thunderfvck - 18-2-2004 at 09:30
Hello!
I had my first test today in organic chemistry II and everytime I finish a test I always need to know how I did on the test before I get it back. Well
there was one question that I was kind of taken back by.
I was asked to rate in order of increasing reaction rate the following brominations on the following substrates:
nitrobenzene
1,3,5,7-cyclooctatriene
toluene
The order of increasing rate (my version) was:
1,3,5,7-cyclooctatriene
nitrobenzene
toluene
Toluene being the most easily brominated because the methyl group is weakly activating the aromatic ring.
Nitrobenzene being the second because the nitro group is strongly deactivating the ring.
The 1,3,5,7-cyclooctatriene being the most unreactive because it is purely cyclic/aromatic. I was thinking it'd be analogous to brominating
benzene, which happens at a slow rate (needs a catalyst).
But I am sure about the nitro being middle. Since it is so strongly deactivated, maybe it should be the least reactive...damn.
What do you guys think?
Triene?
Turel - 18-2-2004 at 10:34
Are you sure you don't mean 1,3,5,7-cyclooctatetraene?
I would rate them as:
nitrobenzene < cyclooctatetraene < toluene.
Toluene brominates the easiest because of the activating ortho/para methyl function, and lack of steric hindrance.
Nitrobenzene is deactivated and also meta directing, so bromination is resisted much more than with toluene.
Cyclyoctatetraene is not aromatic, so standard free radical bromination would occur at the double bonds, which happens more readily than with
nitrobenzene, but I would assume less readily than with toluene, since toluene loves to be substituted.
-T
thunderfvck - 18-2-2004 at 11:06
Yeah, I meant tetraene, sorry.
OHHH.
4n+2 = 8, n isn't an integer! DAMN, you're right. I just lost some points. Mother fucker!
guaguanco - 18-2-2004 at 13:44
Interesting. Without any books around, I would have guessed:
cyclooctatetraene > toluene > nitrobenzene
since non-aromatic polyenes are extremely reactive to halogens
Toluene is reactive to bromine, different conditions will favor aromatic substitution or benzylic substitution
t_Pyro - 21-2-2004 at 11:37
I think this is a weird question. Cyclooctatetraene would undergo bromination by the halonium mechanism, adding to the double bond, and it does not
require any further reagents. The bromination of the benzene ring, on the other hand, is via electophlic aromatic substitution, so some lewis acid
would be required, like FeCl<sub>3</sub>. The conditions for the bromination being different, you can't really compare the
bromination of cyclooctatetraene to nitrobenzene or toulene. Among toulene and nitrobenzene however, toulene is more reactive, since the methyl group
weakly activates the ring, while the nitro group strongly de-activates it.
thunderfvck - 21-2-2004 at 11:45
Yes, the cyclooctatetraene is not aromatic, so it will easily react. It was a trick.
thunderfvck - 23-2-2004 at 20:35
My teacher was nice, he only took one mark out of five for that mistake. 