Question

A general rule for a group of hydrocarbon isomers is that as the amount of branching increases, the boiling point decreases. Explain why this would be true.

Short answer

The branching of hydrocarbons has an impact on the boiling point, among other things. It is based on intermolecular interactions or London dispersion forces in the case of alkanes.

With the increasing branching, surface area decreases among neighboring molecules, and this leads to weaker LD forces and,therefore, lower boiling points.

Step-by-step solution

Define the hydrocarbon isomers

Chain isomers are structural isomers that differ in the chain of constituent atoms.

Carbon and hydrocarbons have a property called chain isomerism.

Step 2: Define London dispersion forces.

The branching of hydrocarbons has an impact on the boiling point among other things. It is based on intermolecular interactions or London dispersion forces in the case of alkanes.

As all hydrocarbons haveLondon dispersion forces as their primary intermolecular forces.

The surface area in contact among the neighboring molecules determines the strength of the LD forces.

Branched alkanes have a reduced surface area for London dispersion forcesbecause they are more compact.

The boiling temperatures of branched alkanes are often lower than those of n-alkanes with the same molecular weight.

Therefore, lower boiling points result from weaker London dispersion forces.