turd
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Pr-Se-Se-Pr (impure)
In the series "Fun for the Whole Family", let me present a simple preparation of dipropyl diselenide and selenide slightly adapted from Deryagina
et al., Russ. J. Gen. Chem., 2003, 73(5), 711-714.
4.0 g NaOH was dissolved in 30 ml 20% aqueous hydrazine solution in a conical flask. After cooling to RT, 7.9 g Se powder was added in one go.
The suspension was stirred until a very dark red/black solution was obtained (don't stopper tightly - overpressure!). The solution was heated at 85°C
for 1 h and cooled back to RT. 12.3g PrBr was added and the suspension stirred over night at RT. In the morning, there were two red/orange phases. The
lighter-coloured lower (heavier) phase was separated with an eye dropper and dried 2 h over freshly baked MgSO4. The liquid was separated from the
MgSO4 with an eye dropper to obtain 10.45 g of a red/orange oil, which probably consists of mostly Pr-Se-Se-Pr with Pr-Se-Pr as impurity.
Smell is intense (but not revolting if you like cuisine with an emphasis on onions and garlic) and very lingering. All used glassware was put in aqua
regia, but the dialkyl selenide and diselenide are surprisingly stable. They swim on top and do what they can do best...stink. 
I'm still undecided between distilling or reducing to the selenol and then distilling. Or organize an NMR spectrum. Time will tell...
For those interested: The selenol can be obtained directly from Se by reduction with NaBH4 to NaSeH and subsequent alkylation. The selenol can then be
easily oxidized to the title compound.
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woelen
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What interesting things can be done with Pre-Se-Se-Pr? This synthesis looks like an easy one if you have the hydrazine. I do have hydrazine
dihydrochloride so I could make a hydrazine/NaCl solution and try with that. It is remarkable that selenium dissolves in such an alkaline solution of
hydrazine. What is the dark red compound when Se is dissolved in N2H4/NaOH solution?
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turd
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Interesting? I plan to make the whole series: disulfide, diselenide (done), ditelluride. And maybe the corresponding thiols / selenols / tellurols.
But I'm a little bit scared of the smell. Apart from that I like the color and will keep it on my bench.
In the paper they use either aqueous hydrazine solution or hydrazine hydrate. In the latter case apparently Te even reacts with dichloromethane to
give [-CH2-Te-Te-]n polymers.
You are the expert on strange oxidation states. So far I've only worked with Se
and Te in the usual oxidation states, viz. IV and VI. My guess would be di- and polyselenides, but the solution was so dark that I couldn't tell if it
was a solution or a very fine suspension...
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woelen
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I do not have PrBr, but I tried the part with the selenium and hydrazine.
I dissolved N2H4.2HCl in an excess amount of a concentrated solution of NaOH, such that the total solution is really concentrated and contains a
slurry of NaCl, which is formed in the reaction between NaOH and N2H4.2HCl. To this strongly alkaline solution (with the NaCl-slurry still present) I
added a tiny amount of powdered selenium. I added a tiny amount because adding too much makes the liquid too dark. The selenium fairly quickly
dissolves and the liquid turns red. While the selenium dissolves, a colorless gas is produced. Next, I added so much water that all NaCl just
dissolves. The liquid I obtained is completely clear and beautifully red with an orange hue. The red liquid is a real solution and not a fine
suspension.
So, I think that using hydrazine salts instead of free base hydrazine certainly is an option. I did not know of the deep red selenium compound and
this certainly is an interesting area of research into which I will go somewhat deeper. I'm not sure about the nature of the deep red selenium
compound, but I think it is a polyselenide. The colorless gas most likely is nitrogen and then the reaction would be something like the following:
2n Se + N<sub>2</sub>H<sub>4</sub> + 4OH<sup>-</sup> --> 2 Se<sub>n</sub><sup>2-</sup>
+ N<sub>2</sub> + 4H<sub>2</sub>O
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woelen
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I have had the red solution standing in contact with air for one day. A very thin dark layer of very fine crystals has formed on the surface of the
liquid. The entire surface is covered by this thin "crust". The liquid itself has become somewhat lighter in color. This is another indication (but no
proof) that selenium indeed is dissolved as polyselenide in the red liquid. Oxygen from the air can oxidize the polyselenide to elemental selenium.
The oxygen only is present at the surface of the liquid, so this explains the presence of the thin dark layer.
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turd
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Alkyl bromides are easily made from alcohol, NaBr and H2SO4, though this needs a classic organic chemistry setup (distillation setup with column,
etc...) and probably makes not much sense on a micro scale.
According to the paper, concentrated H2N4 solutions are not needed. If I read it right, in one experiment they use 30 ml water and 3 ml hydrazine
hydrate, corresponding to an approximately10% H2N4 solution.
I'm quite sure that the redox reaction you propose is correct, since this is the classic reduction with hydrazine. After adding the alkyl bromide
there was practically no more gas evolution, making me believe that after heating at 85°C for 1 h practically all Se was reduced to the diselenide.
