Alkanes Paraffins
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Reactivity and Selectivity
Reactivity and Selectivity
In its attack on alkanes, the bromine atom is much more selective than chlorine atom (with relative rate factors of 1600 : 82 : 1 as compared to 5.0 : 3.8 : 1). It is also much less reactive than chlorine atom (only 1/375 000 as reactive toward methane) This is just one example of a general relationship : in a set of similar reactions the less reactive the reagent, the more selective it is in its attack.
Mechanism of free radical substitution reaction

Puzzle. Show rate of halogenation in following compound.

Finally, how does the mechanism of chlorination occurs ? In fact, a small amount of oxygen slow down the reaction for a period of time, which depends upon the amount of oxygen, after which the reaction proceeds normally?
Oxygen is believed to react with a methyl radical to form a new free radical :

The CH3OO• radical is much less reactive than the CH3• radical, and can do little to continue the chain. By combining with a methyl radical, one oxygen molecule breaks a chain, and thus prevents the formation of thousands of molecules of methyl chloride; this, of course, slows down the reaction tremendously. After all the oxygen molecules present have combined with methyl radicals, the reaction is free to proceed at its normal rate.
A substance that slow down or stops a reaction even though present in small amount is called inhibitor. The period of time during which inhibition lasts, and after which the reaction proceeds normally, is called the inhibition period. Inhibition by a relatively small amount of an added material is quite characteristic of chain reactions of any type, and is often one of the clues that first leads us to suspect that we are dealing with a chain reaction

(ii) Reed’s Reaction or Chlorosulphonation of alkanes

When alkanes reacts with sulphurdioxide and chlorine in ultraviolet medium it forms chlorosulphonated product reaction follows free radical mechanism by the steps of propogation and termination. If the reaction can undergo expansion by reaction with alkali it forms synthetic utility products like detergents (RSO3Na).


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