[Berettagtz]

Can someone tell me why does the nitration of dimethylamine benzene procede meta instead of ortho para?

I thought -N(CH3)2 was an ortho para director.

Thanks in advance.

[macman104]

-N(CH₃)₂ is ortho/para directing.  However, I suppose it's possible that the amine could be become protonated.  If that happens, then it goes from being ortho/para to meta directing.

[Berettagtz]

So, once the amine gets protonated, it will then become an electron withrdrawing group.  And that will make it a meta director. 
is that correct?

[macman104]

Yes, I am only making a guess.  Assuming the nitration does proceed with the meta isomer being formed like you said and not the ortho/para products (which would have been my initial guess), that would be my rationalization.