📝 Alkanes and sources of hydrocarbons
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The alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2 and names ending –ane. Alkanes contain only single carbon–carbon bonds and so are saturated. Alkanes with four or more carbon atoms display structural isomerism because the carbon chain may be either straight or branched. The naming of alkanes depends on whether they are straight or branched. Alkanes are not especially reactive, but they do undergo combustion. With an adequate supply of air, they react to form carbon dioxide and water.
Chlorine atoms or bromine atoms can substitute for hydrogen atoms in alkanes in the presence of ultraviolet light, producing halogenoalkanes. This is called a free radical substitution reaction. A free radical is a species which contains an unpaired electron. They are caused by the homolytic fission of covalent bonds.
The Cl-Cl or Br-Br bond undergoes homolytic fission in ultraviolet light, producing reactive Cl or Br free radicals. This initiation step of free-radical substitution is followed by propagation steps involving a chain reaction which regenerates the halogen-free radicals. Termination steps occur, when two free radicals combine.
The vast majority of carbon-containing compounds in widespread use have been made from crude oil. Crude oil is a mixture of hydrocarbons. Crude oil is separated into its different components by a process called fractional distillation. The separation of oil is based on the difference in the boiling point of the various fractions. The most important fractions of oil are liquefied petroleum gas, petrol or gasoline, naphtha, kerosene or paraffin, diesel or gas oil, mineral/lubricating oil, fuel oil, wax and grease, bitumen or tar.
The products of fractional distillation are often converted into other, even more useful hydrocarbons by a process called cracking. It increases the amount of gasoline and other economically important fractions, increases branching in chains, an important factor for petrol, and produces alkenes, an important feedstock for chemicals.
In a thermal cracking, the bonds are broken using a high temperature (400-900oC) and a high pressure (70 atmospheres). The high temperatures mean that the molecule breaks near the end of the chain, giving a high percentage of small alkenes such as ethene.
Catalytic cracking has several advantages over thermal cracking: it produces a higher proportion of branched alkanes, which burn more easily than straight-chain alkanes and are therefore an important component of petrol. In catalytic cracking, the bonds are broken using a high temperature (450oC, which is generally lower than in thermal cracking), a slight pressure (slightly greater than 1 atmosphere), and a zeolite catalyst.
The use of a lower temperature and pressure means it is cheaper. It produces a higher proportion of arenes which are valuable feedstock chemicals.