fractional
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Chloral, a success, but...
I synthesised chloral using the procedures outlined in Cumming, Hopper, Wheeler "Systematic Organic Chemistry", on LamdaSyn (in German) and also on
this board, by bubbling chlorine-gas through ethanol (99%) under reflux. The reaction-mixture was kept in an ice-bath for about 45 min and then slowly
heated to boiling-point, where it was kept for about 9 hours more, while continuously adding chlorine.
The overall reaction is CH3CH2OH + 4 Cl2 --> CCl3CHO + 5 HCl
One mol of Cl2 is used in a first step to yield acetaldehyde (and 2 mol of HCl). Acetaldehyde is then chlorinated to get chloral and 3 mol of HCl.
Actually, the equation above is incomplete: one more mol of ethanol is needed because an alcoholate is formed of structure CCl3CH(OH)OCH2CH3. An equal
volume of concentrated sulphuric acid (according to Cumming) must be added to split it into chloral and ethanol.
This mixture was then destilled over a short fractioning column. The fraction boiling between 92 and 98 degC was retained. This raw chloral would be
mixed with 1/5 of its weight of H2O to get solid chloralhydrate, which would be re-crystallised from chloroform. I kept just the chloral for the time
being.
So, theoretically, in order to get 1 mol (147 g) of chloral you need 2 mol (92 g) of ethanol and 4 mol (284g) of chlorine.
It is well known that this procedure has a very poor yield and needs a lot (!) of chlorine (plus a lot of time). My yield from 1 mol of ethanol was
about 25g of unpurified chloral (LamdaSyn gives a yield of about 33 g for 1 mol of ethanol). For this I used about 3 mol of Cl2 (142g, from TCCA and
HCl).
So, the yield is simply lousy. Ok, the raw products are cheap (HCl, TCCA, ethanol), but you use a real lot of one noxious gas (Cl2) to generate an
even greater lot of another noxious gas (HCl). Until I decided to bubble the gases generated through a large bucket of water I had a "nice" fog of
hydrochloric acid in my garden.
The poor yield is already inherent in the reaction (only three atoms of chlorine end up in the chloral for eight atoms required), but in addition I am
sure that the acetaldehyde-intermediate does not immediately react further, but instead a lot goes off through the condensor. Also, especially later
in the process, more and more chlorine escapes unreacted.
All in all in my opinion this procedure is unsatisfactory.
Still, I want chloralhydrate, because it forms part of a very useful reagent in mycology ("Melzer's reagent") and is hard to get.
I am planning to split up this one-pot procedure into two steps to increase the yield: a.) synthesise acetaldehyde and react further to paraldehyde,
b.) chlorinate to chloral. There exists a procedure for bromal from paraldehyde (Long, Howard, in Organic Synthesis Vol XVII, 1937) which I think can
be adapted for step b, although it still uses elementary bromine.
Another improvement worth a try is to use TCCA in-situ for both the oxidation and the chlorination step, rather than using it just as an external
reagent to generate chlorine-gas.
Thís experiment is going to happen within a week or two. I'll keep you posted.
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evil_lurker
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Welll... your only as good as your equipment.
I've been working on getting some new gassing equipment lately specifically for chlorine and HCl.
Check these babies out:
http://www.kontes.com/html/pg-746035-D1.html
With one of those you can put one of these in them:
http://www.aceglass.com/page.php?page=7202
So what do you get?
An adapter that will turn any regular jointed flask into a gas wash bottle, and if you cap off the vacuum inlet, you get a gassin adapter that will
fit any flask.
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fractional
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Nice stuff, indeed! That's definitely a much better method for introducing a gas in a liquid than the simple one I used. Thanks for the links!
With this you'll get more efficiency out of the process. An additional option that could be tried is reducing the temperature of the cooling water to
e.g. 5 degC or so in order to hold back more of the acetaldehyde. I used a closed loop system that's basically running at room temperature and which
obviously wan't have much effect on the acetaldehyde.
What I want to try, however, is a process that does not work with elementary chlorine at all, e.g. with TCCA as I mentioned before (which does not
solve the HCl-problem, of course).
Hale, in " The Manufacture of Chemicals by Elektrolysis" makes a passing reference to an electrolytic process by which chloral can be produced using
alcohol in the presence of alkali chloride. There is not much detail, just a reference to a German journal from 1894. I am currently trying to get
hold of the original reference. It would be just great if this could be made to work. I'll post when I know more.
