Sciencemadness Discussion Board

nitration vs oxidation using HNO3

Magpie - 30-12-2009 at 13:45

In light of the recent bombing attempt over Detroit I was curious about PETN's synthesis. Wiki says that 98% nitric acid is used to nitrate pentaerythritol, a 5 carbon polyol with 4 alcoholic functional groups. I don't know what the actual nitrating specie is in this case. The product is a nitrate ester.

When nitrating benzene and analogs I believe nitrating acid, a mixture of HNO3 and H2SO4, is normally used. In this case the nitronium ion, generated as follows, does the ntration through an electrophillic attack. A nitro product is formed.

HNO3 + H2SO4 --> HSO4- + H2O + NO2+

When making oxalic and malonic acids I used a diol and 6M HNO3. In this case oxidation instead of nitration results.

My interest in this subject was piqued when I realized that I might inadvertantly make an explosive ester while trying to do an oxidation. Since I really know very little about this I'm hoping others can enlighten me. Does anyone else share my concern?


hissingnoise - 30-12-2009 at 15:02

Ethyleneglycol is nitrated with a mixed acid similar to that used used for glycerol and for sucrose.
*Dilute* HNO3 is safe enough for oxidising purposes. . .

S.C. Wack - 30-12-2009 at 15:28

Higher temps and water favors oxidation, lower temps and conc. HNO3 favors nitration.

JohnWW - 30-12-2009 at 15:39

This thread should really be in the "Energetic materials" forum. There is, however, a difference between "nitration", which refers to adding a -NO2 group bonded via the N atom to a C atom on usually an aromatic ring (and sometimes other appropriately activated C atoms, or where nucleophilic substitution is possible) by use of a nitrating reagent which generates nitronium cations, NO2+ (either HNO3+H2SO4, HNO3+HClO4, or N2O5); and formation of a covalent nitrate ester, which is the replacement of -OH groups on an alcohol or polyol (or in some cases phenols) with -O-NO2 groups bonded via an O atom. Because of the extra O atom, covalent nitrate esters should usually be more powerful explosives than comparable nitro-compounds.

12AX7 - 31-12-2009 at 03:16

NO3- is resonance stabilized, and essentially inert, so we can safely ignore it (i.e., sugar isn't burned by KNO3 in water).

Oxidation only occurs in acid, so it requires H+. HNO3 is strong, but not too strong (pKa = -1.4), so there will be some HNO3 present even in dilute acid. Any active groups (e.g., alcohol, carbonyl) will also be slightly protonated (pKb ~ 0), and therefore activated towards NO3-. Whichever the oxidant, a proton transfer and break of an N-O bond and off goes NO2 gas, leaving an OH on the substrate (which may dehydrate to a carbonyl, after more proton transfer). Because H2O, H+, NO3- and HNO3 are present, oxidation is favored under these conditions.

Concentrated acid, lacking available H2O (I specify "available" because sulfuric is often used to bind it), leads instead to self-protonation, H2NO3+, which dissociates to water and the nitronium ion, NO2+, a strong electrophile.

Now, if a strong nucleophile is present (e.g., phenol), it won't matter much that the concentration of NO2+ is very small. Hence, phenol is often nitrated in fairly mild conditions. Quite a bit of oxidation occurs, resulting in dark tars along with the desired product.

If a weak nucleophile is present, higher concentrations of NO2+ may be necessary for nitration (e.g., higher nitrobenzenes). However, oxidation occurs less, which must be due to the lower concentration of NO3- in nitrating acid (as far as I know, TNT production isn't renouned for producing tars).

Tim

Ozone - 31-12-2009 at 12:05

An interesting question (which was, is in fact, asked in the conclusion sections of several of the papers that I found while looking for a satisfactory answer). I was left with more questions, but here is the synopsis:

High acid concentration and low temperatures favor nitration. I have included a nice (short!) paper dealing with the preparation of carboxylic acids from alcohols using HNO3.

Whether it oxidizes or nitrates is also a function of the species involved. That is, straight nitric tends to oxidize while mixed nitrating acid tends to, well, nitrate. Mixed acid is enriched with the non-oxidative, electrophillic "nitronium" species while straight nitric is not (e.g. nucleophilic -ONO2??).

