Part 1. ALKANES and the PETROCHEMICAL INDUSTRY
Doc Brown's Advanced A Level Organic Chemistry Revision Notes
1.3 Modification of alkanes by cracking, isomerisation and reforming
What is cracking? Why is cracking important? What are the products of cracking? What are the products of cracking used for? What is reforming? Why is reforming important? What are the products of reforming? What are the products of reforming used for?
Petrol (gasoline) contains a mixture of hydrocarbons, with 5 to 10 carbon atoms, but the products of fractional distillation do not completely suit the desired mixture for a fuel. There are too many larger hydrocarbons, mainly alkanes and no alkenes for polymer production. See basic discussion for the need of cracking see CRACKING - a problem of supply and demand, other products. This discussion does include isomerisation and reforming which are described in detail below.
Cracking is essentially the thermal decomposition of hydrocarbons such as alkanes into molecules of smaller carbon number, namely lower alkanes as chemical feedstock for other processes and alkenes for polymer production.
(a) Steam cracking (thermal cracking) is done at high temperatures e.g. 900oC and high pressure, with no catalyst. Naphtha (C6-C10) and kerosene (C10 - C16) are the chemical feedstocks and the vaporised hydrocarbons are exposed to the high temperature for just a short time.
(b) Catalytic cracking involves passing a fraction like gas oil (C14 - C20 alkanes) over a zeolite catalyst at 500oC. The reactor pressure is slightly above normal.
PLEASE READ my basic notes on CRACKING - a problem of supply and demand, other products
Included are several equation examples of cracking alkanes into alkenes, lower alkanes and hydrogen.
I won't repeat the structural formula equations here, but add I've added more above on the reaction conditions, products and skeletal formula equations below.
e.g. cracking to produce alkenes (for polymer production) and lower alkanes for fuels to reduce the octane number of petrol, and note the 'theoretical' variety of products possible!
(i) hexane ===> ethene + butane/methylpropane or propene + propane
(ii) octane ===> propane + pent-1-ene or propene + pentane or but-1-ene + hydrogen or other combinations
(iii) The production of more branched alkanes from decane C10H22, of various branched alkanes C7H16 and propene C3H6
, ,,, +
Reaction mechanisms for alkane cracking reactions
Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes (maybe needed for UK A Level ?)
Ionic mechanism for cracking hydrocarbons (definitely NOT needed for UK A level chemistry)
Converting linear alkane molecules into branched alkane molecules is an example of an isomerisation reaction because the reactant and products have the same molecular formula. Branched alkanes increase the octane number of petrol fuels. The chemical feedstock is usually C4 to C6 alkanes.
Typical reaction conditions: Pt/Al2O3 at 150oC
Examples of isomerisation reactions
(i) butane ===> methylpropane (all C4H10)
(ii) pentane ===> methylbutane (2-methylbutane) or dimethylpropane (2,2-dimethylpropane, numbers not strictly needed)
(iii) hexane ===> 2-methylpentane or 3-methylpentane
(v) heptane ===> 3-ethylpentane or 2,2-dimethylpentane or 2,3-dimethylpentane
or or or
By changing the reaction conditions you can vary, and to some extent change, the ratio of products to select those most desired from an isomerisation reaction.
Converting linear alkane molecules into cyclic hydrocarbons (cycloalkanes and aromatic molecules - have a benzene ring).
The products have the same number of carbon atoms as the reactants, but less hydrogen atoms and a ring structure.
Naphtha is the chemical feedstock (C6 - C10 alkanes) and passed over a Pt/Al2O3 catalyst at 500oC. The hydrogen formed is recycled to minimise 'coking' (thermal decomposition to give a carbon deposit on the catalyst).
(i) hexane ===> cyclohexane ===> benzene
then + 3H2
This reforming reaction probably goes through the dehydrogenation sequence ?
hexane ==> cyclohexane ==> cyclohexene ==> cyclohexa-1,3-diene ==> benzene
(ii) heptane or 2-methylhexane or 3-methylhexane ===> methylcyclohexane ===> methylbenzene
If asked to write the full equation don't forget the hydrogens! and check whether structural formulae or skeletal formulae are required.
Extra note on an example of an aromatic disproportionation reaction
Disproportionation is said to occur, when a reactant is transformed into two or more dissimilar products.
2 + or or (mixture of three isomeric products)
This is not a redox disproportionation reaction, but it does involve two identical molecules reacting together to give two different products with no other reactant or product involved. The reaction is a way of producing a mixture of 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene from methylbenzene. If you need less methylbenzene and more benzene or dimethylbenzenes, this is the way to do it.
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