UCLEOPHILES AND ELECTROPHILES
The organic reactions proceed by the attack of certain active species on the reactant molecule (generally called substrate). These active species are called attacking species. They may be classified into nucleophiles and electrophiles.
These are neutral or negatively charged species which are capable of donating a pair of electrons to some other molecules. These species act as Lewis bases and attack the electron deficient centres ( electrophilic centres) of the organic molecule. Some examples are:
Neutral electrophiles: BF3, AlCl3, S03.
A free radical may be defined as an atom or group of atoms which contains an unpaired electron. Free radicals are formed by homolytic cleavage of a covalent bond. For example, in the presence of U.V. light, chlorine gives chlorine free radicals.
If the unpaired electron is on carbon atom then they are called alkyl free radicals. Alkyl free radicals are classified as primary (1 °), secondary (2°) or tertiary (3°) after the class of the carbon atom carrying unpaired electron
The order of stability of free radicals is 3° > 2° > 1°.
CLEAVAGE OR FISSION OF COVALENT BONDS
Organic reactions involve the breaking of bonds in the reacting molecules and formation of new bonds to give product molecules. The breaking of covalent bond can take place in two ways depending upon the nature of bond, nature of attacking reagent and conditions of the reaction.
1. Homolytic Cleavage or Homolytic Fission
In this type of cleavage each fragment formed as the result of cleavage gets one electron from the shared pair of electrons.
This type of cleavage results in the formation of species with odd or unpaired electrons. Such species are called free radicals. Some examples of homolytic cleavage are given below:
Homolytic cleavage is favoured by conditions such as non-polar nature of bond, high temperature or presence of high energy radiations.
2. Heterolytic Cleavage or Heterolytic Fission
In this type of cleavage both the shared electrons of the covalent bond are taken away by one of the fragments (while the other fragments do not get any of the shared electrons).
This type of cleavage results in the formation of charge species. Heterolytic cleavage is favoured by the condition such as polar nature of bond and presence of polar solvent