Enkidu
March 7th, 2007, 12:49 AM
Before anyone asks, yes, I have UTFSE and consulted Urbanski. The reason I started a new thread is that the PETN thread is 20+ pages and contains a wide variety of unsupported and often off-topic information. No other threads were appropriate for this discussion.
*Everything that follows, I've only dreamt.*
In my recent dreams, I've been using Sam Barro's method (which I believe BLASTER posted a long time ago in Sam's forum before it disappeared?) for the synthesis of PETN. His method is the easiest for me, since I've got 70% HNO3 on hand. Unfortunately, his method seems to have worked too well, because I have got 130% yields for my last two batches. I noticed in the PETN thread someone mentioned a synth that had "102% yield." The high yields of most users employing this method lead me to believe that they may be experiencing the same problem I have been, except that they may not notice it.
First, a recap of my proceedure. My method followed Sam's method fairly closely. (Not to unduly credit Sam; I say it's his method because I printed Sam's page for my hard copy.) I used a slightly different nitrating mixture. My nitrating mixture consisted of 34mL of ~70% HNO3 and 25mL of 93% H2SO4. I used an acetone/dry ice bath to cool my acids and keep the reaction cool (-15C to 0C). I have a magnetic stirrer, so the PE/nitrating solution was well mixed during the entire synth. When I recrystallized my PETN in acetone, virtually all of it dissolved. When I poured the acetone/PETN solution into cold H2O, it precipitated instantly. This is step where I first became concerned. Not all the precipitate immediately sank to the the bottom of the flask. Some did sink immediately, and formed a dense layer at the bottom; but some sank slowly and formed a very "airy" layer of flakes or long crystals; and some even floated. After Vacuum filtration, some crystals on the filter paper seemed to be very fine and others larger. My feeling is that the larger, majority crystals are PETN.
Here's what I think could be causing the impossible yields:
1) My pentaerythritol might not be pure. However, even if it isn't, I do not believe that would increase my yields but rather decrease them. I don't have a way to easily test the purity of my PE; its melting point is too high to be easily tested. I hate thin layer chromatography, and beside that, I don't have any Rf values. Without the Rf values and the specs of my PE, TLC is just about worthless, because most PE is slightly impure. (I'm reasonably sure I've got PE because I have dreamt of detonating something synthesized from this PE.)
2) The H2SO4 might not have been neutralized properly with sodium bicarbonate solution. I had to run to the store and my faithful lab techs (heh) were supposed to neutralize it. Then, when I dumped the acidic PETN solution into the acetone, I'd get whatever acetone + dilute sulfuric acid produces, if anything. I should have paid more attention in O. Chem.
3) I might have some of what BLASTER calls 'sulfosesters' or what Urbanski calls 'sulphuric esters.' A sulfuric ester is, on the molecular level, much heavier than a nitric ester. This is the cause that I believe is most plausible. Urbanski gives sulfuric esters but little indirect mention on page 122 in Vol. I in reference to nitrating aromatic compounds with a solution of ethyl nitrate in sulphuric acid. He says
Ropuszyliski [174] recently investigated the action of a solution of nitroglycerine in sulphuric acid more fully and inferred that decomposition of nitroglycerine by sulphuric acid to form a nitronium ion was probably taking place by stages:
C3H5(ONO2)3 + H2SO4 <-> C3H5(ONO2)3H+ + HSO4- (77)
C3H5(ONO2)3H+ <-> C3H5(OH)(ONO2)2 + NO2+ (77a)
With an excess of sulphuric acid, glycerine dinitrate was decomposed to give further nitronium ions, etc. Finally a double ester nitric and sulphuric of glycerol was formed:
C3H5(OH)2(ONO2) + H2SO4 -> C3H5(OH)(ONO2)(OSO3H) + H2O (77b)
Now, I suppose, following what Urbanski wrote, PETN would decompose in this manner:
C5H8(ONO2)4 + H2SO4 <-> C5H8(ONO2)4H+ + HSO4- (1)
C5H8(ONO2)4H+ + HSO4- <-> C5H8(OH)(NO2)3 + NO2+ + HSO4- (1a)
With an excess of sulfuric acid, Pentaerythritol Trinitrate could be decomposed to give further nitronium ions, etc. Finally a double ester nitric and sulfuric of pentaerythritol would be formed:
C5H8(OH)3(ONO2) + H2SO4 -> C5H8(OH)2(ONO2)(OSO3H) + H2O (1b)
That is what, I suppose, would happen if PETN were to be put in concentrated sulfuric acid. That's the reason the PETN solution must be neutralized.
