pls. explain me when cyclic intermediates are formed?what is the condtion of formation of cyclic intermediates?

usually cyclic intermediates are formed in two situations :

1. when rxns take place on an aromatic ring

2. when a cyclic ring is formed from a linear chain

as you may know that intermediate formation generally takes place only when the intermediates formed are stable...

looking at the aromatic intermediate you can easily predict whether it is stable or not...

generally the rings formed from linear chain are stable (also depend on the stability of the ring)...

this, perhaps explains your question...

Cyclic intermediates can be formed during addition across carbon carbon double bond. The most familiar example is the formation of cyclic bromonium ion during addition of bromine across C=C. The other examples are hydroxylation with cold dil KMnO4 where cyclic mangnate ester is formed,  with osmium tetra oxide formation of cyclic osmate ester, , formation of epoxide, ozonoid can also be regardes as examples of cyclic intermediates.

The pericyclic reactions are other example of formation of cyclic intermediate.

In both the above examples the common thing is C=C; hence we may say that reactions of C=C can have cyclic intermediates.