Ajantis
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Noob questions about nitration
Why Trinitrotoluene can have no more than 3 nitro groups and benzene can have 6 nitro groups? why many nitro-compounds have NO2 group and others can
have ONO2 groups? why carbon cannot take more than 1 nitro group? How can we determine the number of nitro groups that a hydrocarbon can acquire?
Probably these are very stupid questions but i can't find answers...
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Ramiel
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This is a roundabout way of answering your question, but how do you think acids can have wildly negative pH's?
What do you expect a 'superacid' to do to methane ( CH<sub>4</sub> )?
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Ajantis
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I cannot understand your answer... acids can have negative pH when the concentration is greater than 1 mol/l... I know that with CH4 we can obtain
nitrometane CH3NO2... but why CH3NO2 and not CH3NO3? why only a nitro group even if carbon can have 4 covalent bonds? The presence of more than 1 bond
makes the molecule too instable to exist for appreciable time?
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cyclonite4
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| Quote: | Originally posted by Ajantis
why many nitro-compounds have NO2 group and others can have ONO2 groups? |
Well, NO2 groups and ONO2 groups are two different things. Nitration of aromatics typically yields a compound with nitro (NO2) groups (e.g.
Trinitrotoluene). Nitration of alcohols, glycols, etc (compounds with hydroxyl groups) will result in the nitrate ester (which is your ONO2 group) e.g
Nitroglycerin.
CH3NO3 is better known as methyl nitrate (a.k.a CH3ONO2), and is the result of nitration of methanol. Also, nitromethane can be further nitrated (to
have four nitro groups) to tetranitromethane (there is a thread at RS about this). IIRC tetranitromethane is very toxic, and thus considered
unsuitable for the home experimenter.
\"It is dangerous to be right, when your government is wrong.\" - Voltaire
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vulture
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The answer is, because HNO3 does not deliver a NO3 species, but a NO2+.
HNO3 + H+ <---> H2O + NO2+
So, in order to get a nitrate group in your final molecule, you need a hydroxyl group which the NO2+ will attack.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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neutrino
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A translation for the layman:
Nitrating mixes produce NO<sub>2</sub><sup>+</sup> ions, which then react with organics:
C-H + NO<sub>2</sub><sup>+</sup> -> C-NO<sub>2</sub> + H<sup>+</sup>
C-OH + NO<sub>2</sub><sup>+</sup> -> C-ONO<sub>2</sub> + H<sup>+</sup>
edit: formatting
[Edited on 3-7-2005 by neutrino]
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sparkgap
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And so you know, some of us like to call NO<sub>2</sub><sup>+</sup> the "nitronium ion". 
The explanation thus far is correct: -ONO<sub>2</sub> is what you get if the original compound has an -OH group; but this is for alcohols
only; when your original compound is aromatic, like phenol or toluene, it is (usually) preferred that the
NO<sub>2</sub><sup>+</sup> substitute for one of the -H atoms in the benzene ring.
sparky (~_~)
"What's UTFSE? I keep hearing about it, but I can't be arsed to search for the answer..."
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Ramiel
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I was reffering to acids in general. Note well that certain superacids can 'acidify' CH<sub>4</sub> to
CH<sub>5</sub>.
What I meant to illustrate by this is that rules like "there are eight electrons in the bonding shell of an atom" or "carbon has four
bonds" are untrue simplifications. toluene can have 5 nitro groups (or 7 or 11! ) the reason nitriation 'stops' at three nitro groups is mainly because three is the most stable you can get (although this is a
simplification!   ).
I was just trying to inspire a bit of diverse reading, sorry if the reply was obtuse or unhelpful. Keep educating yourself, and the answers will
become clear.
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Ajantis
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Thanks to all, now it is all clear! i didn't know that CH5 can exist, what is the structure of this molecule?
acetylene can be nitrated to C2(NO2)2?
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sparkgap
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Unfortunately, no one (AFAIK) has been able to synthesize dinitroacetylene. 
IIRC, CH<sub>5</sub><sup>+</sup> has a
trigonal bipyramidal structure. Try to visualize two tetrahedra joined at their faces; that's what it might resemble.
sparky (°_°)
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12AX7
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Makes sense. About the lowest energy form you can get for five, as compared to octahedral for six. Reminds me, how is NH4+ coordinated? What makes
the proton stick to the butt end of the NH3(+0)?
And for that matter, what makes NH3 and H2O kinked...
Dragging back on topic, I'll second that: what forms CH5+?
Tim
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sparkgap
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1) Ammonium ions are tetrahedral, Tim.
2) It's the "lone pairs" that kink water and ammonia.
3) Ramiel mentioned upthread how to make CH<sub>5</sub><sup>+</sup>...  I will also have to say now that it really doesn't last long...
sparky (~_~)
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12AX7
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| Quote: | Originally posted by sparkgap
3) Ramiel mentioned upthread how to make CH<sub>5</sub><sup>+</sup>... I will also have to say now that it really doesn't last long...
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Any specific reaction I mean?
Tim
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Ramiel
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None that one could <html><a href="http://pubs.acs.org/cgi-bin/sample.cgi/joceah/2005/70/i07/html/jo040285o.html">realistically
</a></html> achieve, Tim. 
[Edit - munted html linkage]
[Edited on 5-7-2005 by Ramiel]
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