[ptryon]

Hello!

The graph showing the relationship between boiling points and molecular weight (for the alkanes) shows a non-linear increase (see link: http://www.chemistrydaily.com/chemistry/QSPR)

Can anyone explain this shape? Why is it not linear? Surely adding the same methylene group to each consecutive alkane should result in a linear increase.

[Nobby]

With increasing of the length the module is more able to fold to a tangle. So the melting- and boiling points will not be linear.

[JGK]

Can anyone explain this shape? Why is it not linear? Surely adding the same methylene group to each consecutive alkane should result in a linear increase.

You may be adding the same methylene group to each successive alkane but as the alkane gets larger the addition of a methylene group is an ever reducing percentage of the molecules size and weight and has lesser impact on the boiling point.

[ptryon]

You may be adding the same methylene group to each successive alkane but as the alkane gets larger the addition of a methylene group is an ever reducing percentage of the molecules size and weight and has lesser impact on the boiling point.

Good answer thanks... this is consistent with the shape of the graph. I don't think molecular tangling would account for the shape of the graph.