TNP - Picric Acid
Picric acid finds a wide range of applications as an explosive and chemical reagent; its salts and derivatives also being very useful. There are many different synthesis routes using precursors such as benzene, phenol, salicylic acid, methyl salicylate and, as outlined here, acetylsalicylic acid. The ASA route is preferred by hobbyists because it's budget friendly and uses chemicals that are widely available.
General Info
Formula: C6H2(NO2)3OH
Molar mass: 229.11g/mol
MP: 123° C
BP: 300° C Explodes
VOD: 7480m/s @ 1.76g/cc
Chemicals
10.0g AcetylSalicylic Acid
50.0mL 98% (18 M) H2SO4
30.0g KNO3
Sulfonation
Begin by heating 50mL of H2SO4 to 100° C on a hot plate or water bath. When the acid is hot, slowly stir in 10 grams powdered ASA. Small bubbles or foam may form by a side reaction of trace amounts of water in the H2SO4 causing the ASA to split into acetic and salicylic acids and the subsequent decarboxylation of the salicylic acid to phenol.
Maintain this temperature until all the ASA has dissolved.
White fumes or an acrid smell are normal.
The acid will turn black as it sulfonates.
What's happening in this reaction?
The ASA is reacting with H2SO4 to form acetic and salicylic acids. The salicylic acid is also being sulfonated in the para position, because of the high temperature, lower temperatures yielding ortho, to 4-Sulfonic. All this happens in one elegant step.
ASA + H2SO4 → Acetic Acid + 4-Sulfonic Salicylic Acid
Nitration
This step produces toxic nitrogen dioxide gas. Do not proceed without proper ventilation.
Take the acid off heat and begin slowly adding 30 grams powdered KNO3, stirring constantly. It may take up to an hour to add all the nitrate. HNO3 vapor and CO2 will be produced but if the rate of addition is kept to a minimum little NO2 will evolve. Lots of red NO2 gas is a sign that the nitrate is being added too fast. The acid mix may foam up to many times its original volume so a large reaction vessel is necessary.
The reaction may turn red during nitration.
What's happening in this reaction?
Nitric acid causes the sulfonic group to be replaced by a nitro group, reforming a molecule of H2SO4. HNO3 then attacks the COOH replacing it with a nitro group causing the release of CO2. Finally the third NO2 is added giving a product of 2,4,6-Trinitro phenol.
The reaction happens in a series of steps looking something like this:
-HSO3 + HNO3 → -NO2 + H2SO4
-COOH + HNO3 → -NO2 + H2O + CO2
-H + HNO3 → -NO2 + H2O
When all the nitrate has been added continue heating at 100° for half an hour. This drives off some water giving better nitration and higher yields.
Filtration, washing and purification
After nitration the reaction vessel will be very hot and should be cooled down to 5° C.
A crust of yellow TNP crystals may appear while cooling.
When the acid is cold slowly pour it into 200mL iced distilled water. Fluffy yellow crystals will precipitate out and fall to the bottom. Decant off most of the liquid then add more cold distilled water. Relatively pure picric acid can be filtered out and dried or purified further.
Making use of picric acids water solubility, 1.40g/100mL, and higher solubility of the mono and di-nitro forms, purification is achieved by crystallization from water.
Dissolve the crystals in 75mL boiling water, spooning off any oily drops from the surface, then filter out and discard anything that remains undissolved. Chill and filter to collect high purity crystals.
There is still some residual acid from the nitration stage in the crystals if they do not dry completly after washing. Simply run the crystals through the purification step again to get rid of the unwanted acid.
More TNP can be obtained from the waste liquids by boiling to half the original volume then precipitating and filtering as above.
Pure anhydrous crystals are white to light yellow while the picrate ion is bright yellow to orange. A sample of picric acid will inevitably look yellow because of traces of water absorbed from the air and possible formation of picrate salts.
Safety
Picric acid is not compatible with any metals, metal salts or bases.
Store wet away from heat and light.
Dry crystals are sensitive to impact, flame and friction. Picric acid should be stored under water in a plastic or glass container with rubber stopper. Screw on lids create friction which may cause any crystals in the threads to detonate.
Chemical resistant gloves must be worn at all times when working with picric acid. Never handle TNP or any of its salts with bare hands. Picric acid has been used industrially as a dye and its incredible staining power can color even resistant rubbers and plastics bright yellow.
Skin irritation and yellowing can occur from contact. Scrubbing with hot water and baking soda will lessen the protein dying effects.
Repeated or extended exposure causes damage to the kidneys, liver and blood.
Aside from its extreme toxicity TNP is also highly reactive. Any contaminated TNP should be disposed of.
Uses
Detonation occurs as 2 C6H2(NO2)3OH(s) → 12 CO(g) + 2 H2O(g) + 3 N2(g) + H2(g) OB: -45.39
Being a medium to high sensitivity explosive, TNP is easily detonated by a relatively small amount of primary. This is useful for making high power blasting caps that contain a layer of TNP under a layer of primary.
With a VOD over 7,000m/s picric acid is suitable for use in shaped charges. Uniform density can be achieved by melting the TNP and casting it into a casing.
TNP is readily converted to salts such as Ammonium, Potassium, Sodium, et cetera. It's also the precursor to Picramic Acid and DiazoDiNitroPhenol.
