HOMOLYTIC VS HETEROLYTIC BOND BREAKAGE

In this section we want to discuss the types of bond cleavage with respect to organic reactions. It should be noted that this aspect of chemistry is vital in understanding organic reactions.

According to Dalton's fifth atomic theory, chemical reactions involve separation and recombination of atom. therefore, all chemical reaction involves bond breakage (fission) and bond formation. There are basically two types of bond breakage (fission) namely;

• Homolytic bond breakage

• Heterolytic bond breakage

Homolytic Fission: In this type of bond fission, the two electrons of the nuclei or covalent bond are shared equally, each atom taking one electron as shown below.this type of bond fission results in formation of radicals. the properties of homolytic fission includes;

• Formation of radicals: radicals are formed during homolytic fission because the species involved are symetrical (i.e. non-polar molecule). e.g chlorine molecule,hydrogen molecule etc

• homolytic fission takes place in the presence of ultraviolet light or high temperature.for example, chlorination of methane. This reaction needs UV light or high temperature to break down the chlorine molecule into radicals (i.e to initiate the reaction).

• homolytic fission takes place in the presence of non-polar solvents (i.e it is favoured by non-polar solvents). This is because the species involved are non-polar, and according to the first rule of solubility which states that ''like dissolves like''. Therefore a polar solvent will dissolve a polar specie and non-polar will dissolve non-polar.

                 R - M   →    R^.   +   M^.

                                      radicals

NOTE: Radicals are species with odd number of electrons.

Heterolytic fission: In this type of bond fission, one of the atom takes the shared electrons. This type of fission takes place due to the presence of electronegative atom in a specie, this electronegative atom inductively withdraw electrons in a nuclei. One group of the atom that is formed is negative and the other is positive.The negative specie is known as nucleophile because it is attracted to a positively charged centre while the positive specie is known as electrophile because it is attracted to a negatively charged centre. therefore we can say that heterolytic fission involves the formation of ions.

           R - M        →    R+          +               M-

                       electrophile                  nucleophile

            

           Therefore we can define electrophile as electron loving specie and nucleophile as nucleus loving specie

                

             NOTE: a proper understanding  of inductive effect and electronegativity is needed. This will help students apply this aspect  of chemistry into use.

         Electronegativity: this is the intrinsic ability of an atom to attract electron to it self.it has been observed that fluorine is the most electronegative element due to its small size.

        Inductive effect: this can be defined as the unequal sharing of electrons in a nuclei due to the presence of a nearby electronegative element. we should also understand that inductive effect dies away with distance (i.e the power to inductively withdraw electrons reduces with distance).

        The properties of heterolytic fission includes;

• Formation of ions: ions are formed during heterolytic fission because the species involved are not symetrical (i.e. polar molecule). e.g hydrogen chlorine molecule,hydrogen fluoride molecule etc

• heterolytic fission takes place in the presence of polar solvents (i.e it is favoured by polar solvents). This is because the species involved are polar (i.e have dipole moment), and according to the first rule of solubility which states that ''like dissolves like''. Therefore a polar solvent will dissolve a polar specie and non-polar will dissolve non-polar.