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NEET Chemistry Notes Hydrocarbons – Chemical Properties of Alkanes

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
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 1
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.
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 2

Combustion

NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 3
Isomerisation

n-alkane on heating in the presence of AlCl3 (anhydrous) and HCl(g) isomerises to give branched chain alkanes.
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 4
Aromatisation

It is applicable to alkanes from hexane to decane, e.g.
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 5

Reaction with Steam
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 6
This method is used for industrial preparation of H2 gas

Pyrolysis or Cracking

Higher alkanes on heating to higher temperature decompose into lower alkanes.
NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 7

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.
    NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 8
    NEET Chemistry Notes Hydrocarbons - Chemical Properties of Alkanes 9
    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.

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