By the mechanism of a chemical reaction, we mean the actual series of discrete steps which are involved in the transformation of reactants into products.
Now since organic compounds are covalent, organic reactions involve:
(i) the breaking of the old, and
(ii) making of the new covalent bond.
(a) Breaking of a Covalent Bond.
The first step, i.e. breaking of a covalent bond between two atoms can take place mainly in two alternative ways, viz. homolytic and heterolytic fissions depending upon the relative electronegativity of the two concerned atoms, e.g.
1. Homolytic fission takes place when the two atoms (say A and B) are usually of similar electronegativity.
\(\mathrm{A}: \mathrm{B} \longrightarrow \underset{\text { Pree radicals }}{\mathrm{A} \cdot+.B}\)
Heterolytic fission takes place when the two atoms (A and B) are of different electronegativities. It may again take place in two different ways.
(a) When A is more electronegative than B
(b) When B is more electronegative than A
It is important to note that homolytic fission requires much less energy (e.g., 67.2 \(\mathrm{kcal} / \mathrm{mole}\) for \(\mathrm{C}-\mathrm{Br}\) bond in \(\mathrm{H}_3 \mathrm{C}-\mathrm{CH}_2-\mathrm{Br}\) into \(\mathrm{H}_3 \mathrm{C}-\dot{\mathrm{C}} \mathrm{H}_2\) and \(B{\dot{ r}} )\) than the heterolytic fission (e.g, 183 \(\mathrm{kcal} /\) mole for \(\mathrm{C}-\mathrm{Br}\) bond in \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{Br}\) into \(\mathrm{CH}_3-\mathrm{CH}_2^{+}\) and \(\mathrm{Br}^{-})\).
1. Homolytic fission or homolysis. As clear from the above representation, in homolytic bond fission one electron of the bonding pair goes with each of the departing atom or group resulting in two electrically neutral fragments or atoms generally known as free radicals*, e.g,
\(\mathrm{H}_3 \mathrm{C}: \mathrm{X} \longrightarrow \mathrm{H}_3 \mathrm{C} .+. \mathrm{X}\)
Thus a free radical may be defined as the atom or group of atoms having a single, odd or unpaired electron. Now since the homolytic fission always results in the formation of free radicals, the reactions involving such (homolytic) fission are known as free radical reactions and are said to proceed via a free radical mechanism.
2. Heterolytic fission or heterolysis. In this type of fission the electrons pair forming the covalent bond goes to a single atom or group and thus electrically charged fragments (ions) are formed. Thus the reactions involving heterolytic fission are known as ionic reactions and are said to proceed via ionic (polar) mechanism. The heterolytic fission of the covalent bond can occur in either of the following two ways. Let us examplify our cases by the cleavage of the \(\mathrm{C}-\mathrm{X}\) bond in \(\mathrm{CH}_3-\mathrm{X}\) compound.
(i) When the electrons pair between C and X leaves the organic group and remains with the departing substituent X and thus the latter attains a negative charge (due to gain of electrons) while the former attains a positive charge (due to loss of electrons).
Such organic species which has only six paired electrons (ie., three pairs of electrons) and a positive charge at its carbon centre is known as carbonium lon. Carbonium ions are generally symbolised as \(\overset {\oplus}{R}\). Reactions in by carbonium ion mechanism. which carbonium ions are formed as intermediate are said to proceed
(ii) When the electrons pair between C and X remains with the organic group and the substituent X is devoid of its bonding electron and thus gets positive charge while the organic group (which has gained electrons) is negatively charged.
Such organic species which has eight paired electrons (i.e., four pairs of electrons) and a negative e charge on one of its carbon centre is known as carbanion. Carbanions are generally symbolized as \(\overset {⊖}{R}\). Reactions in which carbanions are formed as intermediate are said to proceed by a carbanion mechanism.
It is important to note that carbon in carbonium ion, carbanion and free radical is trivalent and as observed earlier the number of electrons in the valence shell of carbon of these species is six, eight and seven respectively.
Thus the natural tendency of carbon to have eight electrons in the valence shell to attain stability accounts for the high reactivity of these species.
(b) Bond Formation
The first step of bond fission is followed by the next step of bond formation for which there are again two possibilities, viz. (a) either it may occur in a next and separate step or (b) it may occur simultaneously with the step of bond fission. For example, an ion : OH- may react in substitution reaction with the molecule CH3Cl in either of the following two ways.
So in this case, the ion \(\overset {+}{C}H_3\), an intermediate species with a short life-time, is obtained.
So in this case, the intermediate species has a zero life time and therefore is not formed. In such cases reaction proceeds through an intremediate 'transitory state' in which the old bond is cleaved and new bond is formed at the same time (simultaneously). Such transitory state can best be represented as below.
