How does chain length affect Van der Waals forces?

1 Answer
anor277
Aug 25, 2016

The longer, and less branched the chain, the greater the opportunity for intermolecular interaction......

Explanation:

........and the greater the magnitude of Van der Waals forces.

The best metric to demonstrate this are the boiling points of the alkanes. As the alkanes increase in size the boiling points increase, as there are more electrons in the molecule to contribute to Van der Waals interactions.

But we can appreciate the influence of chain length by considering the isomeric pentanes.

#n-C_5H_12#, a straight chain molecule, has #"b.p."=36.1 ""^@C#.

#"Isopentane, methylbutane"#, its isomer, #"b.p."=27.7 ""^@C#.

#"Neopentane, dimethylpropane"#, the third isomer has #"b.p."=9.5 ""^@C#

Chain to chain interactions are clearly less achievable in methylbutane and neopentane because of the branching, and this manifests in the boiling point.

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