties of haloalkanes : Haloalkanes and Haloarenes MCQ Questions and Answers
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Isomeric Variation
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MCQ Questions
1. Among the following constitutional isomers of C4H9Cl, which is expected to have the highest boiling point primarily because of its greatest molecular surface area?
A.
1-Chlorobutane
B.
2-Chlorobutane
C.
1-Chloro-2-methylpropane
D.
2-Chloro-2-methylpropane
Correct Answer:
1-Chlorobutane
Explanation:
All four compounds have the same molecular formula and similar molar masses, but 1-chlorobutane is least branched. Its extended structure provides greater surface contact between molecules, producing stronger dispersion forces and the highest boiling point.
2. Which sequence correctly represents the expected decrease in boiling point among the following isomeric chlorobutanes?
A.
1-Chlorobutane > 2-chlorobutane > 1-chloro-2-methylpropane > 2-chloro-2-methylpropane
B.
2-Chloro-2-methylpropane > 1-chloro-2-methylpropane > 2-chlorobutane > 1-chlorobutane
C.
1-Chloro-2-methylpropane > 1-chlorobutane > 2-chloro-2-methylpropane > 2-chlorobutane
D.
2-Chlorobutane > 1-chlorobutane > 1-chloro-2-methylpropane > 2-chloro-2-methylpropane
Explanation:
For isomers with the same formula, increased branching generally decreases molecular surface area and weakens dispersion forces. The straight-chain isomer therefore boils highest, while the highly branched tert-butyl chloride boils lowest.
3. Two isomeric haloalkanes have identical molecular formulae and the same halogen atom. Compound X is more highly branched than compound Y. Which comparison is most generally expected for their boiling points?
A.
X has a higher boiling point because branching increases molecular mass
B.
X has a lower boiling point because branching reduces intermolecular contact area
C.
X and Y must have identical boiling points because their molecular formulae are identical
D.
X has a higher boiling point because branching always increases dipole moment
Correct Answer:
X has a lower boiling point because branching reduces intermolecular contact area
Explanation:
Structural isomers have the same molecular formula but different shapes. Greater branching makes a molecule more compact, reducing its surface contact and usually lowering London dispersion forces and boiling point.
4. Which isomer of C5H11Br is expected to have the highest boiling point, assuming comparison under identical external conditions?
A.
1-Bromopentane
B.
2-Bromopentane
C.
1-Bromo-2-methylbutane
D.
2-Bromo-2-methylbutane
Correct Answer:
1-Bromopentane
Explanation:
1-Bromopentane has an unbranched carbon skeleton, giving it the largest effective surface area among these isomers. Consequently, its intermolecular dispersion forces and boiling point are expected to be greatest.
5. Which structural change most directly explains the lower boiling point of 2-chloropropane compared with 1-chloropropane?
A.
Replacement of chlorine by hydrogen
B.
Increase in molecular mass caused by branching
C.
Formation of a more compact, branched molecular shape
D.
Formation of hydrogen bonds in 1-chloropropane only
Correct Answer:
Formation of a more compact, branched molecular shape
Explanation:
The two compounds have the same molecular formula. In 2-chloropropane, the compact arrangement reduces the area over which neighboring molecules can interact, so its dispersion forces and boiling point are lower.
6. Among the following isomeric bromides, which compound is expected to possess the lowest boiling point because its carbon skeleton is most highly branched?
A.
1-Bromobutane
B.
2-Bromobutane
C.
1-Bromo-2-methylpropane
D.
2-Bromo-2-methylpropane
Correct Answer:
2-Bromo-2-methylpropane
Explanation:
2-Bromo-2-methylpropane has a compact, highly branched structure. This minimizes molecular surface contact and weakens dispersion interactions relative to the other C4H9Br isomers, resulting in the lowest boiling point.
7. Two isomeric haloalkanes have equal molar masses, but one is straight-chain and the other is branched. Which observation most strongly supports the effect of isomerism on their physical properties?
A.
They necessarily have identical boiling points
B.
The straight-chain isomer generally has a higher boiling point
C.
The branched isomer necessarily has a higher density and boiling point
D.
Only the straight-chain isomer can exist as a liquid
Correct Answer:
The straight-chain isomer generally has a higher boiling point
Explanation:
Isomerism changes molecular shape without changing molecular formula or molar mass. A straight-chain haloalkane usually has greater intermolecular contact area, leading to stronger dispersion forces and a higher boiling point.
8. Which pair represents isomeric haloalkanes for which branching can be used to rationalize a lower boiling point of the second compound?
A.
1-Chlorobutane and 2-chloro-2-methylpropane
B.
1-Chlorobutane and 1-bromobutane
C.
Chloromethane and chloroethane
D.
1,2-Dichloroethane and 1,1-dichloroethane
Correct Answer:
1-Chlorobutane and 2-chloro-2-methylpropane
Explanation:
These two compounds have the same molecular formula, C4H9Cl, but different carbon skeletons. The second is highly branched, so it has lower surface contact and a lower boiling point than the straight-chain first compound.
9. A student claims that all isomers of C4H9Cl must have the same volatility because they have the same molecular mass. Which response is scientifically most accurate?
A.
Correct, because volatility depends only on molecular mass
B.
Incorrect, because molecular shape changes intermolecular attractions and hence boiling point
C.
Correct, because chlorine atoms eliminate differences caused by branching
D.
Incorrect, because isomers always have different molecular formulae
Correct Answer:
Incorrect, because molecular shape changes intermolecular attractions and hence boiling point
Explanation:
Volatility is related to how readily molecules escape into the gas phase, which depends on intermolecular forces as well as mass. Different shapes of isomeric haloalkanes alter contact area and therefore their boiling points and volatilities.
10. For the isomeric compounds 1-chloropentane, 2-chloropentane, and 1-chloro-3-methylbutane, which prediction is most consistent with the effect of isomeric branching on boiling point?
A.
1-Chloropentane is expected to have the highest boiling point
B.
2-Chloropentane is expected to have the highest boiling point because the chlorine atom is internal
C.
1-Chloro-3-methylbutane is expected to have the highest boiling point because it is branched
D.
All three must have identical boiling points because they contain five carbon atoms
Correct Answer:
1-Chloropentane is expected to have the highest boiling point
Explanation:
1-Chloropentane has the least branched, most extended carbon skeleton among the listed isomers. Its larger effective surface area generally produces stronger dispersion forces and a higher boiling point.
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