How to make explosive (PETN) (for industrial purposes)

Explosives are materials (chemicals/nuclear) that can be initiated to undergo very rapid self-propagating decomposition that results in the formation of more stable materials with liberation of heat and large amount of gases. This heat can be tapped and use in industrial applications e.g generation of steam that can be used to turn turbines which can either generates electricity or do some mechanical works (in machines like mills e.t.c).

Pentaerythritoltetranitrate is one of secondary explosives. It is an explosive booster/high explosive.  Explosive boosters are made of materials which are insensitive to mechanical shocks and flames but explode with great violence when set off by explosive shock. The explosion is achieved by detonating small amount of primary explosives in contact with high explosives like lead azide and lead styphnate which are highly susceptible to initiation. 

Pentaerythritoltetranitrate can be synthesized from methanal/formaldehyde (CH₂O) and acetaldehyde (CH₃CHO) through crossed Aldol condensation.  The product of (pentaerythritol) is then reacted with concentrated nitric acid to yield pentaerythritoltetranitrate. Aldol condensation is a condensation  in which two aldehydes or ketones combined to form beta-hydroxyaldehye or beta-bydroxyketone in the presence of a base. On the other hand, crossed Aldol condensation is a Aldol condensation where different aldehydes or ketones with active alpha-hydrogen react to form beta-hydroxyaldehyde and beta-hydroxyaldehyde ketone respectively. This initial reaction of synthesis of pentaerythritoltetranitrate takes place between aldehyde (formaldehyde) which lack active hydrogen but has carbonyl group which is more reactive to nucleophile addition than carbonyl group in acetaldehyde. The reactions that occur are as follows:

CH₃CHO (acetaldehyde) + CH₂O (formaldehyde) + NaOH(base) = ^-OCH₂– CH₂-CHO (alkoxide ion) + H₂O (water) = OHCH2-CH₂-CHO (beta-hydroxypropionaldehyde)

In this reaction base (^-OH)which is from NaOH,  abstracts H⁺ from alpha-carbon of acetaldehyde to form nucleophile (^-CH2CHO) and in the process water is formed (^-OH + H⁺ = H₂O.  ^-CH2CHO then attacks carbonyl of formaldehyde (CH₂O) to form propionalkoxide ion (^-OCH₂– CH₂-CHO), this ion then abstracts hydrogen (H⁺) from solvent (H2O) to form beta-hydroxypropionaldehyde and in the process ^-OH is liberated from (H₂O – H⁺ = ^-OH).

Beta-hydroxypropionaldehyde still has two alpha hydrogen atoms which can further undergo the same reaction with two moles of formaldehyde to yield trihydric alcohol as shown below:

OHCH₂CH₂CHO (one mole of propionaldehyde) + 2 CH₂O (two moles of formaldehyde) + ^-OH= (CH₂OH)₃CCHO(trihydric alcohol). 


The trihydric alcohol then undergoes canizzaro reaction with one mole of formaldehyde in the present of strong base (NaOH) to form pentaerythritol and formic acid. Canizzaro reaction is a chemical reaction that involves the base-induced disproportionation of aldehyde lacking hydrogen in the alpha position (alpha carbon position). The reaction is as follows:

(CH₂OH)₃ CCHO(trihydric  alcohol) + HCHO(formaldehyde) + ^-OH(from a strong base like NaOH) = C(CH2OH)₄(Pentaerythritol )+ Formic acid (HCOOH). 

The reaction involves nucleophile substitution on aldehyde with a leaving group concurrently attacking another aldehyde. Hydroxide attacks carbonyl hence resulting in tetrahedral intermediate which then collapses  and reform the carbonyl and in the process transferring hydride to another carbonyl. Lastly, the acid and alkoxide formed exchange a proton in the presence of strong base (NaOH). Initially aldehyde forms a doubly charged anion from which a hydride is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ion. The alkoxide ion abstracts a proton from solvent forming pentaerythritol. 

Pentaerythritol is then reacted with concentrated nitric acid to form pentaerythritoltetranitrate(PETN)- explosive  and water (H₂O). The reaction is as shown below:

C (CH2OH)₄(pentaerythitol)(aqueous) + 4HNO₃(concentrated nitric acid)(liquid) = C(CH₂ONO₂)₄ (PETN)(Precipitate)  + 4H2O(water) (aqueous) 

The precipitate can then be recrystallized with acetone to give the final crystals (PETN). When PETN is initiated it decomposes as shown below:

C (CH2OH)₄   (s)=  3CO₂(g) + 2 CO(g) + 4 H₂O(g) + 2N₂(g) + Heat , upon decomposition PETN produces gases that exert tremendous pressure as they expand at high temperature of the reaction. The heat produced  is what is employed in industrial applications.

Calculations

Considering moles in the above equations the exact amount of each chemical required for synthesis of PETN can be calculated. For instance, for synthesis of pentaerythritol requires 4 moles of formaldehyde and 1(one) mole of acetaldehyde and synthesis of PETN requires 1(one) mole of pentaerythritol and 3 moles of nitric acid. The challenge here is how to obtain the amount of base that can be employed in the overall reaction, since it will not be practical to say that because the mole ratios in the above reactions are in the ratio 1:1 with the species involved, therefore base should be used as theoretically stated in the equations.  But it can be done by trial error method to determine the exact amount of base that will result in high yield. It can also be seen in the reactions above that there are by-products and the useful one is formic acid which can be used in other purposes.

Now, take an example on how the amount of desired products can be calculated from the above reactions.  For example, if someone what to make one litre of pentaerythitol then proceeds to make PETN with that amount of pentarythitol.   How much amount of the chemicals starting from acetaldehyde and formaldehyde should she/he employ in the process? Include the amount of concentrated nitric acid (HNO₃) and that of formic acid. 

From mole reactions above, the process needs 4 moles of formaldehyde and 1 mole of acetaldehyde to make 1 mole of pentaerythritol. Therefore considering each chemical with relation to pentaerythritol then the result will be;

1mole acetaldehyde        = 1 mole pentaerythritol ,  hence

                                     ? = 1 litre, will be

(1X 1/1)  = 1 litre

Therefore to produce 1 litre of pentaerythritol,the process will require one litre of acetaldehyde (CH₂CHO).

4 moles of formaldehyde = 1 mole of pentaerythritol, hence

                                         = 1 litre, will be

4 X 1/1    = 4 litres of formaldehyde 

Therefore to produce 1 liter of pentaerythritol,the process will require 4 liters of formaldehyde. 

The amount of nitric acid that can be employed in the process will be;

According to the reaction, one mole of pentarythritol requires 3 moles of nitric acid to produce one mole of PETN and 4 moles of water. Hence, the amount of nitric acid that is required will be:

1 mole of pentaerythritol             =          3 moles of nitric acid

1 litre        =         ?

1X3/1       =         3 litres. 

Three litres of nitric acid will be required and approximately one (1) kilogram of PENT will be produced. This is according to theoretical perspective.  

Considering the cost of production one can easily identify that this process is not cost effective. But even with this small amount of explosive (pentaerythrioltetranitrate) can do great work and also the by-product (formic acid) in the process is also useful. Therefore the process can be worth trying. 

NB: If you wish to try this process please carry it out under supervision of professional, otherwise you can cause great damage!!

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