[Steenrod]

Which of o-chloronitrobenzene(let's call it I) and 1-chloro-2,4,6-trinitrobenzene(let's call it II) is more reactive towards nucleophilic substitution?

My attempt: The next dipole moment of (II) seems to be much lesser than that of (I) due to the electron-withdrawing effect of -NO_2 group. So, perhaps (II) will show more reactivity towards nucleophilic substitution.

Am I right or wrong?

Thanks!

[discodermolide]

Firstly compound II should be named 2-chloro-1,3,5-trinitrobenzene.
Both are certainly reactive towards substitution. I don't think the dipole moment has much to do with it.
Actually I would favour compound I as being more reactive because the chlorine in II is somewhat sterically crowded due to the two ortho nitro groups.

[james_a]

We're talking about nucleophilic aromatic substitution here.

What's the rate determining step? Attack of the nucleophile at the aromatic ring to give a negatively charged species.

Which of those two molecules will be able to stabilize negative charge better? Therein lies your answer.

[NotExactly]

Firstly compound II should be named 2-chloro-1,3,5-trinitrobenzene.
Both are certainly reactive towards substitution. I don't think the dipole moment has much to do with it.
Actually I would favour compound I as being more reactive because the chlorine in II is somewhat sterically crowded due to the two ortho nitro groups.

It's clear that II is more electrophilic than I for electronic reasons and that I is more stericially hindered than II so there would be some competition between these two factors.  From experience, I'd favor II for being more reactive (I'd imagine if you let it sit in methanol long enough at RT you'd get some methoxy displacement of chloride).  If you were using a large nucleophile, like tButoxide or something along those lines, maybe the steric factors would outweigh the electronic factors

[orgopete]

It is II. This reaction is a nucleophilic aromatic substitution reaction. The rate limiting step is the formation of the a ionic intermediate (Meisenheimer complex). The nitro groups stabilize this intermediate. However, I believe the success of attack is due to the electron withdrawal of the nitro groups, thus even groups like a CF3 undergo nucleophilic aromatic substitution, though at slower rates.

It is helpful to draw the intermediate so you don't confuse instances of ortho versus meta attack if thinking simply as electron withdrawing properties. I am reluctant to argue the so called "resonance stabilization" should be the reason for an initial attack.