CBSE Class 11-science Answered

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules

The branched chain compounds have lower boiling points than the corresponding straight chain isomers as follows:

No branching

CH₃CH₂CH₂CH₂CH₃

Pentane (n−pentane) (b.p.=309K)

One branching

CH₃CH(CH₃)CH₂CH₃

2−Methylbutane (Iso−pentane) (b.p.=301K)

Two branches

C(CH₃)₄

2,2−Dimethylpropane (Neo−pentane) (b.p.=282.5K)

This is due to the fact that branching of the chain makes the molecule more compact and thereby decreases the surface area. Therefore, the intermolecular attractive forces which depend upon the surface area, also become small in magnitude on account of branching. Consequently, the boiling points of the branched chain alkanes are less than the straight chain isomers.