Assertion (A): Among 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane, 2-bromo-2-methylpropane has the lowest boiling point.
Reason (R): In haloalkanes, the intermolecular attractions get stronger as the molecules get bigger in size and have more electrons.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): Among 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane, 2-bromo-2-methylpropane has the lowest boiling point. — True. The three compounds are isomers (all C4H9Br). For isomeric haloalkanes the boiling point falls as branching increases; 2-bromo-2-methylpropane is the most branched, so it boils lowest (346 K, against 364 K and 375 K).
Reason (R): In haloalkanes, the intermolecular attractions get stronger as the molecules get bigger in size and have more electrons. — True. This is why boiling points rise from CH3X to larger alkyl halides, and from RF to RI.
Why R does not explain A?
The three compounds are isomers with the same mass and the same number of electrons, so R (bigger molecules boil higher) cannot account for the difference among them.
The correct reason is branching: it makes the molecule more compact, lowers its surface area and so weakens the van der Waals forces, giving the most branched isomer the lowest boiling point.
NCERT (2026-27 edition):
6.6 Physical Properties – “The attractions get stronger as the molecules get bigger in size and have more electrons. … For the same alkyl group, the boiling points of alkyl halides decrease in the order: RI> RBr> RCl> RF.”
“The boiling points of isomeric haloalkanes decrease with increase in branching. For example, 2-bromo-2-methylpropane has the lowest boiling point among the three isomers.”
7.4.3 Physical Properties (alcohols, the same effect of branching) – “…In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area).”