akcapr - 27-5-2010 at 14:34
My chemistry professor proposed a question to me today, and I cant realyl figure it out. We were talkign about Nitration of aromatics, and he told me
that if you take N,N-dimethyl aniline, and add conc. sulfuric acid, and then add a mix of HNO2 and H2so4 you will get one product-
meta-nitro-N,N-dimethyl aniline.
First if find it wierd that it adds nitro, and about the spicific order of first adding just h2so4, then the mix to gain the mono nitrate product. I
rationalize that it only adds once because the nitro group is e-withdrawing, but why does it add meta? N,N-dimethyl aniline is activiating, and should
add to o and p positions, and help/thoughts? thanks
[Edited on 27-5-2010 by akcapr]
psychokinetic - 27-5-2010 at 15:01
The H2SO4 is likely there to make an electrophile (NO2+), and will be 'reformed' at the end.
The meta bit I'm not sure about, sorry. Perhaps it is to do with the stereochemistry of the N(CH3)2
Perhaps also, the addition of conc H2SO4 at the beginning alters the ring's para carbon by sulfonation.
not_important - 27-5-2010 at 15:47
The acid converts the amino group into the corresponding substituted ammonium salt, and NR4<sup>(1+)</sup> is deactivating and meta
directing. You've got a + charge sitting out on the nitrogen, sucking at ring electrons.
It takes strongly acid conditions to insure that most to all the amine is in the salt form, adding H2SO4 first does so without any chance of the free
amine reacting.
One page 10 of the PDFF here http://www.smccd.edu/accounts/lawrencey/232_Fall09/16_Study_... is a nice graph of substitutes rank by their activating/deactivating and meta/o-p
direction; you'll see that +NR2 is about the same as NO2
akcapr - 27-5-2010 at 15:53
thanks i didnt think of that but it makes perfect sense!
[Edited on 27-5-2010 by akcapr]
psychokinetic - 28-5-2010 at 14:12
Ok that makes more sense.