Does more branching lead to lower boiling point in alkanes?

Question

List the following alkanes in order of increasing boiling point.

A. $\ce{CH3(CH2)4CH3}$
B. $\ce{(CH3)2CHCH(CH3)2}$
C. $\ce{CH3CH2CH(CH3)CH2CH3}$

Answer: $C<B<A$, low to high.

Why is it that B has not a lower boiling point than C? Is it not more branched? I believe branching decreases surface area, leading to less intermolecular interactions, and to a higher boiling point.

Possible duplicate of http://chemistry.stackexchange.com/questions/6944/why-do-branched-chain-compounds-have-lower-boiling-points-than-the-corresponding?rq=1 — Geoff Hutchison, Dec 15 '14 at 20:18

score 4 · Answer 1 · answered Dec 15 '14 at 22:01

4

$A$ n-hexane, boiling point 69 degrees C.

$B$ 2,3 dimethyl butane, boiling point 58 degrees C.

$C$ 3-methyl pentane, boiling point 63 degrees C.

$B<C<A$

answered Dec 15 '14 at 22:01

DavePhD

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score 4 · Accepted Answer · answered Dec 16 '14 at 19:14

B has more branching and this means that molecules of B pack less efficiently and so there are fewer Van der Waals forces between them and so B has the lowest boiling point. On the other hand A is a linear molecule and so packs efficiently, has stronger VdWs forces and so has the highest boiling point. As @DavePhD's data shows $\ce{B < C < A}$

Does more branching lead to lower boiling point in alkanes?

2 Answers2