Question

Rank the following molecules in order of increasing preference for an equatorial placement of the non-hydrogen substituent:

a) Propane, Butane, Pentane
b) Methane, Ethane, Propane
c) Ethane, Butane, Pentane
d) Butane, Ethane, Methane

Answer 1

The boiling points of the compounds can be predicted based on their molecular mass and shape. Propane has the lowest boiling point while n-pentane has the highest boiling point. The order of increasing boiling points is propane < 2-methylpropane < n-butane < n-pentane.

Step-by-step explanation:

The four compounds are alkanes and nonpolar, so London dispersion forces are the only important intermolecular forces. These forces are generally stronger with increasing molecular mass, so propane should have the lowest boiling point and n-pentane should have the highest, with the two butane isomers falling in between. Of the two butane isomers, 2-methylpropane is more compact, and n-butane has the more extended shape. Consequently, we expect intermolecular interactions for n-butane to be stronger due to its larger surface area, resulting in a higher boiling point. The overall order is thus as follows, with actual boiling points in parentheses: propane (-42.1°C) < 2-methylpropane (-11.7°C) < n-butane (-0.5°C) < n-pentane (36.1°C).