Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Chlorination of methyl formate
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 26-12-2006 at 03:43
Chlorination of methyl formate


This stuff is very volatile and low boiling. The classical methods of chlorination are:

a. With elemental chlorine in a UV reactor. There is some explosion hazard attendant to mixtures of Cl2 and methyl formate vapor.

b. SO2Cl2 and dibenzoyl peroxide as initiator. Problem here is that SO2Cl2 is verboten and so I would need to make my own from equally verboten Cl2 and SO2.

Any suggestions on alternative ways of chlorinating the formyl H of a formyl ester?

The other route to the product, methyl chloroformate is to esterify methanol with phosgene. I am not enthusiastic about generating phosgene although it is easy to do.

Suggestions?

In case you are wondering, the product is to be further chlorinated to the trichloromethyl chloroformate which is a very useful reagent for making isocyanates among other things. While it is toxic, it is a high boiling liquid with a relatively low vapor pressure and this is a LOT safer to handle than the very low boiling phosgene (b.p. 0 C). If one must employ phosgene it is generally stored in solution in toluene, or else generated as needed by dropping CCl4 into hot fuming sulfuric acid. Oxidizing chloroform with chromic acid is another method. I do not advise working with it unless there's no other option. Trichloromethyl chloroformate is that other option.

Still another is hexachlorodimethyl carbonate which is made by the UV-mediated chlorination of methyl carbonate. It is solid, less reactive than the others, far less than phosgene.
View user's profile View All Posts By User
quino
Hazard to Self
**




Posts: 91
Registered: 19-11-2006
Member Is Offline

Mood: No Mood

[*] posted on 26-12-2006 at 04:31


Dear Sauron

Please read this
copy pest in address bar

http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6...

and

http://www.springerlink.com/content/vp17120kg48m047u/

ok bye regards
View user's profile View All Posts By User
quino
Hazard to Self
**




Posts: 91
Registered: 19-11-2006
Member Is Offline

Mood: No Mood

[*] posted on 26-12-2006 at 04:34


Title
Chlorination with peroxides as catalysts. IV. Chlorination of methyl formate
Yura, Shozo,
Nippon Kagaku Kaishi (1921-47) (1948), 51(Ind. Chem. Sect.), 157-8

In scattered light, chlorination of HCO2Me stopped with the formation of ClCO2CH2Cl (I), a slight amt. of ClCO2CHCl2 (II) being formed on continued chlorination. In the dark no chlorination occurred, but on addn. of 1% Bz2O2 II was obtained, but decompd. on continued chlorination. In the dark completely dry HCO2Me gave II in the presence of 1% Bz2O2 and ClCO2CCl3 in the presence of Ac2O2.
View user's profile View All Posts By User
quino
Hazard to Self
**




Posts: 91
Registered: 19-11-2006
Member Is Offline

Mood: No Mood

[*] posted on 26-12-2006 at 04:35


Chlorination of methyl formate and methyl chloroformate

Grignard, V.; Rivat, G.; Urbain, Ed.

Compt. rend. (1919), 169, 1074-6

Using Pb chlorination chambers illuminated by powerful incandescent lamps, and a reaction temp. of 140-50° it was found impossible to exceed a Cl content of 65.5%, while the theory for ClCO2CCl3 (A) is 71.7%. The same ultimate products are obtained whether starting with formate (B) or chloroformate (C), C being, according to Hentschel (J. prakt. Chem. [2] 36, 99, 209, 305, 469(1887)), the 1st product of the chlorination of B. However, according to Del.acte.epine (private communication) HCO2CHCl2 (D), an extremely un-stable compd., is formed initially, and G., R., and U. have found that this is entirely transformed into the chloroformate derivs. Contrary to H. it is possible to prep. ClCO2CH2Cl (E) even in weak, diffuse light if one starts with slightly superheated vapor of C (75°); there is a slow initial phase until about 15% of E is formed, after which chlorination proceeds actively up to 75-80% of E, followed by a third phase in which the reaction slows down as it approaches satn. Stronger light must be used to carry the chlorination further, but even then the ultimate product, A, is produced very slowly, similar results being obtained in sunlight as well as with powerful incandescent lamps or an Auer burner. Using ultraviolet light, however, preferably from a quartz Hg-vapor lamp, and working in a round-bottom flask of ordinary white glass (trans-parent to about 0.3 .mu.), products containing 69-71% Cl, or 80-90% A, are readily obtained. If B is used, the chlorination is started at 30° and the temp. is progressively raised, while if C is used it is slightly superheated with the aid of gentle pressure; in both cases the main part of the reaction is carried on as about 80°. Explosions are avoided by not admitting Cl before heating and irradiating the liquid. No advantage is gained by chlorinating at higher temps., and at about 117° COCl2 is evolved owing to partial decompn. according to ClCO2CCl3 .fwdarw. 2COCl2. As catalyzers SO2Cl2, "pyrosulfuryl chloride," FeCl3, SbCl5, CuCl2, and S were tried. With FeCl3 and SbCl5 chlorination took place, followed by decompn. after a certain concn., apparently due to the reaction ClCO2CHCl2 (F) .fwdarw. CO2 + CHCl3. F also resulted when S was used. Although Fe and Pb immersed in the liquid are without action, when suspended in the vapors they give rise to chlorides with marked destructive action, so that a chlorination commenced in Fe or Pb vessels cannot be completed in glass or earthenware, owing to the action of these chlorides in soln. For work on a semi-technical scale the following procedure was used: Cement a glass bell with very thin walls (1.5-2 mm.) into the large opening of an earthenware chlorination jar, forming a shield into which is introduced a Westinghouse-Cooper-Hewitt lamp. The other 2 openings carry the Cl tube and an earthenware or quartz reflux condenser. Products of d. 1.63-1.64, containing about 70% Cl, are easily obtained in this way at the ordinary temp.
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 26-12-2006 at 05:16


