CHAPT 4.1 : Free radical substitution Flashcards Preview

CHEM III organic chemistry > CHAPT 4.1 : Free radical substitution > Flashcards

Flashcards in CHAPT 4.1 : Free radical substitution Deck (8)
Loading flashcards...
1

how are free radicals formed

by homolysis of a covalent bond

2

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

3

what is free radical substitution

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

4

compare halogenation reactants

useful: Cl2 , Br2
violent: F2
slow : I2

5

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

6

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

7

when a bond breaks..

2 half arrow arrow must come out

8

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