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				Syntheses of Phenylhydrazines
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Phenylhydrazine [Org Syn Coll Vol 1, pp 442-445]

In a 12-liter round-bottomed flask, fitted with a mechanical stirrer, is
placed 1045 ml of concentrated commercial hydrochloric acid (d 1.138). The
flask is surrounded with a freezing mixture of ice and salt, and, when the
contents are at 0øC, stirring is started and 500g of cracked ice is added,
or more ice can be added and the external cooling dispensed with; then 372g
(364 ml, 4 moles) of aniline, also cooled to 0øC, is run in during five
minutes. The mixture is treated with 500g more of cracked ice, and a cold
solution (0øC) of 290g (4 moles) of technical sodium nitrite dissolved in
600 cc. of water is allowed to run in slowly (twenty to thirty minutes)
from a separatory funnel, the end of which is drawn to a small tip and
reaches nearly to the bottom of the flask. During this addition, the
stirrer is operated rather vigorously, and the temperature is kept as near
0øC as possible by the frequent addition of cracked ice (about 1 kg).

In the meantime, a sodium sulfite solution is prepared by dissolving 890g
(20 moles) of sodium hydroxide, of about 90 per cent purity, in about 1 l.
of water and then diluting to 6 l. A few drops of phenolphthalein solution
are added and sulfur dioxide passed in, first until an acid reaction is
indicated and then for two or three minutes longer. During the addition of
the sulfur dioxide, the solution is cooled with running water. On account
of the strong alkaline solution, the original color produced by the
phenolphthalein is very faint, but this slowly increases until it becomes
deep just before the acid point is reached. It is best to remove a small
sample of the liquid from time to time, dilute with three or four volumes
of water, and add a drop more of phenolphthalein (It is very important that
there is no excess alkali left, or the yields of phenylhydrazine will be
greatly diminished).

The sodium sulfite solution is placed in a 12-l. flask and cooled to about
5øC. Approximately 500g of cracked ice is added, and then, with mechanical
stirring, the diazonium salt solution is run in as rapidly as possible. The
mixture becomes a bright orange-red. The flask is now warmed to about 20øC
on a steam bath, until the solid sodium sulfite, which has separated while
cooling, redissolves. The total amount of liquid is now about 10 l.
One-half of this is poured into another 12-l. flask, and both halves are
warmed on the steam bath to 60-70øC, until the color becomes quite dark
(thirty to sixty minutes). Sufficient hydrochloric acid (300-400ml) is now
added (*) to each flask to make the solutions acid to litmus. The heating
is continued and the color gradually becomes lighter until, after four to
six hours, the solutions have become nearly colorless; they may be heated
overnight, if desired.

To the hot solutions is now added about one-third of their volume of
concentrated hydrochloric acid (2000ml to each portion), and the mixtures
are cooled, first in running water, then in a freezing mixture, to 0øC. The
phenylhydrazine hydrochloride precipitates in the form of slightly
yellowish or pinkish crystals which may be filtered off and dried.
The phenylhydrazine hydrochloride may be purified by crystallizing from
water. A 600 ml. portion of water is used for 100g of crude hydrochloride,
and the solution boiled a short time with a few grams of activated
charcoal. After filtering, 200 ml of concentrated hydrochloric acid is
added, and the mixture cooled to 0øC. Pure white crystals in a yield of
85-90g are obtained.

The free base is liberated by adding to the phenylhydrazine hydrochloride
1000ml of a 25% solution of sodium hydroxide. The phenylhydrazine separates
and is taken up with benzene (2x300 ml portions). The combined extractions
are well dried with 200g of solid sodium hydroxide (it is important that
there are no trace of water or phenylhydrazine hydrochloride left), poured
off, and distilled. Most of the benzene may be distilled under ordinary
pressure, and the remainder, and any low-boiling impurities, under
diminished pressure. The pure phenylhydrazine boils at 137-138øC at18 mmHg,
and is obtained as a pale yellow liquid. It can be crystallized on cooling
in an ice bath; the crystals melt at 23øC. The crude phenylhydrazine from
two lots of aniline (744g) is best distilled at one time and gives 695-725
g. of pure product (80-84 per cent of the theoretical amount).

(*) If the sodium sulfite-diazonium salt mixture is acidified before
    warming or before becoming dark, the red color of the solution does
    not disappear on heating, and the precipitated phenylhydrazine
    hydrochloride obtained is colored red.
    Most published directions for the preparation of phenylhydrazine
    specify the use of zinc dust and acetic acid following the reduction
    with sodium sulfite. No improvement in the quality or quantity of the
    product was obtained by using zinc and acetic acid.

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Phenylhydrazine [Mann & Saunders, Practical Organic Chemistry, 3rd Ed]

Phenyl diazonium chloride reacts in solution with sodium sulfite (Na2SO3)
to give phenyl diazonium sodium sulfonate, which when treated with
sulphurous acid undergoes reduction to phenylhydrazine sodium sulphonate.
The latter readily hydrolyses in the presence of concentrated hydrochloric
acid to give phenylhydrazine hydrochloride, from which the free base can be
isolated by the action of caustic alkalis.

