evil_lurker
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Photo-Catalytic chlorination of Toluene...
I have done a search on the board and have yet to find any in depth information here on how the process is conducted, however I did find a site that
explains it in a simplified manner.
Suprisingly to me, UV light doesn't actually do anything to the toluene molecule, it is used to break the bonds of the Cl2 molecule. The CL2
molecule is split into two CL "free radicals". For the purposes of this post a free radical will be written with a "¤".
So using the sybol above chlorine reaction is Cl2 --UV---> 2CL¤.
Now what happens when the CL¤ hits a another molecule? Well basically you have non-productive, productive, and useful collisions.
Non productive collisions are when two Cl¤ bump into each other. In this scenario, things are a complete waste because the two free radicals cancel
each other out.
2Cl¤ ---> Cl2
Other non-productive collisions are when a Cl¤ molecule bums into a CL2 molecule. While not productive, nothing is lost or gained.
Cl¤ + Cl2 ----> Cl2 + Cl¤
In a productive collision, the Cl¤ will hit the CH3 side chain of the toluene molecule, knocking off a Hydrogen molecule and leaving a phenylmethyl
radical. The toluene molecule will be represented with a "©-" for this post.
©-CH3 + Cl¤ ----> ©-CH2¤ + HCI
So what happens to the above formed radical? There are three possibilities. On the first, the phenylmethyl radical combines with a chlorine free
radical, which is what you want, but not the reaction you want.
©-CH2¤ + Cl¤ -----> ©-CH2Cl
The reaction you want is where the phenylmethyl radical bumps into an unreacted Chlorine molecule. In this case, the Chlorine molecule binds with the
phenylmethyl radical, and releases another Chlorine radical, which then goes on to react with something else.
©-CH2¤ + Cl2 ----> ©-CH2Cl + Cl¤
There is one reaction you don't want, it forms a side product, and the reaction is terminated. This happens when two phenylmethyl radicals react
with each other.
©-CH2¤ + ©-CH2¤ ----> ©-CH2-2HC-©
Most folks that I know of want to use the chlorinated product to make benzaldehyde. For this, it is neccessary to take the reaction further by adding
on another Cl molecule to the side chain. This is easy as it is conducted in the same way as above, by letting the reaction continue as all the
hydrogen molecules of the side chain can be replaced by Cl.
©-CH2Cl + Cl¤ ----> ©-¤CHCl + HCI
©-¤CHCl + Cl2 ----> ©-CHCl2 + Cl¤
If the reaction is allowed to continue till the trichlroide is formed, then the final product will be benzoic acid.
So how does one build a reactor? The best way would probably be to use a 55 gallon olive drum with a screw on lid as a chlorine resevoir, a glass
flask is set upon a hot plate equipped with a magnetic stirrer on the outside of the drum with supply and return lines to the top and bottom of the
drum respectively, an aquarium pump is used to pump the chlorine into the lighted CL2 reactor, and the free radicals are pumped into the flask.
I'll try to get some plans drawn up soon and post them.
Some neccessities:
1. The reaction container needs to be inert material as iron or aluminum will contribute to ring chain chlorination. A 1-2 liter ehrlenmeyer
(spelling?) flask would be perfect.
2. The best light source is that producing the blue spectrum around 420nm. Actinic power compacts used in the aquarium industry would be well suited
for this, as they run cool and emit light of the proper spectrum. Prices for a 55 watt light runs under $75. Sticking the light bulb into some PVC
pipe (think light bulb in a condenser) immediatly before the flask comes to mind.
3. The tempurature of the toluene needs to be 80C+ preferably 100-110C to prevent ring chlorination. The hot toluene will probably want to evaporate,
thus making a small condenser on the exit of the flask would take care of that.
Anyhoos, hopefully this will give some of ya'll some ideas.
But what about the chlorine? The best way would be to make it chemically, then to purify it via distillation. Ok, that is crazy... or is it.
Some air compressors use filters that are threaded. If one takes off the filter and replaces it with pipe, then one can compress the crude chlorine
gas. Of course, this is NOT good for the pump but hey, ya gotta do what ya gotta do (best to get one with an extended warranty  ).
Anyways, Chlorine gas is liquified at approx 86 PSI at 70F. By re-routing the compressor outlet to a portable air tank, and then packing that tank in
ice, the condensation pressure will be much lower.
The crude gas produced is routed into a water trap/condenser, then into 55 gallon trash bags via PVC pipe. Once they are nearly full, the compressor
is kicked on and the tank filled up to 125 PSI. After the CL2 liquifies, the pressure should drop. The trick then is to then vent off some of the gas
impurities, and then repeat the process. Viola, after numerous compression cycles, ya got your liquified CL2.