Thanks, those are useful and informative.

[Edited on 26-12-2006 by Sauron]
View user's profile View All Posts By User
leu
National Hazard
****




Posts: 368
Registered: 13-10-2005
Member Is Offline

Mood: No Mood

[*] posted on 27-12-2006 at 19:28


The articles abstracted above:

Compt. rend. 169, 1074-7 (1919)

Kogyo Kagaku Zasshi 51; 157-8 (1948)

:cool:

Attachment: C2H3O2Cl.zip (148kB)
This file has been downloaded 559 times





Chemistry is our Covalent Bond
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 27-12-2006 at 22:01


Another free radical chlorinating agent (besides elemental Cl) is SO2Cl2. Besides the classical peroxide initiators AIBN is commonly used, even instructional labs have students perform this chlorination on semicroscale (but not with methyl formate as substrate.)

I am just thinking out loud.

DMF undergoes an interesting reaction with oxalyl chloride forming dimethylammonium chloroformyl chloride. In short, the formyl H of DMF can be chlorinated.

DMF is simply an amide of formic acid.

Methyl formate is simply an ester of formic acid.

So, will oxalyl chloride chlorinate the formyl H of methyl formate?

Methyl chloroformate is just an intermediate to be free-radical chlorinated to the perchloro ester (trichloromethyl chloroformate, "diphosgene"). This is very useful for some classes of work as enumerated in Org.Syn., Feiser etc. Basically it is a safer replacement for phosgene in the lab. It has a low vapor pressure.

In some notes I made I have a reaction which I found but did not reference, probably I have the journal article for it on hard disk somewhere.

Dimethyl carbonate + PCl5 = methyl chloroformate + POCl3 + MeCl. The reaction requires some heating.

Problem here of course is the PCl5 unless one has enough red P (or as in my case any red P).

Dimethyl carbonate slowly perchlorinates under UV with Cl2 to hexachlorodimethyl carbonate "triphosgene" which is a solid. It is also a useful phosgene substitute and unlike diphosgene it is not a CWC scheduled compound. It is however far less ractive than diphosgene and far, far, far less than phosgene.

It is commercially available but bloody expensive.

FWIW I do not recommend anyone work with any of these without a good efficient hood, using a caustic scrubber as well.
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 28-12-2006 at 04:04


@ quino

That Springerlink, concerning zeolite Y catalyzed chlorination of formaldehyde. In abstract it states that methyl formate was an unchlorinated byprouct. The products were chloromethane and dichloromethane. So of little use to me in this particular scheme.

But thanks anyway.
View user's profile View All Posts By User
leu
National Hazard
****




Posts: 368
Registered: 13-10-2005
Member Is Offline

Mood: No Mood

[*] posted on 28-12-2006 at 17:03


Two more publications on chlorination of methyl formate:

J prakt Chem 38, 209-15 (1887)

and

DE297933

:D

Attachment: C2H3O2Cl!.zip (142kB)
This file has been downloaded 576 times





Chemistry is our Covalent Bond
View user's profile View All Posts By User
leu
National Hazard
****




Posts: 368
Registered: 13-10-2005
Member Is Offline

Mood: No Mood

[*] posted on 3-1-2007 at 20:44


Diphosgene can be made from triphosgene instead of from methyl formate or methyl chloroformate.

