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Tuesday, 14 February 2017

Hydrocarbon III


Features of Halogenations:-
(i) The reactivity of Halogens:- F2 > Cl2 > Br2 > I2.
(ii) The rate of replacement of Hydrogens of alkanes is:   3° > 2° > 1°

Mechanism:- Halogenations reaction take place by free radical mechanism.
The reaction proceeds in the following steps:
Initiation
(i) Chain initiation step:-
(ii) Chain Propagation step:-
(iii) Chain Termination step:-
Nitration:-

  • The reaction takes places by free radicals mechanism at high temp (4500C).
  • At high temp C—C bond is also broken so that mixture of nitro alkanes is obtained.


Sulphonation:- Replacement of hydrogen atom of alkane by – SO3group.



Aromatization:-

This method is also called dehydrogenation or hydroforming. Similarly, heptane gives toluene, 
n-Octane give o-xylene and 2, methyl heptane give m-xylene.

Thermal decomposition or Pyrolysis or cracking or Fragmentation:- 

When higher alkanes are heated at high temp (about 700-800k) in the presence of alumina or silica catalysts, the alkanes break down to lower alkanes and alkenes.

Action of steam:- Catalyst: nickel, alumina Al2O3


This reaction is used for the industrial preparation of hydrogen from natural gas.
Isomerisation
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