Question

Arrange in increasing order of boiling points: (A) 2,2-dimethylpropane, (B) 2-methylbutane, (C) n-pentane, (D) n-butane.

• A. A $<$ B $<$ C $<$ D
• B. C $<$ A $<$ B $<$ D
• C. D $<$ A $<$ B $<$ C
• D. D $<$ C $<$ B $<$ A

Hint:

More branching decreases boiling point because branching reduces molecular surface area and weakens van der Waals attraction.

Answer 1

The Correct Option is C

To determine the order of boiling points for the given compounds, we need to understand how molecular structure impacts boiling point. Boiling points generally increase with molecular size and decrease with increased branching due to reduced surface area.

1. First, let's identify the compounds:
   • (A) 2,2-dimethylpropane
   • (B) 2-methylbutane
   • (C) n-pentane
   • (D) n-butane
2. Boiling points are affected by the following factors:
   • Chain Length: Longer carbon chains generally have higher boiling points due to increased surface area, leading to stronger van der Waals forces.
   • Branching: More branching in a molecule reduces boiling point since it decreases the surface area.
3. Let's evaluate the given compounds:
   • (D) n-butane has the shortest chain with no branching. It is expected to have the lowest boiling point.
   • (A) 2,2-dimethylpropane is highly branched, thus also having a low boiling point, but higher than n-butane due to slightly more total molecular mass.
   • (B) 2-methylbutane is less branched than 2,2-dimethylpropane and has a longer chain length, resulting in a higher boiling point.
   • (C) n-pentane, an unbranched compound with the longest chain among them, should have the highest boiling point.
4. In increasing order of boiling points, the arrangement is:
   • \(D \lt A \lt B \lt C\)

Therefore, the correct answer is: D < A < B < C