J prakt Chem 7 101-119 (1873)

:D

Attachment: geuther.zip (608kB)
This file has been downloaded 532 times





Chemistry is our Covalent Bond
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 3-1-2007 at 21:20


Thanks! although triphosgene is a bitch to make. It is commercially available but horribly expensive. It is solid, almost nonvolatile and unrestricted. Diphosgene is liquid, not very volatile, but restricted.

So I thank you for this procedure but it is the long way round the block. The chlorination in UV of dimethyl carbonate is long and tedious and doubtless wasteful of chlorine.
View user's profile View All Posts By User
garage chemist
chemical wizard
*****




Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline

Mood: No Mood

[*] posted on 9-5-2007 at 13:11


I have thought about the possible uses of methyl chloroformate. In Ullmann, it is stated that chloroformates have the properties of acyl chlorides, decomposing to HCl with water. Methyl chloroformate and carboxylic acids are said to give carboxylic anhydrides, for which a JACS reference is given:
E.Schipper, J.Nichols, JACS 80 (1958) 5714

Also, methyl chloroformate reacts with methanol at the boiling point to give dimethyl carbonate, providing a phosgene-free method for the synthesis of this compound.
Dimethyl carbonate is of low toxicity and is useful as a methylating agent, despite its much lower reactivity than e.g. dimethyl sulfate.

Methyl chloroformate is the first chlorination product of methyl formate, the chlorination best being carried out in the gas phase as indicated in the german Hentschel article from 1887 that leu posted as an attachment.
So methyl formate is boiled under reflux while chlorine is slowly intruduced into the gas phase. The product mixture is fractionated later, methyl chloroformate boiling at 71°C.

An excess of methyl formate over chlorine should be used, in order to avoid chlorination of the methyl group. Also, the chlorine must be introduced very slowly as sudden decomposition with the production of soot may happen when chlorine is allowed to accumulate in the reaction mix.

[Edited on 9-5-2007 by garage chemist]




www.versuchschemie.de
Das aktivste deutsche Chemieforum!
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 9-5-2007 at 14:00


All of the 20th century literature on chlorination of methyl formate indicates that methyl formate vapor forms explosive mixtures with chlorine.

Hence this chlorination is carried out in the cold, UV irradiation is commenced BEFORE chlorine is introduced, and the rate of chlorination is regulated so that chlorine does not accumulate either in the liquid phase (as indicated by absence of yellow-green color) or in the headspace. Very efficient cooling is necessary to reflux the methyl formate because it is extremely volatile and otherwise much is lost. The reaction is exothermic, and the Hg high pressure lamps always introduce some heat.

Dimethyl carbonate is cheap and readily available. Is it not so in Germany?

Methyl chloroformate is very valuable as a feedstock for perchlorination to the very versatile trichloromethyl chloroformate (diphosgene) and I would not advise squandering it on making the carbonate, nor on making acid anhydrides. It is also pretty damned toxic, much more so than diphosgene. See THE WAR GASES; also the Org.Syn. prep of trichloromethyl chloroformate.

The toxicity issue is manageable with a good hood. What bothers me is the prospect of blowing up a $2000 photochemical reactor and scattering some chloroformates around beyond the hood into the bargain.

The chlorination is a long slow tedious one and those are bad parameters for a hazardous procedure. It's a white knuckler. And that is why I am interested in alternatives.

Even making the chloroformate from phosgene and methanol is perhaps less risky. Assuming a good hood of course. No explosion hazard, and the reaction does not take long.

If the target is diphosgene, the chlorination of methyl chloroformate (obtained from methanol and phosgene) takes much less time than starting with the formate, and lacks the explosion hazard.

I believe I posted a JACS article on chlorination of methyl formate at the start of this thread. Don't see it now though, so here it is.

Incidentally other chloroformates are commercially available; only methyl chloroformate (and the chloromethylated homologs)is a CWC listed compound. Ethyl chloroformate, and other alkyl chloroformates are available. Benzyl chloroformate is a very widely used N-protecting group reagent in peptide synthesis, the famous Z reagent of Max Bergmann and Emile Zervas. Any of those should give same dehydrating reaction that you are referring to.



[Edited on 10-5-2007 by Sauron]

Attachment: ja01328a022[1].pdf (434kB)
This file has been downloaded 776 times

View user's profile View All Posts By User
garage chemist
chemical wizard
*****




Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline

Mood: No Mood

[*] posted on 9-5-2007 at 17:21


Dimethyl carbonate should be cheap, but I am interested more in its synthesis and less in the product itself.
Methyl chloroformate would indeed be a valuable raw material for both diphosgene and acyl chlorides/anhydrides but if you were prepared to work with phosgene, then you could just use that for chlorinations as it is an acyl chloride itself.

