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Rosco Bodine
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Quote: Originally posted by PHILOU Zrealone  | | Quote: Originally posted by Rosco Bodine |
In COPAE there is an ordering of substitution hierarchy activity given by Davis for promoters or inhibitors of nitration of the benzene ring, as
compared with hydrogen, that follows the scheme as follows with the greatest promoter being (phenolic) hydroxyl on the left with decreasing activity
towards promotion to the right, with all substituents on the right of Hydrogen actually serving to hinder substitution of a ring hydrogen, (or
anything else to the left of themselves?) more greatly. That same activity for hindering substitution of a ring Hydrogen would make such substitutents
vulnerable to preferential substitution themselves (as compared with a ring hydrogen), if I understand correctly.
-OH> -NH2> -CH3> -Cl> -H> -NO2> -SO2(OH)> -COOH
I am not certain the ordering which Davis gives is correct. This issue has come up before and it may be a perfect example of a "textbook error".
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The order given is simply a relative order for the speed of nitration in relation to benzene as being taken as unity of reference. So hydroxy is maybe
10E6 times faster than benzene and nitro 10E-3 times slower than benzene but combination of those groups depends on orienting effect (additive or
antagonist)...if additive the speed remains on the side of the more activating effect.
If for example -CO2H is taken as unity speed and -OH is 10000 x unity speed; then:
-speed of mononitration of phenol is 10000.
-speed of mononitration of benzoic acid is 1.
-speed of mononitration of salicylic acid (2-phenol-benzoic acid; thus additive effect) would be 10000+1 =10001
-speed of mononitration of 3-phenol-benzoic acid (antagonist effect) would be something between 10000 and 1 but much closer to 10000!
This order has nothing to do with substitution ability!
Otherwise you would:
-never be able to get dinitro or trinitrobenzene from benzene and you would stop at nitrobenzene, substituting endlessly the NO2 by NO2 instead of H!
-never be able to get metanitrobenzoic acid or 3.5-dinitrobenzoic acid from benzoic acid nitration and the only result (what is never obtained in
normal nitration process) would be nitrobenzene.
[Edited on 31-3-2015 by PHILOU Zrealone] |
You think Davis is completely wrong? I don't, the only issue I have is the specific order may not be exactly right for all cases.
Respectfully I disagree this ordering for groups as promoters or inhibitors of entering nitro groups has nothing to do with substitution ability
because yes the speed of reaction which you reference decreases to near zero as further substitution is inhibited for the same reaction condition.
This explains why for trinitro benzene each entering nitro group is more difficult to introduce, and why for nitration of toluene and for other
aromatic nitrations the same general rule applies. The conditions of nitration can require being made more intense to introduce the second nitro, and
still more intense for the third, likewise for a fourth. This is not written in stone with no exceptions, but is a general rule.
[Edited on 31-3-2015 by Rosco Bodine]
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PHILOU Zrealone
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Why does nitrous acid allow substitution of a carboxylic acid function in ortho or para of a phenol?
If you take a look at keton chemistry with nitrous acid or nitrosating agents, you will notice the following:
1°)Butanone turns into butandione monoxime:
CH3-C(=O)-CH2-CH3 + HONO --> CH3-C(=O)-CH(-N=O)-CH3 + H2O
CH3-C(=O)-CH(-N=O)-CH3 <-=> CH3-C(=O)-C(=N-OH)-CH3
or
CH3-CO-CH2-CH3 + HONO --> CH3-CO-CH(-N=O)-CH3 + H2O
CH3-CO-CH(-N=O)-CH3 <-=> CH3-CO-C(=NOH)-CH3
2°)Pentan-2.4-dione turns into pentan-2.3.4-trione-3-oxime:
CH3-C(=O)-CH2-C(=O)-CH3 + HONO --> CH3-C(=O)-CH(-N=O)-C(=O)-CH3 + H2O
CH3-C(=O)-CH(-N=O)-C(=O)-CH3 <-=> CH3-C(=O)-C(=N-OH)-C(=O)-CH3
or
CH3-CO-CH2-CO-CH3 + HONO --> CH3-CO-CH(-N=O)-CO-CH3 + H2O
CH3-CO-CH(-N=O)-CO-CH3 <-=> CH3-CO-C(=NOH)-CO-CH3
So hydrogen atoms in alfa of a ketonic group are favourized (the stability of the enol form CH3-C(-OH)=CH-CH3 explains the attack of the nitrosation
on the viccinal CH2 and not on the terminal CH3 what would have lead to a aldo-ceton oxime)!
Hydrogen atoms in alfa of two ketonic groups are even more favourized especially if the final compound generate an extended resonance of pi bonds (sp2
carbons) this is the case with the 3 successive keto groups.
What has it to do with phenol and salicylic acid?
-Phenol is easily nitrosated in para-position...because phenol is a discrete form of keton...phenol contains "enol" what means it is the enol form of
a keton  .
