Chemistry, Third edition

17 · ORGANIC CHEMISTRY: HYDROCARBONS

Sources of alkanes

Alkanes are obtained from crude oil and natural gas. These relatively simple organic

chemicals can be used to make other, more complicated, organic compounds.

Oil

Oil and gas are trapped in rock and are the decayed remains of tiny marine plants and

animals. Natural gas contains 60–90% methane. Crude oil is a mixture of many carbon

compounds with different chain lengths and is not very useful if left untreated. The oil

is therefore separated into groups of compounds with roughly the same chain length in

anoil refinery. The technique that is used to refine the oil is called fractional distillation.

The oil is heated and passed into a tall fractionating column(Fig. 17.2). Compounds

with large molecules and high boiling points are collected at the bottom of the tower,

whereas the more volatile components, with smaller molecules, are collected near

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Physical properties of alkanes

Using data in the following table, plot a graph of the number of carbon atoms contained in

the straight chain alkane (xaxis) against boiling point of the alkane (yaxis). Draw a curve

to join up the points.

Alkane Number of carbon atoms Boiling point/K

Methane 1 109

Ethane 2 185

Propane 3 231

n-Butane 4 273

n-Pentane 5 309

n-Hexane 6 342

n-Heptane 7 372

n-Octane 8 ***

n-Nonane 9 424

n-Decane 10 447

(i) Which alkane with the most number of carbon atoms is a gas at room temperature?

(ii)Estimate the boiling point of n-octane.

Exercise 17F

Physical properties of the alkanes

1.The alkane molecules contain carbon and hydrogen atoms covalently bonded

together. There is very little difference between the electronegativities of these

atoms and so the C–H bond is only weakly polar. Furthermore, in a symmetrical

molecule such as methane, the weak polarities of the C–H bonds cancel out and

the molecule is non-polar overall. Therefore, alkane molecules are either non-

polar or weakly polar and the intermolecular forces between them are weak van

der Waals’ forces, so that the lower alkanes are volatile. As the molecular masses

of the alkanes increase (and thus the number of electrons in the molecules), the

intermolecular forces between the molecules of the heavier alkanes become

increasingly stronger. As the molecules of the alkanes get larger, therefore, their

melting and boiling points increase in a reasonably regular fashion.

2.Because they are non-polar and covalent compounds, the alkanes are soluble in

non-polar solvents such as benzene or ethoxyethane (diethyl ether) and insoluble

in water.