NEET Chemistry Notes Hydrocarbons – Chemical Properties of Alkanes
Chemical Properties of Alkanes
Chemical Properties of Alkanes
Alkanes being saturated compounds undergo only substitution reactions at C—H bond.
Halogenation
Halogenation of alkanes is a free radical substitution reaction which occurs in the presence of UV light.
Reactivity towards halogenation is
Bromination and chlorination proceed approx same way. Iodination is reversible but fluorination of alkanes is too vigorous.
Mechanism of Halogenation
The mechanism of halogenation can be discussed in the following three steps:
Initiation The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat.
Combustion
Isomerisation
n-alkane on heating in the presence of AlCl3 (anhydrous) and HCl(g) isomerises to give branched chain alkanes.
Aromatisation
It is applicable to alkanes from hexane to decane, e.g.
Reaction with Steam
This method is used for industrial preparation of H2 gas
Pyrolysis or Cracking
Higher alkanes on heating to higher temperature decompose into lower alkanes.
Uses of Alkanes
Higher alkanes in the form of gasoline, kerosene oil, paraffin wax and lubricating oils are used.
The quality of petrol or gasoline is measured in terms of octane number while that of diesel is measured in terms of octane number. Octane number gives the percentage of iso-octane (by volume) in a mixture of n-heptane and iso-octane while octane number gives the percentage of octane (by volume) in a mixture of octane (C16H34) and a-methyl naphthalene.
Conformations
Spatial arrangements of atoms which can be converted into one another by rotation around a C— C single bond are called conformations or conformers or rotamers.
Ethane has three conformations:
- Eclipse Conformation
The conformations in which the hydrogen atoms attached to two carbons are as closed together as possible. This conformation is highly unstable and has maximum energy. - Staggered Conformation
The conformations in which hydrogens are as far apart as possible. It is highly stable and has minimum energy. - Skew Conformation
It is intermediate conformation of the above two conformations.
The energy difference between two extreme forms is of the order of 12.5 kj/mol which is very small, therefore eclipsed and staggered form cannot be isolated.