Question

why does nitrogen dioxide show its effect only at ortho- and para- position and not at meta-position?

Answer 1

I assume that you mean the nitration of benzene with an electron-donating group (such as -OH or -CH3)? This is normally carried out by reaction with concentrated HNO3 and H2SO4, which form the nitronium ion, NO2+.
The electron donating group (on carbon 1 of the benzene ring) causes carbons 2, 4 and 6 to become slightly more negative, and so more susceptible to attack by electrophiles such as NO2+.

Answer 2

my question is : why no2 doesn't occupy 3,5 position

Answer 3

Is your question related to nitration of benzene of the activating effect of nitro group on benzene?

Answer 4

nitro group is a deactivator

Answer 5

Its a deactivator so this nuetralizes it

Answer 6

ok you have to do a SIGMA COMPLEX look up examples but basically the Resonance of the meta is the most stable for ELECTRON withdrawing groups and the RESONANCE of the ortho n para are most stable WITH ELECTRON DONATING groups, the "+" charge lands or doesnt land on those specific groups, hope this helps