Costanza9
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Organic Reaction with benzene
Hey, i'm kind of confused on what is happening in this reaction. I know that N20 is meta directing, but I don't know if that is relevant to this
reaction. I would greatly appreciate if anyone could explain the steps in this reaction.
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Ozone
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Ah, I remember these.
Assume that the nitro group on nitrobenzene, Ar-NO2 (not N2O), is a very strong meta director (for extra credit ask why this is).
Then ask:
1. What happens to the activity of the ring (see extra credit above) when a nitro froup is added to it?
2. what is the usual problem with Friedel Crafts alkylation?
3. What sort of director do you expect the methyl group(s) to be?
Good luck,
O3
[Edited on 13-4-2010 by Ozone]
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Nicodem
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The answer is "none". As far as I know nobody ever succeeded in alkylating nitrobenzene using the AlCl3 catalysed Friedel-Crafts reaction (it is even
commonly used as solvent for this reaction). Special conditions are required for ethylation (see DOI: 10.1021/jo00299a051) while methylation under
these conditions does not work even on less deactivated substrates (see DOI: 10.1021/jo00025a037). So the product of the first reaction is
nitrobenzene (the recovered starting material). Its bromination (with heating) gives 1-bromo-3-nitrobenzene, which is however not depicted among the
answer options. Hence the only correct answer is not among the given options.
Interestingly though, even if the dimethylation of nitrobenzene would somehow be possible, the main dimethylated product would be
3,4-dimethyl-1-nitrobenzene which should monobrominate to give mainly 1-bromo-2,3-dimethyl-5-nitrobenzene. Yet, this is not even among the answer
options, therefore additionally confirming the only correct answer is "none" (unless the person who composed the question is clueless about the basics
of the aromatic electrophilic substitutions, which is quite likely if this person is a university professor).
Edit: Never mind my opinion that the dimethylation of nitrobenzene would give the 3,4-isomer as the main product. The ethylation described in the
above refereed paper gives a wide distribution of mono- and diethylated nitrobenzenes. The 3,4-diethylated product expected from my interpretation of
the electronic effects is only the second prevalent dialkylated isomer after the 2,5-isomer, but the differences are small (it appears the inductive
effect of the nitro group is not as important as I thought, and the steric effects of the ethyl and the strong mesomeric effects of the nitro are more
important, thus giving almost 50% more of the 2,5-isomer and some 25% less 3,5-isomer - but possibly within measurement error!).
Just a few reminders for our newcomers: The pedagogic and homework related threads go to the Beginnings section (where I'm moving this). We also have
a "Short questions..." thread, so that you can use that one if all you need is short answers (for example, one letter answers like A, B, C... qualify
as short). When posting pictures, it is expected you check for them are not being too wide, so that they don't distort the page frames of those who
use normal screen widths.
[Edited on 13/4/2010 by Nicodem]
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Nicodem
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Thread Moved
13-4-2010 at 07:50 |
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