Synthesis of chloroalkanes

FREE-RADICAL SUBSTITION= alkanes do not react with chlorine at room temp or in the dark. They do in UV light, however a mixture of alkane and chlorine will react at room temp, forming HCL and a mixture of chlorinated alkane.

FREE RADICAL= is a species which contains an odd nom. electrons with one electron unpaired.
they can be single atoms or groups of atoms.
radicals are formed when a covalent bond breaks with an equal splitting of the bonding pair of electrons (fission)

STEPS OF FREE-RADICAL MECHANISM:

Initiation-

Cl2-->2Cl*
the uv light provides the energy to start the reaction by splitting some chloride molecules into radicals. process occurs first because the Cl-Cl bond in chlorine is weaker than the C-H bond in methane

Propagation-

Cl* + CH4-->*CH3 + HCL
*CH3 + Cl2-->CH3Cl Cl*
in each step a radical is used and a new radical formed-chain reaction. its exothermic so the chain reaction may lead to an explosion.
CH4 + Cl2-->CH3Cl + HCl

Termination-

when 2 radicals combine, they form a stable molecule and the sequence of reactions stop, unpaired electron in the radicals pair up to form a covalent bond. 2 possible termination steps are:
Cl* + *CH3-->CH3Cl
*CH3 + *CH3-->CH3CH3

Such termination steps can lead to trace amounts of impurities e.g.ethane in the final product
Cl* + Cl*-->Cl2
it is possible but unlikely because the chlorine radicals collide with enough energy to separate again immediately

Further substition-

reaction of chlorine radical with methane extracts a hydrogen radical to form HCL. chloromethane still contains 3 H atoms, so further propagation is possible, leading to dichloromethane (CH2Cl2), trichloromethane (CHCl3) and tetrachloromethane (CCl4).

the propagation steps for CH2Cl2=
Cl* + CH3Cl--> CH2Cl + HCl
*CH2Cl + Cl2-->CH2Cl2 + Cl*

	Created by s_phie5 about 10 years ago

AS-level Chemistry (Haloalkanes) Note on Synthesis of chloroalkanes, created by s_phie5 on 04/11/2014.

Synthesis of chloroalkanes