Boffis
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p-Nitroso-dimethylaniline
Let me say before I continue, yes I am totally aware of the toxicity and carcinogenic nature of this compound and all I can add is that I suggest that
you don't eat it; intentionally or otherwise. I used the Vogel procedure because I expected it to be a tried and tested method and it allows the
reaction to be carried out in an enclosed system that minimised operator exposure to both the base and the benzene solvent.
I needed some of this compound so I followed the procedure in Vogel's "Textbook of organic chemistry" 3rd ed. The first stage, the preparation of the
hydrochloride salt, worked fine and I got a good yield of the yellow salt. The next stage, the liberation of the free base, however, was a disaster
and I feel it necessary to report my results and suggest alternatives to help others avoid the pitfalls of this method.
In Vogel's method 30g of the crude hydrochloride are suspended in 100ml of water in a 500ml separating funnel and 10% NaOH solution until the solution
is alkaline and all of the yellow salt has turned green (the free base). The suspension of the free base is then extracted twice with 60ml portions of
benzene. Distil off half the benzene and cool; filter of the scaly crystals of free base. Easy??
No; the problems is that the slurry is never homogeneous enough and the first 60ml of benzene is not enough to dissolve all of the suspended solid
which then choke the funnel tap. Nor is 120ml added in one go enough. In fact some small scale experiments show that the benzene must be hot or you
need closer to 500ml for the 1st extraction. Sloshing around 500ml quantities of benzene is not my idea of fun!
I had to pour the liquid out into a buchner funnel to collect the free base, vacuum filter, separate the benzene, distil off half the benzene, allow
to cool and filter of the crystals. Then you have a lot of benzene residue, lots of contaminated equipment to clean up and lots of handling of benzene
in the air. Even under a fume hood this is no fun and nor risk free.
The final insult was to discover that the beautiful deep green crystals to 6-7mm in diameter with an amazing purple iridescent are not
p-nitroso-dimethylaniline but a benzene solvate which appears to be p-n-diMeAn+2Bz ie 1 molecule of the nitroso compound and 2 of benzene. Over the
next two days the benzene evaporated slowly and the crystals crumbled to a green powder so probably filling the air in my lab with unnecessary amounts
of benzene vapour! They don't tell you this in Vogel!! Beautiful as it is I don't really feel inclined to investigate the benzene solvate complex
further.
Right, so whats a better method? Its simple.
Dissolve your amount of the hydrochloride salt you wish to convert to free base in 10ml of 1% hydrochloric acid per g of hydrochloride, this will
require heating. When dissolved add cold 5% sodium hydroxide solution until it is alkaline to phenolphthalein (about 4.2ml per g of hydrochloride,
avoid a large excess) with rapid stirring and cool quickly by plunging the container into ice and water (this inhibits the decomposition of the free
base if excess NaOH has been added- dimethylamine is evolved otherwise). When cold, vacuum filter and wash the bright green cake with a little cold
water and dry.
The base is best recrystallised using a simple alcohol, IPA, rectified spirit etc.
I hope these notes save someone else from having to learn of the hard way!
One final note; the recrystallisation of the hydrochloride is not as simple as you might believe either. Here is my method:
Use 10ml of hot, 1-2% hydrochloric acid per g of crude salt, allow to cool and filter off the crystals. Unless you used freshly vacuum distilled
dimethylaniline in the original preparation the crystals will be either greenish or brown due to the presence of acid soluble tar.
Redissolve the crystals in hot water (10ml per g) containing just a few drops of conc HCl, treat with active charcoal powder (0.05-0.07g per g),
filter hot and cool. Finally chill to near freezing overnight and filter of the orange yellow crystals. If the original dimethylaniline was not
redistilled you may need to use several recrystallisation with charcoal treatment to obtain the orange yellow salt.
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Boffis
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Has anyone else ever prepared this compound? I prepared the p-nitroso-N,N-dimethylaniline hydrochloride from 99-100g of freshly vacuum distilled
dimethylaniline and obtained about 190g of orangy ochre yellow powder but this is about 125% recovery! I attempted to dry it further but much HCl
vapour came off and the solid started to turn green so part of this I converted into the free base by treatment with 5M NaOH in aqueous solution. The
remainder I tried to recrystallise from water. Thinking that the impurity is probably salt I dissolved 10g of the crude hydrochloride salt in 35ml of
hot water (about 7ml per g of crude material or about 10ml per g of "theoretical" hydrochloride content). It dissolved completely to yield a dark
brown solution that crystallised on cooling. I filtered of the crystals and washed then with a little cold 50% aqueous ethanol and dried to get a pale
yellow green mass of crystals. Under the microscope they are small pale green glassy prisms; not the golden yellow crystals I was expecting! When
dissolved in water they yield a deep orange yellow solution though.
