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):

INTRODUCTION:

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!

MATERIALS:

Reactants:

Glassware:

98% Concentrated Sulfuric Acid
(H2SO4(l))

250mL Glass
beaker.

68% Concentrated Nitric Acid
(HNO3(aq))

Pipette

Phenol
(C6H5OH(s))

Stirring
Rod

Ethanol (C2H6O(l))

Funnel/filter
paper

Distilled Water (H2O(l))

Ice
bath

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.

PROCEDURE:

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

C6H5OH(s)
+ H2SO4(l)  =>
(HO.C6H4.SO3H + H2O) +
HNO3(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:

PURIFICATION:

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.

RESULTS:

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.

ALTERNATIVE
SYNTHESIS:

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.

Questions? Comments? Suggestions? E-mail me!

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