The alkanes are a homologous series of compounds containing only carbon and hydr

Question

The alkanes are a homologous series of compounds containing only carbon and hydrogen that have the general formula CnH2n+2. Members of this series include butane (C4H10), 3,3-dimethylpentane (C7H16), hexane (C6H14), and heptane (C7H16).

Part A

The boiling points for a set of compounds in a homologous series can be qualitatively predicted using intermolecular force strengths. Using their condensed structural formulas, rank the homologous series for a set of alkanes by their boiling point.

Rank from highest to lowest boiling point. To rank items as equivalent, overlap them.

Explanation / Answer

To answer this question, we need to remember two key facts:

1. The boiling point of alkanes increase with their molecular weight.

2. Branching in alkanes lead to lowering of boiling points.

As the molecular weight of an alkane increases, the boiling point increases. The reason can be attributed to an increase in Vander Waal’s forces due to increase in molecular weight. Vander Waal’s forces are proportional to the surface area. As the molecular weight increases, the chain length increases and this leads to increased ability of the molecules to attract each other. Therefore, the boiling point increases.

Branching leads to molecules becoming more compact. As branching increases, the molecules condense into a sphere-like shape. The surface area decreases and hence, the Vander Waal’s forces and thus the boiling point decreases.

Therefore, the order of boiling point (from highest to lowest) is

heptane > 3,3-dimethylpentane > hexane > butane

3,3-dimethylpentane exhibits a higher boiling point than hexane because it has greater surface area than hexane. However, despite branching, the surface area is lower than that of heptane and thus the boiling point is lower than heptane.

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