free radical substitution of alkanes

what is a free radical/radical

any species with an unpaired electron

shown as a dot

highly reactive species - react with alkanes

reactions involving free radicals are called

free radical substitution

stages for reaction between any alkane and chlorine/bromine

3 stages:

• initiation

• propagation

• termination

describe the initiation between any alkane and chlorine/bromine

shine ultraviolet light onto reaction mix

energy causes single covalent bond between two chlorine/bromine atoms to break

one electron now goes to each chlorine/bromine atom - these are now chlorine/bromine radicals. this is called homolytic fission

what is homolytic fission

in an initiation reaction, one electron now goes to each chlorine/bromine atom - these are now chlorine/bromine radicals

describe the STEP 1 propagation between any alkane and chlorine/bromine

a chlorine/bromine radical reacts with an alkane molecule

the radical takes a hydrogen atom plus one electron from the alkane

describe the STEP 2 propagation between any alkane and chlorine/bromine

the alkane radical now reacts with the bromine/chlorine molecule producing a bromo/chloroalkane and a chlorine or bromine radical

propagation steps 1 and 2 form a chain reaction - the radical formed in step 2 can react with the alkane in step 1 - this continues until termination

describe termination between any alkane and chlorine/bromine

two free radicals react together to form a molecule with no unpaired electrons

problem wit free radical substitution

can form a wide range of side products, including different isomers - need to separate products at end