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evil_lurker
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Be extremely careful with TCCA in situ!
What'll happen is the HCl formed as a by-product of the reaction will react with the TCCA releasing more CL2 which releases more HCl so on and so
forth speeding up the reaction...
The end result is a runaway!
Also for working with things that have very low boiling points you might want to invest in a dewar condensor, they run around $120 USD from www.unitedglasstech.com
That will keep your CL2 in the reaction flask where it belongs along with everything else, while letting the HCL escape.
Personally for the low boiling stuff I use a 400mm double surface coil condenser I snagged off of Ebay for around $70USD. For cooling I take non-stick
bread loaf pans filled with water and put them in the freezer forming large blocks that weigh around 2kg each. Then I put them in a large cooler with
a recirculating pump. 4 blocks will run about 4-6 hours depending on ambient temperature and other variables such as length of your coolant lines. If
you don't use a cooler you can cut the length of time they will last by 30-50%.
[Edited on 3-10-2006 by evil_lurker]
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fractional
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I agree that the risk of a runaway reaction when using TCCA in-situ must be seriously taken into account when attempting any reaction on a scale
larger than in a test-tube. I will not try anything without having at least a large pot of ice-water at hand. When looking at the complete reactions,
including the production of chlorine the following schemes apply:
a.) 2 CH3CH2OH + 4 Cl2 --> CH3CH2OH + CCl3CHO + 5 HCl
b.) 4/3 TCCA + 4 HCl --> 4/3 CA + 4 Cl2
a.) is the combined oxidation/chlorination to chloral, including 1 extra mol of EtOH for the formation of the alcoholate;
b.) is the production of Cl2 from TCCA and HCl scaled up to generate just the required amount of chlorine.
And as you say reaction a.) will generate more HCl than is consumed in reaction b.) --> runaway
Interestingly it also shows a theoretical possibility of a one-pot reaction with 2 mol EtOH, 4/3 mol TCCA and just a catalytic amount of HCl
just to get the reaction started.
Having said that I think that this is very attractive in theory, but I do not think it's going to work (leaving the risk of a runaway reaction aside
for the moment).
The paper "TCCA: A Safe and Efficient Oxidant", which is frequently cited on this board gives an overview both for oxidations and chlorinations with
TCCA. It does not say a whole lot about oxidations of primary alocohols, but the general conclusion is clear: chlorinations take place under acidic
conditions, and you don't get much more acidic than when the reaction generates even more acid; oxidations require alkaline conditions, and this means
that the HCl must be scavanged somehow, and many of the reactions quoted use pyridine as a scavanger. So the different Ph-regimes make me believe that
this wonderful one-pot reaction is not going to work (although I am definitely going to try).
I did a few quick and dirty tests on a test-tube scale and it showed quite clearly that the results of the reaction strongly depend on the conditions,
e.g. 96% EtOH with TCCA and some (limited) heating led to an exothermic reaction generating at least some acetaldeyde. 99% EtOH with a few drops of
muriatic acid (30%) and TCCA led to a gentler reaction which much less acetaldehyde smell, but I believe some chloral.
So there are a number of parameters worth tweaking: some water in the mixture or not; HCl in solution, or as dry gas, temperature, presence of a
scavanger or not...
So that's my programme set out for the next couple of weeks.
Working with a Dewar and (I guess) liquid nitrogen would surely do the trick of "disciplining" the volatile stuff; that's something I can only dream
of right now, and you are absolutely right: keeping the cooling water really cold for hours is easier said than done and needs a lot of ice!
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12AX7
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Clearly, the trick must be to add a dash of TCCA as you go. Let the residual (or nascent) HCl catalyze it, bit by bit.
Equivalent to slowly bubbling Cl2 into the alcohol, or dripping limited amounts of HCl on TCCA, in your original reaction.
Tim
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fractional
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Yes, this might do the trick, definitely one option I want to try.
Another one I had in mind was to dissolve the TCCA in the EtOH and add it dropwise to a small amount of muriatic acid. If the solubility of TCCA in
ethanol works out at all (I haven't checked yet) it must be done in the cold. Upon heating you get heat + acetaldehyde + possibly all sorts of stuff.
But that might even be option 3, if the yield of acetaldeyde is ok, but this of course makes the overall synthesis of chloral a 2-step reaction.
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