We must keep in mind what happens to the nitrating solution and the nitration itself.
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O (2)
However, the alcohol is simultaneously being dehydrated by the hydrogen sulfate and converted to a nitric ester by the nitronium ion. Therefore, we end up with sulfuric acid again.
C5H8(OH)4 + 4NO2+ + 4HSO4- + 4H2O -> C5H8(ONO2)4 + 4H2SO4 + 8H2O (3)
Doesn't (3) look similar to (1a)?
If nobody knows what's going on right off the bat, here are a couple questions that may lead us in the right direction.
Why does BLASTER's method specify heat? Is it because with heat the equilibrium shifts to the left?
What are the exact conditions that lead to sulfuric esters in this PETN synth? Obviously, there is an excess of sulfuric acid. Is the time in the hot water bath too long or too short?
What governs equations 1, 1a, and 1b? What governs the reaction between a nitric ester and sulfuric ester in general?
What products are of eq. 1, 1a, and 1b are soluble in acetone or water? I guess that they are and are not, respectively.
What concentration H2SO4 is required to decompose a nitric ester?
I'll leave any speculating about acetone + hydronium + hydrogen sulfate to you all. Keep in mind that the H2SO4 is dilute. Like I said, I should have paid more attention in O. Chem.
If anybody does know what is going on, please also post a method of purification of the PETN along with your analysis of the problem.
Next weekend, I'll do the synth myself from start to finish, and maybe I'll post some pictures. If I still get huge yields, we can rule out cause number two. Thanks in advance for any intelligent replies.
*Everything that follows, I've only dreamt.*
In my recent dreams, I've been using Sam Barro's method (which I believe BLASTER posted a long time ago in Sam's forum before it disappeared?) for the synthesis of PETN. His method is the easiest for me, since I've got 70% HNO3 on hand. Unfortunately, his method seems to have worked too well, because I have got 130% yields for my last two batches. I noticed in the PETN thread someone mentioned a synth that had "102% yield." The high yields of most users employing this method lead me to believe that they may be experiencing the same problem I have been, except that they may not notice it.
First, a recap of my proceedure. My method followed Sam's method fairly closely. (Not to unduly credit Sam; I say it's his method because I printed Sam's page for my hard copy.) I used a slightly different nitrating mixture. My nitrating mixture consisted of 34mL of ~70% HNO3 and 25mL of 93% H2SO4. I used an acetone/dry ice bath to cool my acids and keep the reaction cool (-15C to 0C). I have a magnetic stirrer, so the PE/nitrating solution was well mixed during the entire synth. When I recrystallized my PETN in acetone, virtually all of it dissolved. When I poured the acetone/PETN solution into cold H2O, it precipitated instantly. This is step where I first became concerned. Not all the precipitate immediately sank to the the bottom of the flask. Some did sink immediately, and formed a dense layer at the bottom; but some sank slowly and formed a very "airy" layer of flakes or long crystals; and some even floated. After Vacuum filtration, some crystals on the filter paper seemed to be very fine and others larger. My feeling is that the larger, majority crystals are PETN.
Here's what I think could be causing the impossible yields:
1) My pentaerythritol might not be pure. However, even if it isn't, I do not believe that would increase my yields but rather decrease them. I don't have a way to easily test the purity of my PE; its melting point is too high to be easily tested. I hate thin layer chromatography, and beside that, I don't have any Rf values. Without the Rf values and the specs of my PE, TLC is just about worthless, because most PE is slightly impure. (I'm reasonably sure I've got PE because I have dreamt of detonating something synthesized from this PE.)
