UV radiation provides initial energy for reaction to take
place
E.g// Methane +
Bromine
CH4 (g) + Br2 (l) --> CH3Br (g) + HBr (g)
Forms
bromomethane
Substitution reaction
Hydrogen atom int he alkane has been substituted by a halogen
atom
Mechanism for bromination of
Alkanes
Free Radical
Substitution
Step 1: INITIATION
Covalent bond in a bromine molecule is broken by HOMOLYCTIC
FISSION
Each bromine atom takes one electron from the pair in the covalent
bond
This forms two highly reactive bromine
radicals
Br-Br --> Br' + Br'
Step 2:
PROPAGATION
Annotations:
These 2 propagation steps can keep reacting over and over in a chain reaction. Similar to that reaction with ozone.
Propagation step 1: CH4 + Br' --> CH3' + HBr
Propagation step 2: CH3' + Br2 --> CH3Br +
Br'
Each methyl radical reacts with another
bromine molecule, forming bromomethane
(CH3Br) and a new bromine radical (Br')
Bromine radical reacts with a C-H bond in the
methane to form a methyl radical (CH3') and a
molecule fo hydrogen brominde
Step 3:
TERMINATION
Annotations:
There can be a number of different termination reactions between two radicals
Two radicals collide, forming a molecule with all electrons paired
Br' + Br' --> Br2
CH3' + CH3' --> C2H6
CH3' + Br' --> CH3Br
Limitations of radical substitution in organic synthesis
Bromomethane CH3Br was formed from radical substitution. Another bromine radical can
collide with a bromomethane molecule, substituting it further. This can carry on in a kind of
chain reaction.
Results in a mixture of compounds, E.g/ CH3Br --> CH2Br2 --> CHBr3 --> CBr4
Substitution at different points in the carbon chain
methane = only
one
monobromo
compound
possible as one
carbon
Ethane = only on
monosubstituted product
C2H2Br possible
However pentane could have three monosubstituted isomers