1) Halogenation of an alkane (or cycloalkane) with equivalent hydrogens:
H 3 CC
CH 3
CH 3
CH 3 H 3 CC
CH 3
CH 3
++Cl 2 CH 2 Cl H Cl
heat
or light
Neopentane Neopentyl chloride
(excess)
- Bromine is generally less reactive toward alkanes than chlorine ⇒ bromination is
more regio-selective.
10.4 CHLORINATION OF METHANE: MECHANISM OF REACTION
- Several important experimental observations about halogenation reactions:
CH 4 + Cl 2 CH 3 Cl + HCl (+ CH 2 Cl 2 , CHCl 3 , and CCl 4 )
1) The reaction is promoted by heat or light.
i) At room temperature methane and chlorine do not react at a perceptible rate as
long as the mixture is kept away from light.
ii) Methane and chlorine do react at room temperature if the gaseous reaction
mixture is irradiated with UV light.
iii) Methane and chlorine do react in the dark if the gaseous reaction mixture is
heated to temperatures greater than 100°C.
2) The light-promoted reaction is highly efficient.
10.4A A MECHANISM FOR THE HALOGENATION REACTION:
- The chlorination (halogenation) reaction takes place by a radical mechanism.
- The first step is the fragmentation of a chlorine molecule, by heat or light, into two
chlorine atoms.
- The frequency of light that promotes the chlorination of methane is a frequency
