Why do branched chain compounds have lower boiling points than the corresponding straight chain isomers?

Question

> The branched chain compounds have lower boiling points than the corresponding straight chain isomers. For example, > >- $\\ce{CH_3CH_2CH_2CH_2CH_3}$ - No branching-Pentane (n-pentane) ($\\mathrm{b.p.}=309~\\mathrm{K}$) >- $\\ce{CH_3CH(CH_3)CH_2CH_3}$ - One branching-2-Methylbutane (Iso-pentane) ($\\mathrm{b.p.}=301~\\mathrm{K}$) >- $\\ce{C(CH_3)_4}$ - Two branches-2,2-Dimethylpropane (Neo-pentane) ($\\mathrm{b.p.}=282.5\\mathrm{K~}$) > >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.* The above extract from my book, mentions clearly that branching makes the molecule more compact and thereby decreases the surface area. Even for isomeric alkyl halides, the boiling points decrease with branching. The reason is said to be because of the decrease in surface area, same as explained before. How does decrease in the surface area make the intermolecular forces small in magnitude?

Answer 1

The answer lies with the number of intermolecular forces between nonpolar compounds. As you may know, these intermolecular forces are called as London dispersion forces( Another force is dipole-dipole interactions which is for molecules with pernament dipole. To simplify the answer, I will only focus on the nonpolar intermolecular forces. A reference will be included in comments if you want more information).

In nonpolar compounds, temporary fluctuations in electron distribution within nonpolar molecules in the solid state results in formation of short-lived instantaneous dipole moments. These instantaneous dipole moments can induce nearby molecules and pull the electrons of the other molecule toward the molecule's positive end. The number of the temporary dipole-induced dipole interactions a molecule has is determined by its surface area.

For example, consider neopentane and n-pentane with boiling points 9.5°C and 36.1°C. Neopentane has the structure of a sphere whereas n-pentane is a straight chain. Therefore, n-pentane will be able to induce significantly more molecules than neopentane. Because more thermal energy will be needed to overcome these temporary attractive forces, the molecule with the most interactions will have a higher boiling point.