PowerLabs Picric Acid Synthesis

Original synthesis procedure removed from “Advanced Practical Organic Chemistry 3rd edition, by J. Leonard, B. Lygo and G. Procter”. pages 171 / 172. Picric Acid (2,4,6-Trinitrophenol) Laboratory Synthesis (industrial and laboratory methods):

Picronitric acid, Carbazotic acid, nitroxantic acid. C₆H₃N₃O₇; Molar Weight; 229.11. Percentage composition; C: 31.46%, H: 1.32%, N: 18.34%, O: 48.88%. Picric Acid is used in electric batteries, leather industry, dies, pigments, inks, paints, manufacture of colored glass, textile mordants, as a laboratory reagent, in matches and explosives. It consists in pale yellow, odorless, intensely bitter crystals of density 1,763 and melting point 122 – 123C. It explodes at 300C. One gram dissolves in 78mL water, 15mL boiling water, 12ml alcohol, 10ml benzene, 35ml chloroform, 65ml ether. It is incompatible with all oxidising substances, albumin, alkaloids and gelatin. It should be kept in a cool place and remote from fire. (Source: Merck Index 12th edition).
Picric Acid was one of my first Organic Synthesis ever, and was performed at High school, under supervision of my Higher Level I.B. chemistry teacher when I was 16 years old. It was part of an assignment and I was rather surprised that our textbook would carry the instructions for the synthesis of a potentially explosive compound, and more surprised still that we would be given this particular synthesis in class as an assignment. Since we were, I don’t see any problem in replicating it here for the general information of the POWERLABS public. However, the usual warnings apply:
As with any other nitration, this procedure is hazardous and involves the use of highly corrosive acids. The final product is toxic and dangerous. This should only be attempted by experienced chemists!

  Here all the chemicals used in the synthesis are seen, from left to right, back to front: Absolute Ethanol, Phenol, Sulphuric Acid (98%), Nitric Acid (68%), Distilled water.

 A note on proportions: 0.8grams of phenol are reacted for every 1.5ml of Nitric acid and 1ml of sulfuric acid.

C₆H₅OH_(s) + H₂SO_4(l)  => (HO.C₆H₄.SO₃H + H₂O) + HNO_3(aq) => C₆H₃N₃O₇

25grams of powdered phenol are weighted and added to a 250ml beaker containing  31.25ml  (57.81g) of concentrated sulfuric acid. The mixture is stirred and than heated on a hotplate at 100C for 30 minutes until it becomes dark colored, indicating the formation of phenol-4-sulphonic acid. This is than chilled to -5C on a water bath with salt added, becoming a cold viscous syrup, to which 47ml of chilled (-5C) nitric acid are added inside a fume cupboard, making sure that the mixture is not allowed to warm up prior to the addition of the acid. Within a few minutes a vigorous reaction will occur which will produce large amounts of nitrogen dioxide, a red colored (poisonous!) gas. One must be very careful with sputtering during this reaction. 

The reaction is allowed to subside and than the mixture (which has now become a thin, orange/yellow liquid) is heated on a water bath for one and a half hours, with occasional shaking.

When the heating is complete the mixture is allowed to cool down and 313ml of cold water are added, causing the Picric Acid to crystallize out of the mixture. The mixture is cooled to 5C so as to precipitate most of the remaining Picric acid, and than is quickly filtered and washed with 1200mL of cold distilled water so as to remove any traces of the nitration acids.

The filtrate appears as the fine yellowish mass of crystals seen to the left. Yield is approximately 40grams for the 25 grams of phenol that was started out with. These crystals are high purity and have all of the properties attributed to reagent grade Picric Acid. However, they can be purified:

Making use of Phenol’s high water solubility, and the higher solubility of dinitro and mononitroPhenol, the Picric Acid can be purified to near analytical grade by re-crystallization from a solvent mixture of 1 volume ethanol and 2 volumes  water, roughly 9ml of solvent being required per gram of Picric Acid.
The crystals are than removed by vacuum filtering, are vacuum dried in a dissector, and form into a nearly yellow mass of mp 123C.
It is advisable to store Picric Acid wet with at least 30% water and in rubber stoppered flasks.

