Free redical halogenation takes place in presence of light or at high temperature (above `500^(@)C`). Formation of halogen free radical intermediate takes place in first step called chain initiation step.
`Cl_(2)overset(hv)rarr2Cl^(**)`
This reaction is mainly given by those compounds which have at least one hydrogen atom present at `sp^(3)` -hybrid carbon. Reactivity of `sp^(3)`-hybrid carbon depends on the reactivity of reaction intermediate.
The relative rate of formation of alkyl radicals by a chlorine radical is:
`underset("(5)")("Tertiary")gtunderset("(3.8)")("Secondary")gtunderset("(1)")("Primary")`
`"Percentage yield of the product"=("Relative amount"xx100)/("Sum of relative amounts")`
`"Relative amount"="Number of hydrogen atoms on the respective carbon"xx"relative reactivity".`
NBS(N-bromosuccinimide) is used for bromination at allylic and benzylic carbon, whereas `Br_(2)//hv` gives bromination at benzylic, allylic and allyl carbons.
Select most reactive compound for chlorination in presence of light:

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