Phenol is cyclo(-C(-OH)=CH-CH=CH-CH=CH-) and cyclo(-CO-CH2-CH=CH-CH=CH-) or cyclo(-CO-CH=CH-CH2-CH=CH-) are the derived ketonic forms. Have you
noticed the CH2 in ortho position or in para position?
Nitrosation will thus happen in ortho or para position. Usually in para but if position is already taken by a non labile group, then it goes in ortho.
-Salicylic acid contains the sequence HO-C=C-CO2H (rest of the aromatic ring left aside for clarity). Note that -CO2H is -CO-OH...
The associated ketonic form will contain the sequence O=C-CH-CO-OH and the CH is thus between two "keto" groups.
Nitrosation will occure at that place very easily.
O=C-CH-CO-OH --> O=C-C(-N=O)-CO-OH
Alfa keto-carboxylic acid usually easily lose their carboxyl group into CO2 (by temperature increase), this effect is facilitated by the nitrosation
(lowering of the decarboxylation temperature).
Malonic acid forms acetic acid. HO2C-CH2-CO2H --> CH3-CO2H + CO2
Pyruvic acid forms ethanal. CH3-CO-CO2H --> CH3-CH=O + CO2
In our case O=C-C(-N=O)-CO-OH --> O=C-CH(-N=O) + CO2
Then the enol form is formed again HO-C=C-N=O to form 2-nitrosophenol!
[Edited on 31-3-2015 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Rosco Bodine
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The nitroso group should have been included but was not provided in the sequence which Davis described ( more or less correctly  ) and I think it should probably be inserted there just to the right of the NO2
group and before the SO2(OH)
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PHILOU Zrealone
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Mood: Bis-diazo-dinitro-hydroquinonic
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| Quote: Originally posted by Rosco Bodine |
You think Davis is completely wrong? I don't, the only issue I have is the specific order may not be exactly right for all cases.
Respectfully I disagree this ordering for groups as promoters or inhibitors of entering nitro groups has nothing to do with substitution ability
because yes the speed of reaction which you reference decreases to near zero as further substitution is inhibited for the same reaction condition.
This explains why for trinitro benzene each entering nitro group is more difficult to introduce, and why for nitration of toluene and for other
aromatic nitrations the same general rule applies. The conditions of nitration can require being made more intense to introduce the second nitro, and
still more intense for the third, likewise for a fourth. This is not written in stone with no exceptions, but is a general rule.
[Edited on 31-3-2015 by Rosco Bodine] |
No!
I agree with COPAE/Davis...so we both agree on that  .
But I do not agree with what you wrote:
"That same activity for hindering substitution of a ring Hydrogen would make such substitutents vulnerable to preferential substitution themselves (as
compared with a ring hydrogen), if I understand correctly."
[Edited on 31-3-2015 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Rosco Bodine
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Registered: 29-9-2004
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Mood: analytical
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For the nitration of phenolsulfonic acid the sulfonic group is easily replaced by a nitro, and similarly will occur replacing a carboxyl with a
nitroso for a nitrosophenol. The nitroso may follow somewhat the same scheme as a nitro, almost as if it were an "incomplete" or partially formed
nitro.
Interestingly for the sulfonation of phenol the first entering sulfonic group occurrs easily, but the second sulfonic group is harder to introduce due
to the inhibiting effect of the first. And with salicylic acid the introduction of a second sulfonic does not occur at all due to the combined effect
of a first sulfonic made even greater by a carboxyl, the inhibiting effect on sulfonation so complete that sulfonation stops at the monosulfonic acid
derivative of salicylic acid.
Yet as you have pointed out, the carboxyl of salicylic acid sulfonate is very easily and preferentially itself vulnerable to substitution by
replacement with a nitroso.
Thinking further about the sequence of Davis and my suggesting that nitroso should be inserted there would in this case actually put it between the
SO2(OH) and the COOH as a ranking in that hierarchy offered by Davis.
The nitroso itself seems to be easily even preferentially replaced by a nitro which I think is probably more of an oxidation to a nitro in that case,
since the "substitution" by a nitro is in part already accomplished. I think that is a "special case exception" which is an exception to the
simplified linear hierarchy that is a general rule shown by Davis. I think there are other exceptions which can occur for combined effect of more
than one substituent already present on the ring so the simplified linear ranking shown by Davis does not hold as an absolute rule, for which there
are exceptions.
| Quote: Originally posted by PHILOU Zrealone |
But I do not agree with what you wrote:
"That same activity for hindering substitution of a ring Hydrogen would make such substitutents vulnerable to preferential substitution themselves (as
compared with a ring hydrogen), if I understand correctly."
[Edited on 31-3-2015 by PHILOU Zrealone] |
The only plausible explanation for what I have observed / guessed to be probably occurring which would give the illusion such substitutions are
preferential, would involve unspecified abstract and algebraic ring resonance effects or "steric influences" at work. Probably elves make it happen
that way so that what is observed remains mysterious
[Edited on 31-3-2015 by Rosco Bodine]
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