Phenyl diazonium chloride also undergoes direct reduction to phenylhydrazine
when treated with an acid solution of stannous chloride (e.g., a solution
in hydrochloric acid) but the yields are not so high as those obtained by
the above sulphonate method.

Required: Aniline 20ml, hydrochloric acid 50ml, sodium nitrite 17g, sodium
          carbonate (anhydrous) 17g, sodium sulphite (7H20) 115g.

First prepare a solution of phenyl diazonium chloride from 20ml (20.5g)
aniline by adding a solution of 17g of sodium nitrite in 40ml of water in
small portions to a well-stirred solution of the aniline in 50ml of
concentrated hydrochloric acid and 50ml of water, the temperature of the
mixture being kept between 5-10øC. While this solution is standing,
dissolve 17g of anhydrous sodium carbonate in 70ml of warm water in a 800ml
beaker, and then add 115g of finely powdered crystalline sodium sulphite
(Na2SO3 * 7H2O) and continue warming the solution at 40-50øC for a few
minutes, so that the greater part of the sulfite dissolves. Then cool the
solution in ice-water until the temperature is between 5-10øC, some of the
sulfite meanwhile crystallizing out again. Now add the diazonium solution
slowly to the well-stirred sulfite solution: the mixture becomes red in
colour, and a deep red oil separates and later solidifies as
orange-coloured crystals (sometimes the orange crystals separate without
the intermediate formation of the oil). When the addition of the diazo
solution is complete, continue stirring the mixture for 5-10 minutes, and
then warm it to 30øC on a water-bath (in a fume-cupboard) and pass in a
stream of sulfur dioxide (made by dropping H2SO4 on NaHSO3 or Na2SO3
crystals in a kipp's apparatus) until the solution is saturated and no more
gas is absorbed: the excess of sodium sulfite and the orange crystals
dissolve up at this stage. Now heat the solution to 70øC and add 150ml of
concentrated hydrochloric acid with stirring. Within a few minutes,
crystals of phenylhydrazine hydrochloride separate out: cool the mixture in
ice-water for 30 minutes with occasional stirring and then filter at the
pump and drain thoroughly.

Suspend the crude hydrochloride in some water in a separating funnel and
add 20% sodium hydroxide solution until the mixture is definitely alkaline
and the crude phenylhydrazine base floats as a deep red oil on the surface.
Now extract the phenylhydrazine twice with benzene (using about 30ml of
benzene on each occasion) and dry the united benzene extracts with powdered
anhydrous potassium carbonate. After the mixture has been allowed to stand
for 20-30 minutes, filter at the pump, and wash the residual carbonate with
a small quantity of dry benzene. The benzene has now to be distilled off at
atmospheric pressure and the residual phenylhydrazine at reduced pressure.
For this purpose, fit a small dropping-funnel to the main neck of a 60ml
Claisen flask, cork the other neck, and fit a water-condenser to the
side-arm. Run about 30ml of the benzene solution into the flask, and heat
the latter in an oil-bath, controlling the temperature of the bath so that
the benzene distills gently over. Allow the remainder of the solution to
run in from the dropping-funnel as fast as the benzene itself distills
over. When the benzene has been almost entirely removed, set up for a
vacuum distillation. Distil the phenylhydrazine carefully from an oil-bath
and collect a fraction boiling over a range of about 3øC, e.g., at
127-130øC at 22 mmHg. The phenylhydrazine is thus obtained as a very pale
yellow (almost colourless) oil of d 1.10: it has a characteristic odour and
is only slightly soluble in water. Yield, 16-17g. Pure phenylhydrazine has
mp 23øC; it boils at 242-243øC at atmospheric pressure with partial
decomposition. Phenylhydrazine on exposure to light slowly darkens and
eventually becomes deep red in colour: salts of the base share this
property but to a lesser degree, the sulphate and acetate (of the common
salts) being most stable to light.

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p-Methoxyphenylhydrazine
JCS 125(1), 313 (1924)

The most satisfactory method of preparing this substance appears to be the
following: p-Anisidine (38 grams) is mixed with concentrated hydrochloric
acid (300 ml), when a thick, crystalline paste of the hydrochloride
separates. This is well stirred and diazotised at 0øC with a solution of
sodium nitrite (20.7 grams in 90 ml of water). A well-cooled solution of
stannous chloride (210 grams in concentrated hydrochloric acid, 180 ml)
is added to the purple diazonium solution, and this causes the separation
of a yellow, crystalline solid which, after standing for two or three
hours, becomes colourless. After being collected and thoroughly pressed,
the mass is decomposed by shaking with caustic potash solution (400 ml of
25 per cent.), care being taken to keep the mixture well cooled. The free
base separates as a light brown, crystalline solid, which is collected,
washed with a very little water, and dried over sulfuric acid in a vacuum
desiccator. The hydrazine is purified by dissolving in the minimum of hot
benzene, filtering, and adding about three volumes of light petroleum,
which precipitates the base in shining plates, mp. 65øC. Yield 44%.

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