THE HALOGENATION OF ALKANES AND CYCLOALKANES

Topics: Atom, Carbon, Alkane Pages: 9 (1939 words) Published: February 24, 2014
Alkanes

The reaction between alkanes and fluorine

This reaction is explosive even in the cold and dark, and you tend to get carbon and hydrogen fluoride produced. It is of no particular interest. For example:

The reaction between alkanes and iodine

Iodine doesn't react with the alkanes to any extent - at least, under normal lab conditions.

The reactions between alkanes and chlorine or bromine

There is no reaction in the dark.

In the presence of a flame, the reactions are rather like the fluorine one - producing a mixture of carbon and the hydrogen halide. The violence of the reaction drops considerably as you go from fluorine to chlorine to bromine.

The interesting reactions happen in the presence of ultra-violet light (sunlight will do). These are photochemical reactions, and happen at room temperature.

We'll look at the reactions with chlorine. The reactions with bromine are similar, but rather slower.

Methane and chlorine

Substitution reactions happen in which hydrogen atoms in the methane are replaced one at a time by chlorine atoms. You end up with a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane. What is cracking?

Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.

The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporised before cracking.

There isn't any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C15H32 might be:

Or, showing more clearly what happens to the various atoms and bonds:

This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline).

Catalytic cracking

Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge. They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners.

The alkane is brought into contact with the catalyst at a temperature of about 500°C and moderately low pressures.

The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol (gasoline). It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene.

For UK A level (and equivalent) purposes, you aren't expected to know how the catalyst works, but you may be expected to know that it involves an ionic intermediate.

Note: You should check your syllabus to find out exactly what you need to know. If you are studying a UK-based syllabus and haven't got one, follow this link. Use the BACK button on your browser to return quickly to this page.

The zeolite catalyst has sites which can remove a hydrogen from an alkane together with the two electrons which bound it to the carbon. That leaves the carbon atom with a positive charge. Ions like this are called carbonium ions (or carbocations). Reorganisation of these leads to the various products of the reaction.

Note: If you are interested in other examples of catalysis in the petrochemical industry, you should follow this link. It will lead you to information on...
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