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:
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.
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.
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.
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. + HCl||Propagation|
|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. ® HCl||Termination|
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 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 + HCl||Propagation|
|Step 3||.CH3 + Cl2 ® CH3Cl + Cl.||Propagation|
|Step 4||.CH3 + Cl.® CH3Cl||Termination|
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.