Physical Properties of Alkanes

Alkanes are non-polar (or only very weakly polar) molecules: carbon and hydrogen have similar

electronegativities, so individual $\text{C--H}$ and $\text{C--C}$ bonds carry negligible dipole

character, and the molecule as a whole has no significant permanent dipole moment. The only

attractive force holding alkane molecules to one another in the liquid or solid state is,

therefore, the weak, instantaneous-dipole/induced-dipole attraction called the London (or

dispersion) force.

Boiling and melting points. Because dispersion forces act over the whole surface of a

molecule and grow stronger the more surface area (and the more polarisable electron cloud) is

available for contact, boiling points rise steadily as the carbon chain lengthens: methane boils

at $-162\,°\text{C}$, propane at $-42\,°\text{C}$, hexane at $69\,°\text{C}$, and decane at

$174\,°\text{C}$. Each additional $-\text{CH}_2-$ unit adds roughly $20$–$30\,°\text{C}$ to

the boiling point for the lower members, an increment that gradually shrinks for very long chains.

Effect of branching. For a fixed molecular formula, branching lowers the boiling point,

because a branched, more nearly spherical molecule has less surface area available for contact

with neighbouring molecules than a long, thin, straight chain does, weakening the net dispersion

attraction. Among the $\text{C}_5\text{H}_{12}$ isomers, n-pentane ($36\,°\text{C}$) boils

higher than isopentane ($28\,°\text{C}$), which boils higher than the almost-spherical

neopentane ($9.5\,°\text{C}$).

State at room temperature and solubility. The lightest alkanes ($\text{C}_1$–$\text{C}_4$)

are gases at room temperature; $\text{C}_5$–$\text{C}_{17}$ are liquids; and alkanes from about

$\text{C}_{18}$ upward are waxy solids. Being non-polar, alkanes are essentially insoluble in …