CHAPT 4.1 : Free radical substitution Flashcards Preview

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Flashcards in CHAPT 4.1 : Free radical substitution Deck (8)
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how are free radicals formed

by homolysis of a covalent bond


stability of free radicals and its factor

methyl< primary< secondary < tert
the lower the bond dissociation energy for a C-H bond, the more stable the resulting carbon radical


what is free radical substitution

in the presence of light or heat, alkanes react with halogens, both in gaseous phase to form alkyl halides


compare halogenation reactants

useful: Cl2 , Br2
violent: F2
slow : I2


With an alkane that has more than one type of hydrogen atom, a mixture of alkyl halide may result .
what kind of alkyl halide would bromination and chlorination form

Bromination is selective towards the formation of product derived from the most stable radicals ( tert radicals)
- it is usually the tert carbon hydrogen that is replaced by Br
chlorination can replace any hydrogen atoms until CCL4 forms


mechanism of free radical substitution

1. initiation : homolytic fission of chlorine molecule by heat or UV light into 2 chlorine radical

2. propagation ( 2 steps) :
- formation of methane radical
- formation of alkyl chloride and chlorine radical

3. termination ( 3 steps) : 2 radicals react with each other to get rid of all radicals
halogen radical +halogen radical
alkane radical + alkane radical
alkane radical + halogen radical


when a bond breaks..

2 half arrow arrow must come out


explain propagation step

- abstraction of a H atom from methane molecule by chlorine radical to form hydrogen chloride and methyl radical
- methyl radical reacts with a chlorine molecule to form cloromethane and chlorine radical