2) The H2SO4 might not have been neutralized properly with sodium bicarbonate solution. I had to run to the store and my faithful lab techs (heh) were supposed to neutralize it. Then, when I dumped the acidic PETN solution into the acetone, I'd get whatever acetone + dilute sulfuric acid produces, if anything. I should have paid more attention in O. Chem.
3) I might have some of what BLASTER calls 'sulfosesters' or what Urbanski calls 'sulphuric esters.' A sulfuric ester is, on the molecular level, much heavier than a nitric ester. This is the cause that I believe is most plausible. Urbanski gives sulfuric esters but little indirect mention on page 122 in Vol. I in reference to nitrating aromatic compounds with a solution of ethyl nitrate in sulphuric acid. He says
Ropuszyliski [174] recently investigated the action of a solution of nitroglycerine in sulphuric acid more fully and inferred that decomposition of nitroglycerine by sulphuric acid to form a nitronium ion was probably taking place by stages:
C3H5(ONO2)3 + H2SO4 <-> C3H5(ONO2)3H+ + HSO4- (77)
C3H5(ONO2)3H+ <-> C3H5(OH)(ONO2)2 + NO2+ (77a)
With an excess of sulphuric acid, glycerine dinitrate was decomposed to give further nitronium ions, etc. Finally a double ester nitric and sulphuric of glycerol was formed:
C3H5(OH)2(ONO2) + H2SO4 -> C3H5(OH)(ONO2)(OSO3H) + H2O (77b)
Now, I suppose, following what Urbanski wrote, PETN would decompose in this manner:
C5H8(ONO2)4 + H2SO4 <-> C5H8(ONO2)4H+ + HSO4- (1)
C5H8(ONO2)4H+ + HSO4- <-> C5H8(OH)(NO2)3 + NO2+ + HSO4- (1a)
With an excess of sulfuric acid, Pentaerythritol Trinitrate could be decomposed to give further nitronium ions, etc. Finally a double ester nitric and sulfuric of pentaerythritol would be formed:
C5H8(OH)3(ONO2) + H2SO4 -> C5H8(OH)2(ONO2)(OSO3H) + H2O (1b)
That is what, I suppose, would happen if PETN were to be put in concentrated sulfuric acid. That's the reason the PETN solution must be neutralized.
We must keep in mind what happens to the nitrating solution and the nitration itself.
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O (2)
However, the alcohol is simultaneously being dehydrated by the hydrogen sulfate and converted to a nitric ester by the nitronium ion. Therefore, we end up with sulfuric acid again.
C5H8(OH)4 + 4NO2+ + 4HSO4- + 4H2O -> C5H8(ONO2)4 + 4H2SO4 + 8H2O (3)
Doesn't (3) look similar to (1a)?
If nobody knows what's going on right off the bat, here are a couple questions that may lead us in the right direction.
Why does BLASTER's method specify heat? Is it because with heat the equilibrium shifts to the left?
What are the exact conditions that lead to sulfuric esters in this PETN synth? Obviously, there is an excess of sulfuric acid. Is the time in the hot water bath too long or too short?
What governs equations 1, 1a, and 1b? What governs the reaction between a nitric ester and sulfuric ester in general?
What products are of eq. 1, 1a, and 1b are soluble in acetone or water? I guess that they are and are not, respectively.
What concentration H2SO4 is required to decompose a nitric ester?
I'll leave any speculating about acetone + hydronium + hydrogen sulfate to you all. Keep in mind that the H2SO4 is dilute. Like I said, I should have paid more attention in O. Chem.
If anybody does know what is going on, please also post a method of purification of the PETN along with your analysis of the problem.
Next weekend, I'll do the synth myself from start to finish, and maybe I'll post some pictures. If I still get huge yields, we can rule out cause number two. Thanks in advance for any intelligent replies.