Radical substitution

A simple example of substitution is the reaction between methane and chlorine in the presence of UV light, with generation of hydrocloric acid and a mixture of halogenated products. In the example only the chloromethane formation is described even in reality the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms, depending on the reagents quantity and reaction time.

CH₄ + Cl₂ CH₃Cl + HCl

Background and actual explanation

The mechanism

1. Chain initiation
The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.

Cl₂2Cl

Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.

2. Chain propagation reactions

These are the reactions which keep the chain continuity:

CH₄  + ClCH₃  +  HCl

CH₃ +  Cl₂CH₃Cl  +  Cl

3. Chain termination reactions
These are reactions which remove free radicals from the system without replacing them by new ones.

2ClCl₂

CH₃ +  ClCH₃Cl

CH₃ +  CH₃CH₃CH₃

PROPOSED MECHANISM

1. Initiation Step:

The reaction begins with an initiation step, which is the separation of the halogen (X₂) into two radicals by the addition of UV light.

In proposed theory a radical is an atom with a single unpaired electron, but in order to characterize its comportment, the magnetic moment of this electron is very important.

Having in mind the new concept of covalent bound, where a simple covalent bound represent only a coupling of two electron magnetic moments, in the first step, this linkage between chloride atoms is broken and two halogen radicals with opposite orientation of electron magnetic moments are obtained.

After generation, due to the thermal agitation of free radicals, the directions of magnetic moments of radicals are arbitrary orientated.

2. Propagation Steps:

Methane will react with chlorine radical, and a molecule of hydrochloric acid is released. In reaction another chlorine radical is regenerated, so this reaction can, in theory, go on forever as long as there are reagents.

The methane radical then reacts with another one of the chlorine molecules to form the product and a new chloride radical.

Termination Steps:

Side reactions that can stop the chain reaction are called termination steps. These termination steps involve the destruction of the free-radical intermediates, typically by two of them coming together.

A comparative analysis of proposed mechanism with existent one can be made in case of allylic or benzylic substitution.

For allylic case, the general reaction is:

The actual explanation of mechanism is available in every medium text about organic chemistry and is not worth to be reminded here.

In proposed explanation the stability of allyl or benzyl radical is related to magnetic interaction of electron magnetic moments.

In the allyl radical, the allylic carbon, remain with a free electron magnetic moment generated in the propagation chain step. This magnetic moment, after a small spatial rotation, can interact with the magnetic moments of double bound and the stability of free radical is increased as is presented in fig. 2

If a marked isotopic carbon is used, after reaction a mixture of compound a) and b) must be obtained due to the position of chlorination at 1 and 3 carbon atom.