When haloalkanes reach the stratosphere, they are exposed to intense UV radiation, and undergo homolytic fission to form ozone-depleting chlorine radicals. \[CH_3Cl + hv\Rightarrow CH_3^\bullet + Cl^\bullet\]
More common under normal lab conditions
\(C\)-\(H\) bond is polar
Can break to form a negative halide ion, and positive carbocation
Sometimes caused by a negatively charged substance reacting with the positively polarised carbon atom
Causes a substitution reaction