BREAKING OF A COVALENT BOND
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 and medium of reaction.


(i) Homolytic fission
takes place when the two atoms (say A and B) are usually of similar electronegativity

(ii) Heterolytic fission
take 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 kcal/mole for C — Br bond in H₃C — CH₂— Br into H₃C —) than the heterolytic fission (e.g., 183 kcal/mole for C — Br bond in CH₃— CH₂— Br into CH₃— CH₂⁺and Br^-)
[1] Types of reactions



(i) Electrophilic addition reactions



Theses occur reaction in alkene and alkyne only where primary attack of electrophile takes place due to thick

in between two carbon atoms

polarity in alkene and alkyne is created by + I and – I effect of different groups than attacking reagent add with opposite charge.

(II) reaction is not an example of electrophilic addition reaction because no addition occurs across the double bond while reaction (I) is an example of electrophilic addition reaction where addition occurs.

Due to dense

and delocalisation of electrons aromatic compounds undergo substitution reaction rather than addition reaction.