After the reaction when I poured aliquots of the aqueous phase into aqua regia, there was massive evolution of a colourless gas after a few seconds of
incubation period, probably due to decomposition of excess hydrazine. This raises the question why the reduction stops at the diselenide level. Maybe
due to the stoechiometric amount of NaOH? Unfortunately I don't have access to the other articles of the same authors.
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woelen
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I'm more into inorganic chemistry, so making PrBr is not my first thing to do, but I have done a little more selenium chemistry experiments.
I noticed that selenium also dissolves in a solution of sodium sulfide. In such a solution, the selenium dissolves and a beautiful red solution is
obtained with an intense color. I tried the sulfide, just to check whether selenium forms polyselenosulfides, just as sulphur forms polysulfides with
sulfide.
S<sup>2-</sup> + n Se --> SSe<sub>n</sub><sup>2-</sup>
This experiment again confirms that selenium can form polyselenide compounds, this time with sulphur.
It is remarkable that my book Chemistry of the elements from Greenwood and Earnshaw does not mention at all the existence of such sulphur/selenium
compounds. Apparently only very few people have taken the effort to do more research on this kind of compounds.
[Edited on 13-2-10 by woelen]
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Nicodem
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That is an interesting experiment about Se dissolving in aqueous sodium sulfide. There are relatively few R-Se-S-R compounds to be found in the
scientific literature. It would be interesting to see if it would be possible to get them by adding some organic electrophiles to that solution.
However, I'm not convinced there is much plain SSe<sup>2-</sup> ions there even when you take a 1:1 stoichiometry. You can have a complex
mixture of di- and polysulfide, di- and polyselenide, with various selenosulfides in there. I think you would get a rapid equilibration of all species
into a thermodinamic equilibrium (therefore a complex mixture with few components prevailing). Something like in a solution of polysulfides where you
have all the S<sub>n</sub><sup>2-</sup> anions with varying values of n distributed according to redox potentials. For
example, in a solution of Na<sub>2</sub>S<sub>2</sub> you have S<sub>n</sub><sup>2-</sup> anions with
n from 1 to whatever (and also the HS<sub>n</sub><sup>-</sup>, etc.). One way to check this would be polarography and IR
spectroscopy. It would be interesting to see if the shape of the Se-Se, S-Se and S-S regions in IR. I bet they would be wide continuous peaks of weird
shapes (like the H-bond regions usually are). Capturing all the species with some very reactive electrophile like dimethyl sulfate and analysing with
GC-MS would be another way, but the kinetics of selenosulfides equilibration would scramble such results to some degree, so this could only be used as
further but nonconclusive evidence of the nature of chemical species involved. Just an idea for a scientific paper if there are any takers.
Edit: Wrote polarimetry instead polarography.
[Edited on 15/2/2010 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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turd
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The authors in the reference I posted tried to make mixed dichalcogenides. Et2SSe formed readily, whereas Et2STe and Et2SeTe were only obtained as
minor products.
A project on mixed S/Se/Te polychalcogenides would indeed be very interesting. Se/Te are very usable for NMR studies, this could be complemented by
X-ray diffraction data and of course vibrational spectroscopy. Unfortunately you would have to do this under disguise of some CdSe/CdTe nanoparticle
nonsense studies. Real chemistry seems not to be wanted anymore. 
My copy of Holleman/Wiberg (101st ed.) contains a short paragraph on polyselenides, but mostly crystallographic data and solutions in NH3 and DMF. No
information on aqueous solutions. It says (to be taken with a grain of salt, HW is known to contain outdated information):
* Se(2-) is red
* Se(2-) does not react in water with Se8 to Sen(2-), but does react in NH3 and DMF
* Se2(2-)-Se7(2-) form helical chains. >=Se8(2-) form cyclic systems (I guess thats from crystals?)
* Solutions in NH3 are: Se2(2-) and Se3(2-): red, Se4(2-): green, Se4(2-) and Se5(2-): red/green (whatever that looks like).
[Edited on 14-2-2010 by turd]
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woelen
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Se(2-) is not red, it is colorless. I have pure CdSe and when this is dissolved in aqueous HCl, then a colorless solution is obtained (if the solution
is kept free from oxygen by adding a small amount of Na2S also). So, the book of Holleman/Wiberg is wrong on this one. Higher selenides, e.g. Se2(2-)
of course probably are red, as the experiment with hydrazine shows.
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turd
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Yeah, Na2Se solutions definitely are colourless and they do dissolve (amorphous) Se.
The NMR did a short meditation on the protons of my sample. There are four propyl species inside and practically nothing else. The main species
(presumably the diselenide) is exactly 75%. Most of the rest is probably the selenide, then there is a small amount which compares well to the shifts
given for the bromide and very minor amounts of what could be the triselenide(?). This corresponds very well to the results of the Russian group.
Unfortunately Te does not dissolve in 20% hydrazine (as was also noted by the Russians) and I don't feel like concentrating it to N2H4.H2O. Dissolving
in water with an excess of NaBH4 works quite well. First you get a dark purple solution which clears up and finally becomes colourless. On contact
with air it reoxidizes promptly. But that doesn't help me, as propyl bromide added to such a solution would probably react with excess NaBH4. And I
don't want to work in abs. alcohol and Na, because that means vacuum evaporation of the solvent.
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