Hi. I am reviewing some practice questions and I'm having a hard time understanding how to know which isomer would form using electrophilic aromatic substitution. I followed Huckel's rule so I know all of the options are aromatic so I have decided that is not how you figure it out. Please help!

 

I attached a copy of the problem (the answer is A)

http://www.cem.msu.edu/~reusch/VirtualText/benzrx1.htm

 

Everything you need is on that page. Feel free to ask if you need it explained better.

The methyl group is more or less neutral with regard to electron donation/withdrawal. As such the first subsitution is likely to occur at the para position for steric reasons. Beyond this you must consider the effect of the highly electron withdrawing nitro group on the mechanism of the reaction. (If you're not convinced it's electron withdrawing, try delocalising a negative charge into it using 'curly arrows' from the para position).

 

Try pushing some charges round the ring and see which positions have the most electron density (i.e. no partial positive charge). This should tell you why you get the 1,3,5- compound.

 

Bonus points: at room temperature, what product would you predict? Why? To get the tri-nitro compound you need relatively high temperatures and long reaction times, as the electron withdrawing nitro group deactivates the ring to electrophilic attack.