Monday, January 28, 2008

IIT JEE Revision Alkanes Chemical Reactions

Chemical properties or reactions

1. Substitution reactions of alkanes
Involves the replacement of one or more atoms of hydrogen by the halogen atoms.

b) Nitration
Involves the replacement of a hydrogen atom of alkane with -NO2 group.
Nitration of higher alkanes (hexane or higher) is carried out by boiling alkane with nitric acid.

Nitration of lower alkanes can be carried out in vapour phase by heating alkane and nitric acid to very high temperatures in the range of 723-773 K. At such higher temperature C-C bonds of alkane breakes, and hence a mixture of nitroalkanes may obtained.

For example in the reaction between ethane and nitric acid at very high temperature, both nitroethane and nitromethane(due to breaking of one C-C bond in ethane) are obtained as mixture.

c) sulphonation
This involves the replacement of a hydrogen atom by -SO3H group.
Fuming sulphuric acid reacts with alkane at higher temperature.

Higher alkanes (hexane and above) only give this reaction.

Lower alkanes particularly methane, ethane do not give this reaction.

2. Oxidation
a) Complete combustion
b) Incomplete combustion
c) Controlled oxidation

3. Action of steam
This reaction is used for the production of hydrogen from natural gas.

On passing a mixture of steam and methane over heated nickel(over alumina Al2O3) catalyst at 1273 K, methane gets oxidized to carbon monoxide and all hydrogen atoms get released.

4. Isomerisation

branched isomers of alkanes are obtained by heating alkanes with anhydrous aluminium chloride (AlCl3) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere.

5. Aromatization

Alkanes containing 6 or more carbon atoms get converted to aromatic compounds, when heated at about 773 K under higher pressures of the order of 10-20 atm in the presence of catalysts - like oxides of chromium, molybdenum or vanadium supported on alumina gel.

6. Thermal decomposition or fragmentation
When higher alkanes are heated to high temperatures (700-800 K) in the presence of alumna or silica catalysts, they break down to lower alkanes and alkenes.

If methane is heated to high temperature up to 1500 K, it breaks down to its elements (carbon and hydrogen)

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