Question

How can I explain the monochlorination of alkanes?

Answer 1

Because each `Cl_2` molecule reacts to form a monoclorinated alkane plus `HCl`.

You would expect to see only di-chlorinated compounds, because chlorine is a di-atomic molecule, but:

The hydrogen atom to be replaced binds to one of the chlorine atoms, while the other one takes its place.

This is not an easy, spontaneous reaction, and it takes heat and pressure. It goes like this:

`CH_4 + Cl_2 ->CH_3Cl+HCl`

In order to help the reaction work, the resulting products have to be taken out of the reaction vat constantly.
If enough `CH_3Cl` is left (and enough chlorine) there may be a second (`CH_2Cl_2`) or third chlorination, but this can be reasonably controlled by pressure and temperature.
A purifying step (by destilling for instance) is always necessary.