Anyhoos, just to avoid flaming, there is a lot of finer details I have left out and some of the proofing/spelling is off. I'm kinda busy at the
moment with other things, so don't flame me too bad.
[Edited on 13-5-2005 by evil_lurker]
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12AX7
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Is compression really necessary? What happens to the aluminum and iron parts? (I'll forget about the later consequences of ring leakage
corroding the crankcase with accumulated moisture, as this is hopefully a short one-time procedure  ).
Tim
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unionised
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You don't need to store the chlorine and storing it (by compression or otherwise) would be difficult and dangerous. Make it as you use it. If you
make it from HCl and KMnO4 or MnO2 it will be fairly clean. The major impurities will be water and HCl. The HCl won't matter since it's a
product of the reaction you want. You could take out the water with CaCl2.
Don't forget that, as HCl is one of the products you will need some sort of trap/ scrubber for the off-gasses.
A mercury lamp is traditional for photochemistry. Medium pressure at 366 nM or low pressure at 254 would both work but the toluene will absorb a lot
at 254.
Overall this is a fairly complex project, with lots of opportunities to go dangerously wrong.
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evil_lurker
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Actually the Mn02 is a good way to release Cl2 gas from a generator. However instead of using hydrochloric acid, one uses a solution of NaCl and
H2S04. In this reaction, sulfuric acid is added to a mixture of table salt and manganese dioxide. The HCl that is formed from NaCl and sulfuric acid
is immediately oxidized to chlorine, and the unreacted sulfuric acid should act to absorb any water formed.
The reaction is:
MnO2 + 2Cl- + 4H+ -----> Mn2+ + Cl2^ + 2H2O
I'm not so sure of the mechanics of this one, so I don't really know what the final products are, but it would be great to be able to
recycle the MnO2. I'm guessing that the end products are NaS04, and Mn. If that is the case, then the NaS04 should be water soluble, and the Mn
insoluble so a good washing and filtering would get most of the catalyst back, and upon drying and exposing to moisture in the air, then subsequently
heating the product to drive off the intermediate formed you would end back up with MnO2.
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12AX7
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Well, MnSO4 (or other valance?) is a stable, existant salt, so..
Tim
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Organikum
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In situ chlorination of toluene with hypochlorites or TCCA is covered elsewhere on this board.
Works well.
For chlorination up to benzalchloride a two step procedure is advised: First to benzylchloride, separation and then this to benzalchloride. Or using
an inert solvent.
(Just because the amount of hypo/TCCA gets to large in relation to the compound to chlorinate)
Hypochlorites in the dark, TCCA with light.
Both reactions are not for the unexperienced as there is a certain risks of explosion.
Benzal and benzylchloride are unhealthy compounds.
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evil_lurker
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I was unaware that anyone actually got the TCCA halogenation to work...
Anyhoos, your suggestion about separating the formed benzylchloride is duly noted. I was originally hoping for a "one pot synthesis" wherein
the reactor would not have to be opened due to the lachrymatic properties of the product formed. However, by only reacting part of the toluene, it
will reduce the formation of di- and tri- products. And, if I am also understanding correctly, that by heating the formed benzylchloride with lead
nitrate it will reduce it down to the benzaldehyde without having to add on another chlorine molecule.
Anyone have any information regarding this?
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Nicodem
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| Quote: | | I was unaware that anyone actually got the TCCA halogenation to work... |
Off course somebody had it work. It is even reported in a scientific paper:
Juenge, Beal, Duncan. Chlorination of Aromatic Systems with Trichloroisocyanuric Acid under Polar and Free-Radical Conditions.
The Journal of Organic Chemistry Vol. 56, No. 3, March 1970.
But I'm sure there are already threads on this forum discusing this reaction.
…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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evil_lurker
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I meant "anyone" as defined as a person who visits this board. 
It would be very helpful if somone who has free access to the article were to post it on this board.
My budget is tight at the moment, and I can't really spare the $25 to purchase the article on something that may/may not be practical.
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Organikum
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I didn´t get it working first, with light and a little heating, I got it working with better equipment, at 80°C+ and a halogen burner (150W) as
light source.
3-neck flask with efficient reflux condensor. TCCA is added in chunks during the reaction. Adding all at once causes a runaway near to an explosion.
The addition of chunks of TCCA to a boiling mixture of toluene with a unshielded halogen burner nearby is NOT trivial.
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S.C. Wack
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The article.
Attachment: joc_35_719_1970.pdf (634kB)
This file has been downloaded 1168 times
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