Apparently, the oxidation mechanism is poorly characterized (B. A. A. van Woezik and K. R. Westerterp. Chemical Engineering and Processing Volume 39, Issue 6, 1 November 2000, Pages 521-537), so an interesting study regarding the oxidation of TNT in nitrating acid (probably via a radical mechanism) is also attached.

Under highly acidic conditions with little water, expect (perhaps even a primary alcohol, and *certainly* 2° and 3° hydroxyls) the OH to protonate to give +OH2. Water is a nice leaving group--and leave, it will, to give the carbocation. This will either eliminate H+ to give the olefin, or it will rearrrange to a more stable (more tertiary) form and then either yield the olefin or some nucleophillic species might add to it (e.g. :OH2 or -ONO2).

Dehydration cannot yield a carbonyl (unless, perhaps, you remove one of the hydroxyls in a vic-diol to create the enol which could tautomerize to yield the aldehyde or ketone. Under these conditions, any aldehyde would either be immediately oxidized to yield the corresponding acid or "aldol-ed" to yield crud and water.)

Cheers,

O3

Attachment: [O]ofalcoholswithHNO3_Svetlakov_etal_2007.pdf (105kB)
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Attachment: [O]ofTNT_mixedacids_Qi-zhao_1982.pdf (397kB)
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smuv - 31-12-2009 at 13:05

I experienced the difference between nitration and oxidation in a preparation of ethyl nitrate. You may want to read this thread.

Essentially from nicodems posts and my observations at the time, formation of a nitrate ester is much faster than oxidation in mixed acids free of nitrogen oxides. In fact oxidation under non radical conditions proceeds through the nitrate ester. In nitric acid containing even small amounts of nitrogen oxides the radical oxidation begins which can be a very fast and self perpetuating reaction, which once begun becomes the major pathway trumping formation of the ester.

The easiest way to get around these problems is to add urea or urea nitrate along with the alcohol to be nitrated, which destroys nitrogen oxides in situ, and to perform the esterefication in the cold to minimized NOx formation, and to ensure that any radical reaction that does occur does not become a self sustaining runaway.

not_important - 31-12-2009 at 18:35

The only 'problem' with the urea method is the start of this thread
Quote:
When making oxalic and malonic acids I used a diol and 6M HNO3. In this case oxidation instead of nitration results.

My interest in this subject was piqued when I realized that I might inadvertantly make an explosive ester while trying to do an oxidation.


which is to say Magpie is after the very thing that urea inhibits :-)

Useful information to spread about, though. As you said, simple and effective if the esters are your goal.

vulture - 1-1-2010 at 02:41

I wonder, is it really the HNO3 that does the oxidizing or is it the NO2? High temperature and HNO3 essentially boils down to lots of NO2...

smuv - 1-1-2010 at 13:53

@ vulture, with NO2 present, as a free radical, it does H* abstraction from the alcohol therefore it would act as an oxadizing agent (though HNO3 will regenerate NO2 from the NO formed in the oxidation, regenerating the oxidizing agent). Without NO2, the HNO3 is the formal oxidizing agent.

@not_important: thanks for pointing that out. Essentially if no steps are taken to scavenge for NO2 there will be very little formation of the nitrate ester and most of the ester that transiently does form will probably undergo oxidation via H* abstraction forming R2C*ONO2 followed by O-N homolysis (or homolysis then H* abstraction). Therefore, any nitrate esters that form will only survive transiently.

[Edited on 1-1-2010 by smuv]

Ozone - 1-1-2010 at 17:14

"...oxidizing or is it the NO2?"*

Good point, it was bothering me, as well.

In the Zhao paper, it is mentioned that nitrosonium ion may be involved. The paper also describes the nitronium species as non-oxidative (which is difficult to believe when regarding such an electron deficient species). Methinks it has a hard time abstracting electrons from species in a highly protonating media (maybe why dilute solutions are regarded as more efficient oxidizers)?


*Ah, I was thinking +NO2 rather than NO2(g). Oops.

Cheers,

O3