Hydrocarbons:Relation of Structure and Physical Properties

Weight Within a homologous series melting points and boiling points increase with increasing molecular weight. The larger the molecule, the more sites for intermolecular attraction. The heavier the substance, the greater the energy needed to break intermolecular forces. Bp are more regular than mp which also depends on crystal lattice formation.

Structure Boiling point decreases with branching because it is more compact so there is less surface area. Melting point increases because it is smaller so it fits easier into a crystal lattice.

Petroleum In 1859 Edwin Drake drilled the first oil well in Oklahoma. Crude oil is black, viscous, and foul smelling. To convert crude oil into various fractions use fractional distillation. Over 40% is made into gasoline. Less than 5% goes to the petrochemical industry.