Is this normal for the hydrochloride salt? Does the hot aqueous solution decompose (it is fairly acidic and appears to contain significant free HCl)?
The solution was not boiled, the maximum temperature was about 70 C.
The free base was recrystallised from rectified spirit (3.5ml per g of crude, dried base) to give 42g of beautiful, almost metallic green, platy
crystals that stain everything instantly. Evaporating down the filtrate to 40% of its original volume has produced a further crop of slightly browner
crystals (work in progress).
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Pumukli
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I quickly checked my local org. chem. preparations manual but unfortunately this particular compound was not part of the curriculum in those days 50
years ago when the book came out. There were other three different nitroso compounds listed but not this one so I can't help.
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AvBaeyer
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Boffis,
Unfortunately, Vogel is not always a reliable source of experimental procedure as you have found. Much of what is in Vogel is "stolen" from Organic
Syntheses and other sources without attribution.
You will find a well described synthesis of p-nitroso-N,N-dimethylaniline hydrochloride in Organic Syntheses, COll. Vol 2, page 223. It is part of a
synthesis of 2,4-dinitrobenzaldehyde. Note 1 of this preparation describes isolating the free base of p-nitroso-N,N-diETHYLaniline which should be
useful to you.
Here is the link: http://orgsyn.org/demo.aspx?prep=cv2p0223
AvB
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Boffis
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Many thanks AvBaeyer, this is an interesting since this is one of my intended uses of the freebase. I intend to react it with o-nitrotoluene to
produce o-nitrobenzaldehyde, the procedure I am intending to use is base catalysed. I notice in the "Notes" section it says "technical"
p-nitrosodimethylaniline hydrochloride (by this I presume they mean previously prepared and stored) does not work which rather suggests that it is
somewhat unstable. This is certainly my observation, recrystallising 10g of my crude hydrochloride gave only 3.05g of pale green (?) crystals, when
the filtrate was basified with sodium carbonate only a further 0.301g of green nitrosodimethylaniline precipitated. Where as the direct ppt of the
free base from 10g of the hydrochloride gives about 5g.
When I attempted to recrystallise again some of the pale green recrystallised crystals the product was darker green and the addition of extra
hydrochloric acid didn't help.
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Pumukli
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This is from hyperlab. Google translate can translate it. They suggest petrolether for recrystallization.
Синтез N,N-диметил-4-нитрозоанилина.
В фарфоровом стакане, снабженном мешалкой, термометром и капельной воронкой,
смешивают 10,3 г (8,6 мл) соляной кислоты с 5 г диметиланилина (диметиланилин вносят в
кислоту), прибавляют около 40 г измельченного льда и медленно, при постоянном
перемешивании и температуре не выше 10°С, приливают из капельной воронки в течение
1 ч раствор 3,1 г нитрита натрия в 10 мл воды. Смесь окрашивается в оранжевый цвет и из
нее выпадают желтые кристаллы гидрохлорида п-нитрозодиметиланилина. Содержимое
перемешивают еще в течение 1 ч, выпавший осадок отфильтровываюь, хорошо отжимают
стеклянной пробкой, промывают 5 мл этилового спирта и высушивают в эксикаторе.
Выход гидрохлорида 7,2 г (90% от теоретического).
Для выделения свободного основания гидрохлорид п-нитрозодиметиланилина
обрабатывают в делительной воронке 5%-ным раствором карбоната натрия и 20 мл эфира.
Основание переходит в эфирный слой, который отделяют от содового раствора, а
затем отгоняют эфир. N,N-диметил-4-нитрозоанилин выделяется в виде зеленых чешуек.
Продукт можно получить более чистым, перекристаллизовав его из петролейного
эфира. Получаются зеленые кристаллы с Тпл = 85°C.
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Boffis
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Hi Pumukli, I had a go at recrystallising some of my least pure p-nitrosodimethylaniline from pet ether boiling range 100-120 C and the results were
excellent. Only one small problem it requires close to 100ml per gram. The product is a beautiful grass green crystalline powder that filters easily
and the filtrate contains very little nitroso derivative its almost colourless. Another slight problem is that the impurities form a dark brown oil
from which the clear hot solution must be decanted as it can't be removed by filtration.
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