Murexide - 15-12-2018 at 20:10
Chloroform, on the amateur level, is generally prepared from the halo form reaction, most commonly with acetone and sodium hypochlorite. Sodium
hypochlorite can be obtained in concentrations 10-15% (based on Cl) at pool stores in 15L plastic jugs, which is sufficient for general purposes.
The plan is to titrate the hypochlorite using sodium thiosulfate after addition of potassium iodide and acidification.
Then 90% stoichiometric equivalent of acetone can be added (recommendations are 97% due to decomposition of chloroform in basic conditions, but the
15L volume is probably not very reliable and I do not want acetone in my chloroform).
There are two main methods here: Some cool the hypochlorite in a freezer (not possible in my case as chemicals in a household freezer leads to
contamination problems), or cool it in an ice bath (this takes forever, unfortunately, and the volume of ice required is huge)
The alternative method is to empty the hypochlorite into a huge bucket/container, add lots of ice directly. This advantages of this method is it
avoids much of the risk of chloroform boiling, but it reduces yielded at the rate of 10g chlorform/ 1kg ice which is significant. However, this is
likely the method I will attempt.
Acetone can be added dropwise using an addition funnel to the bucket.
Chloroform seems to be able to diffuse through plastic. However, this doesn’t appear to be a problem for those doing the reactions in household
bleach containers so I assume it is safe in plastic jugs too, so long as the a huge amount of water is above the chloroform
Safety_Lucas - 15-12-2018 at 22:10
I don't recommend performing this reaction in a bleach bottle as chloroform readily dissolves HDPE. I know it's been done before, and I myself have
done it before but best practice dictates glass or another suitable material. If this is your first time performing this reaction, I recommend
starting with a smaller batch. (<1L) In larger volumes, it can be difficult to dissipate heat fast enough to keep the chloroform from boiling and
the sodium hypochlorite from decomposing, even with prior chilling. This problem is worsened if plastic is used for the reaction vessel as most
plastics have poor thermal conductivity. I agree with your decision to add the acetone drop-wise as this will greatly reduce the risk of thermal
runaway. (Just be careful to make sure the stopcock is closed before adding the acetone. I know it seems obvious but I forgot that once.
) As for the addition of ice directly to the reaction, this shouldn't decrease yield
as the amount of sodium hypochlorite in solution is the same regardless of dilution. The only caveat I can foresee is that chloroform is slightly
miscible in water so you would likely lose a small amount to that. However, as long as acetone is added slowly and the batch size is small, it should
be practical to cool the reaction indirectly. I would keep ice close by though just in case it is necessary to cool the reaction directly. Of course,
when working with chloroform, it is advisable to have a well ventilated area. Finally, I would recommend you distill and dry the final product as it
will be saturated with water and possibly a small amount of acetone. Chloroform has a beautiful high refractive index but it must be stored in a dark
container free from air as it decomposes in light and oxygen to form highly toxic phosgene gas. If desired, chloroform can be stabilized by the
addition of a small amount of certain alcohols, ethers, or hydrocarbons.
Murexide - 15-12-2018 at 22:57
Thank you for your reply, Safety_Lucas. I am able to obtain soda-lime glass containers 1-4L locally, so that would appear to be a good option.
For the synthesis, some sources suggest adding some NaOH as they claim the reaction to be a net acidified. Other sources suggest that it is a net
basifier, and that the leftover aqueous waste can be left to stand in bleach bottles for a week and the basic conditions will hydrolyse the dissolved
chloroform.
The distillation does seem to be necessary (after MgSO4 or 3A molecular sieves drying), as one of my uses of the chloroform cannot tolerate chloride
contamination within the dissolved water. A hot water bath seems to be the best option, but my only distillation apparatus available is a retort flask
(I am worried that lack of cooling will result in large amounts of chloroform escaping into the air - which is of course undesirable). Standard
procedure is wet cloths around the long tube of the retort leading to flask cooled with ice (successful for distilling 50% AA bp around 100, but not
sure about chloroform BP 60).
Storage should be safe in a small Schott bottle, filled (hopefully) to the top, wrapped in aluminium and stabilised with 1% ethanol (95%, since 100%
does not appear necessary - although it is available with molecular sieves 3A).
Murexide - 15-12-2018 at 23:18
Also, my calculations (based on 50% yield and 1:3 acetone: hypochlorite ratio) puts the yield at an amazing 48mL for 1L of 12.5% hypochlorite based on
chlorine (it says % active chlorine). That seems completely impossible, so either my calculations are wrong, the hypochlorite % is based on NaClO
(much more reasonable yield - guessing around 20mL), or the labelled concentration is wrong (if this is the case, I will find out after titration).
Safety_Lucas - 16-12-2018 at 10:36
The concentration printed on the label is seldom the actual concentration but I suppose that's the purpose of titration. It is not necessary to add
sodium hydroxide as most bleach already contains a significant percentage of it as a stabilizer. As for distillation, a retort should work so long as
you go slow with the heating. Wet rags can help but a flowing stream of cooling water would be better. I don't suppose you have access to a condenser
but you could probably rig something up to keep cool water flowing over the retort stem. Perhaps some sort of ring at the bottom to force the water to
drip off could work. I would only bother drying the chloroform after it's been distilled. Just watch the internal temperature carefully and stop when
it gets above 61°C. Lastly, ethanol cannot be used to stabilize chloroform. You need a larger alcohol.
Murexide - 16-12-2018 at 12:19
The chlorine I am using is pool chlorine and it is claimed to be stabilised with cyanuric acid (and possibly with NaOH, pH 12.5 (1% w/w) is listed so
I’m assuming the 1% is NaOH), although I have seen NaOH used to stabilise many commercial cleaning bleach solutions. Cyanuric acid is normally quite
safe, there is the risk of cyanide formation but it should be minimal since it is insoluble in chloroform.
I do have access to an Ahlin condenser intended for refluxing, but I suppose that will work too for some cooling. It could be attached in a rough fit
to the end of the long retort arm (non-ground glass) sealed by some tape, to a gas adapter (to open the system) and to a ground glass 250mL (gas
adapter used to avoid heating a closed system). I am a bit worried about chloroform gas leaking out of the non-ground glass joint and dissolving the
plastic of the tape, but that shouldn’t be too significant (most of it should be condensed in the air condenser of the retort arm even before it
reaches the condenser.
I have seen ethanol and methanol used on many instances, interestingly. I imagine it would just form the dialkyl carbonate. What would make a larger
alcohol necessary?