Preparation
The main source of alkanes is petroleum.
The word PETROLEUM comes from Greek and means 'rock oil'. It is a dark, strong-smelling substance, lighter than water, of a very complex and variable composition, mainly constituted by hydrocarbons, that can be found in nature in underground reservoirs in the upper layers of the Earth's crust.
Fractional distillation of petroleum yields many products of great industrial importance, such as gasoline, kerosene, tar, solvents and many more.
Practically nothing of petroleum is wasted. The heaviest and lightest residues are submitted to different processes, i.e. CRACKING and REFORMING, respectively.
By means of CRACKING, the large compounds present in the heaviest residues are broken by the action of high temperatures and special catalysts (a well-kept secret of the refining companies) among which one can find ZEOLITES.
By REFORMING of the lightest residues containing relatively small molecules, more complex molecules can be obtained. The use of special (and secret) catalysts is also essential to this process.
The hydrocarbon with the highest 'octane number' is isooctane. In turn, heptane is the one with the lowest. The 'octane number' measures the resistance of gasoline to self-ignition under pressure: the higher the octane number, the more difficult the self-ignition, which is what car engines require.
Gasoline would be outrageously expensive if it were made almost entirely of isooctane in order to reach a suitable octane number. The octane number of a mixture of alkanes can be raised with additives that prevent the premature self-ignition of gasolines in the explosion cycle of an engine. One of the most common additives in the past was tetraethyllead. It has long been banned because of its high toxicity ('unleaded gasolines'). The search for less harmful additives has been frantic and led to the use of methyl tert-butyl ether (MTBE).