Bond Fission, Radicals and Photochemical Reactions

Bond breaking is also known as bond fission.The picture below represents a diatomic molecule with a shared pair of electrons between the atoms forming a single covalent bond. You can investigate the different ways in which this bond can break:

Break the covalent bond...
   

Click here for a summary of bond fission.

Heterolytic fission results in the formation of two different chemical species in the sense that one is a cation and the other an anion. Homolytic fission results in two electrically uncharged radicals.

Radicals have an unpaired electron...

Radicals are particles that have an unpaired electron. They may be single atoms (e.g. chlorine radical, Cl.) or groups of covalently bonded atoms (e.g. methyl radical,.CH3).

Some radicals, called biradicals, have two unpaired electrons, for example, .O. (1s22s22p4) and .O2.. (A more advanced theory of chemical bonding is needed to explain the biradical structure of dioxygen.)

Because of the unpaired electron, radicals can be very reactive. However, there are some that are relatively stable and behave somewhat like ordinary molecules. An example is nitrogen monoxide.

Some radicals are illustrated below:

Heterolytic fission versus Homolytic fission...

Reaction conditions are important...

The hydrogen chloride molecule (H-Cl) is polar owing to the greater electronegativity of the chlorine atom. Heterolytic fission is more common where a chemical bond is already polar. Hydrogen chloride is highly soluble in water and becomes fully ionised; it is a strong acid. Solvents with polar molecules favour heterolytic fission.

HCl(aq) + H2O(l) ® H3O+(aq) + Cl-(aq)

Homolyic fission is favoured by non-polar solvents, or by gaseous conditions, and the presence of visible or ultraviolet light.


Reactions involving homolytic fission...

The reaction between Hydrogen and Chlorine

H2(g) + Cl2(g) ® 2HCl(g)

A mixture of hydrogen and chlorine gases kept in the dark reacts only very slowly if at all. Now subject it to a pulse of ultraviolet light and an explosive reaction takes place.

Having determined the equation for a chemical reaction and made observations such as the conditions under which it takes place, the nature of any intermediates, and gained information about its rate, the chemist is interested to understand the route the reaction takes in going from reactants to products. This is described by the mechanism of the reaction. The mechanism of a reaction is proposed based on the evidence available to the chemist. New discoveries about the reaction may require the suggested mechanism to be changed. A mechanism may involve a single step or several steps.

Initiation, propagation, termination...

The reaction of hydrogen and chlorine is a typical photochemical chain reaction involving radicals. The reaction involves three stages: initiation, propagation, and termination. It requires photons of light only to get it started (Initiation of the reaction) after which it rapidly reaches completion. These photons, absorbed by a few of the chlorine molecules, cause the Cl-Cl bonds to break homolytically.

Step 1  Cl2 + hn ® 2Cl.Initiation  

The reaction now has to keep going, or propagate itself. The next two steps in the mechanism involve propagation. A propagation reaction involves the loss of a radical, but also the formation of another radical. Two propagation steps are required otherwise the reaction would come to a stop before completion.

Step 2  Cl. + H2 ® H. + HClPropagation  
Step 3  H. + Cl2 ® HCl + Cl.Propagation  

The propagation steps repeat over and over in a chain reaction. Radicals also come together forming covalent bond in Termination steps. Here is one of them.

Step 4  H. + Cl. ® HClTermination  

Curly arrows...

Curly arrows are used to show the movements of electrons in chemical reaction, particulary for organic reactions. They show where the electrons begin and where they finish. A curly arrow with a single head shows the movement of a single electron and one with a double head the movement of a pair of eletrons.

Here is an example of the use of the single-headed arrow for Step 2 in the above mechanism:

The Chlorination of Methane

The chlorination of methane is another photochemical radical chain reaction. The reaction is a substitution reaction; a hydrogen atom of methane is swapped for a chlorine atom.

CH4 + Cl2 ® CH3Cl + HCl

Here is the mechanism for the reaction...

Step 1  Cl2 + hn ® 2Cl.Initiation  
Step 2  CH4 + Cl. ® .CH3 + HClPropagation  
Step 3  .CH3 + Cl2 ® CH3Cl + Cl.Propagation  
Step 4  .CH3 + Cl.® CH3ClTermination  

In the reaction of methane and chlorine, chloromethane (CH3Cl) is not the only organic product. A mixture of organic products (also CH2Cl2, CHCl3, CCl4) is obtained, corresponding to the substitution of each of the hydrogen atoms of methane. The formation of these arises from steps 2 and 3 above repeating. The formation of the disubstituted derivative, dichloromethane (CH2Cl2) is shown:

CH3Cl + Cl. ® .CH2Cl + HCl
.CH2Cl + Cl2 ® CH2Cl2 + Cl.

Finally, to be more precise about the name of the mechanism for this reaction: it is a radical substitution reaction.