Question

Arrange the compounds in order of their increasing boiling points. (A) 1-bromoethane (B) 1-bromobutane (C) 2-bromo-2-methylpropane (D) 2-bromobutane

• A. (A), (C), (D), (B)
• B. (A), (D), (C), (B)
• C. (C), (D), (A), (B)
• D. (C), (A), (D), (B)

Hint:

For alkyl halides of comparable molecular mass, greater branching means lower boiling point because of reduced surface area available for intermolecular attractions.

Answer 1

The Correct Option is A

Concept: Boiling point depends mainly upon:
• Molecular mass
• Surface area of molecules
• Strength of van der Waals forces
• Degree of branching For isomeric alkyl halides, greater branching decreases surface area and therefore decreases boiling point.

Step 1: Compare molecular sizes.
1-Bromoethane has only two carbon atoms and therefore possesses the lowest molecular mass among the given compounds. Hence it will have the lowest boiling point.

Step 2: Compare the C\(_4\)H\(_9\)Br isomers.
Among the butyl bromides: \[ \text{tert-Butyl bromide} < \text{sec-Butyl bromide} < \text{n-Butyl bromide} \] because branching decreases intermolecular attraction.

Step 3: Write the increasing order.
\[ \text{1-Bromoethane} < \text{2-Bromo-2-methylpropane} < \text{2-Bromobutane} < \text{1-Bromobutane} \] Thus, \[ (A)<(C)<(D)<(B) \]