Question

How do the large surface area of pentane leads to its high boiling point then iso pentane? ?

Answer 1

The branched chain compounds have lower boiling points than the corresponding straight chain isomers. For example,

• CH3CH2CH2CH2CH3 - No branching-Pentane (n-pentane) (b.p.=309 K)
• CH3CH(CH3)CH2CH3- One branching-2-Methylbutane (Iso-pentane) (b.p.=301 K)
• C(CH3)4 - Two branches-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.