This acid may be used for other synthesis, such as Ammonium Picrate (C₆H₂.ONH₄.(NO₂)₃) Ammonium picrate, also called ammonium piconitrate, Explosive D, or carbazoate is a commonly used safety explosive which is insensitive enough to be used in shells that penetrate armor and detonate inside it. It requires a substantial shock to cause it to detonate, slightly less than that required to detonate ammonium nitrate. In many ways it is much safer than picric acid, since it does not have the tendency to form hazardous unstable salts when placed in metal containers. Vdet 7100M/s. It is made by dissolving picric acid crystals in 15 parts hot, steaming distilled water and adding clear ammonia in excess, allowing the excess ammonia to evaporate (excess ammonia should not be used as it will form a red precipitate that is more sensitive than pure Ammonium Picrate. Yellow ammonium picrate can be obtained from red picrate by recrystallizing it several times from water. It also eventually changes into the yellow form if stored as a concentrated solution. Keep this material as dry as possible). The powder remaining should be ammonium picrate. The water should not be heated, as ammonium picrate is sensitive to heat. Vacuum distillation and open evaporation are relatively safe ways to extract the picrate. Ammonium picrate most commonly appears as bright yellow crystals, and is soluble in water. These crystals should be treated with the care due to all shock sensitive materials. Some illegal salutes have been found to contain ammonium picrate, which makes them much more hazardous. Equation goes as follows:
NH₃�H₂O + C₆H₂(NO₂)₃OH —> C₆H₂�(NO₂)₃�O�NH₄ + H₂O
Melts at 271�C, impact sensitivity is 20% less impact than TNT, and relative strength is 93 – 100% TNT in the trauzl test.

Picric Acid can also be produced from Phenol derivatives such as Salicylic and Acetylsalicylic acids. The procedure is exactly the same as the one outlined above, except a smaller amount of Salicyclic (0,6grams per 1mL H₂SO₄ and 1,5mL HNO₃) our Acetylsalicylic (0,5g per 1mL H₂SO₄ and 1,5mL HNO₃) acids are used. This is important because Acetylsalicylic acid is the active ingredient in aspirin and can be obtained by dissolving them in hot ethanol (20 500mG pills per 100mL), filtering out the buffers, and allowing the pure acid to crystallize out. The reaction than goes:
                H₂SO₄
C₉H₈O₄  ——->  C₂H₄O₂ +  C₇H₄O₂
(The sulfuric acid breaks down the acetylsalicylic acid to acetic and salicylic acid)

 Or if Salicylic acid is used,
 C₇H₄O₂ + H₂O —> C₆H₆O + CO₂
(Salicylic acid with heat source converts to carbolic acid (phenol) and carbon dioxide; (in presence of water from 98% H₂SO₄)
Or the formed phenol just sulfonates rapidly. From Phenol-4-sulfonic acid onwards the reaction proceeds as outlined above.
The Nitric Acid can also be substituted by adding 15 grams Sodium Nitrate per 80mL H₂SO₄.
With the NaNO₃/H₂SO₄ nitrating system (H₂SO₄ already present) without any external cooling; the phenolsulfonic acid
compound readily nitrates to a polynitro compound (2,4,6-trinitrophenol).

Yet another way of preparing picric acid is by the conversion of benzene directly into picric acid in one step. This was patented by Mueller Teeters in 1948 (US pat. 2,455,322): 1g of mercuric nitrate is added to 60mL of 70% nitric acid which is than stirred and added to 23 mL of benzene. The mixture is than heated for several hours and distilled. Benzene will distill over first, then nitrobenzene, and finally a mixture of dinitrobenzene and dinitrophenol will come over slowly. The distillation is continued until these volatile organics are removed. The remaining residue is filtered while hot to remove any impurities and allowed to crystallize. the picric acid that should have crystallized can be purified by recrystallizing from hot water.

Picric Acid can be used alone or in the synthesis of picrate salts such as, for example:
Potassium Picrate
Lead Picrate