Question on Nitro Groups

[Hexon]

EK says they're 'electron withdrawing' which i find confusing; i mean i understand that they're located on the more electronegative side of the Periodic table, but usually nitrogen has a paif of electrons, even if it is a tertiary nitrogen.

How can the Nitrogen still be 'electron withdrawing' if they're not positively charged and are still hanging onto that pair of electrons?
is it because of the influence of the oxygens in the nitro group? and does the same apply to amines? are amines also 'electron withdrawing'?

Thanks guys and gals.

---

Members don't see this ad.

 

[gettheleadout]

There exists a resonance form allowing the R-N single bond to become a double bond by drawing electrons from the adjacent pi system and the one N=O double bond to bump the pi electrons out to the oxygen.

 

[Hexon]

There exists a resonance form allowing the R-N single bond to become a double bond by drawing electrons from the adjacent pi system and the one N=O double bond to bump the pi electrons out to the oxygen.

thanks for the explanation;

so let me get this straight; for amines, the nitrogen is generally basic (electron donating), but in the case of nitros, since the adjoining atoms are more electronegative than the nitrogen, it leaves the nitrogen in Nitro groups, feeling rather electronnegative?

 

[gettheleadout]

It's not primarily the electronegativity of the oxygens; an RNF2 substituent would be more inductively withdrawing than an amine because the F's draw electron density away from the N while the H's don't, but RNF2 would still be far less withdrawing than a nitro because even though F is more electronegative than O, the possibility for lone pair movement for resonance is not present in RNF2 the way it is in a nitro.

 

[N1st]

thanks for the explanation;

so let me get this straight; for amines, the nitrogen is generally basic (electron donating), but in the case of nitros, since the adjoining atoms are more electronegative than the nitrogen, it leaves the nitrogen in Nitro groups, feeling rather electronnegative?

The N in the nitro group is positive, so it withdraws electrons merely because of induction, and as said above, it also is electron withdrawing by resonance. Thus it is a really good electron withdrawing group.

 

[gettheleadout]

The N in the nitro group is positive, so it withdraws electrons merely because of induction, and as said above, it also is electron withdrawing by resonance. Thus it is a really good electron withdrawing group.

This brings up a good point that I missed from your OP, OP, in that the nitrogen does have a partial positive charge in the nitro resonance hybrid.

 

[Hexon]

i see, thanks guys.
So does this mean the inclusion of a nitro group in a molecule renders the molecule more acidic, due to the electron withdrawing nature of the nitro group?

 

[gettheleadout]

Yes, but the extent to which it does depends on the molecule. If the nitro is a substituent on an aromatic system then the resonance effect will accompany the inductive effect to make it highly electron-withdrawing, but if it's on a molecule without an accessible pi system then its influence will be limited to its inductive effect. In the latter case the nitro's placement within the molecule relative to the acidic proton will also determine the extent to which induction influences acidity.

 

[Hexon]

thanks for the 'splanation!