A problem is also that there is no good laboratory preparation of phosgene. The CCl4/oleum method is wasteful of precious chemicals. CO+Cl2 requires complicated apparatus. Do you know of a good method to generate phosgene in the lab?
I have heard of "phosgenation cartridges", those produce phosgene by decomposition of triphosgene. Really expensive.




www.versuchschemie.de
Das aktivste deutsche Chemieforum!
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 9-5-2007 at 20:43


The phosgenation cartridges are very small submolar scale and expensive, all they are is triphosgene and a catalyst in a delivery system, just apply heat.

I think the CCl4 method also works with conc H2SO4, see the older lit on this.

And the alternative that works just as well is chromic acid and chloroform. Chloroform at least had advantage of being more easily made than CCl4.

If you can buy benzyl chloroformate it is a convenient source of phosgene by thermal breakdown. In fact that is a problem with it in storage.

Ullmann's contains some refs to non phosgene preps of alkyl chloroformates, one is Houben-Wyl, which I have no access to. The damned refs are interesting but inaccessible for me.

"Chloroformic esters can also be made by reacting alcohols with carbon monoxide and chlorine under pressure [22] , by chlorinating formates or thioesters, by reacting phosgene with aldehydes, or by reacting phosphorus pentachloride with cyclic carbonates [2] , [4]."

[22] Ube Industries, JP-Kokai 54/61 121, 1979 (S. Nakatomi, Y. Kamitoku). (I think this is a Japanese patent.)

[2] Houben-Weyl, 8, 101 – 105.Houben-Weyl, E4, 15 – 28.

[4] R. D. Concez, Inf. Chem. 106 (1972) 139.

Basically this dilemna with methyl chloroformate is why I started this thread and so far there's no way out. Let's find one together.

1. The proposition that a cyclic carbonate (like ethylene carbonate) will react with PCl5 to give ethyl chloroformate is interesting because it opens up the possibility that oxalyl chloride or TCT might do the same job.

2. If alcohols react with CO and Cl2 under pressure to give chloroformates, how about saturating or supersaturating methanol in the cold with CO then introducing Cl2? I guess the trick is to keep phosgene in excess, otherwise you just make carbonate.

3. Chlorination of thioesters? This is intriguing.

Me-C(=O)-S-Me is this what they mean?

I;m a little vague about this; is the proposition that S-Me is a good LG, perhaps to dimerize to the sulfide Me-S-S-Me?

Worth looking into.

[Edited on 11-5-2007 by Sauron]
View user's profile View All Posts By User
smuv
International Hazard
*****




Posts: 842
Registered: 2-5-2007
Member Is Offline

Mood: Jingoistic

[*] posted on 11-5-2007 at 21:06


would potassium permanganate or chromic acid oxidize tetrachloroethylene to phosgene? Also, I think I remember reading trichloroethylene had some problems with stabalization, maybe like chloroform it too decomposes to phosgene. Of course there is nothing wrong with the standard methods, it just might be nice to use readily available chemicals.
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 11-5-2007 at 21:58


"Readily available chemicals" has no fixed meaning, it varies from place to place and time to time.

@G C objected to CCl4/fuming sulfuric acid on the grounds that both are too precious. Actally Vogel calls for 100% H2SO4 @120-130 C and not fuming sulfuric acid. The poin is still valid re CCl4.

I already posted about chromic acid/CHCl3, I assume you are right, but here both are unavailable. CHCl3 at least can be made from acetone and hypochlorite. CCl4 is more difficult to make and if I were to do so I wouldn't make phosgene out of it.

CO is readily generated and so is Cl2.

Vogel recommends hot (70-80 C) conc sulfuric acid to turn a drip of formic acid into CO. Brauer recommends conc phosphoric acid at 80 C, both are cheap.and available. I hardly need to tell anyone here how to generate Cl;2 do I?

Mix themand pass through a cooled column of granular AC and you have phosgene (see Sartori's book THE WAR GASES in forum library.) Bubble it into toluene, if I remember correctly a liter of toluene holds about 12.5% by weight phosgene at RT. Now drip in dry methanol slowly with stirring and cooling, so that phosgene is always in excess. Get the stoichiometry right (by weighing the phosgene, and not adding too much methanol) and you have a soln of several hundred g methyl chloroformate in toluene. Do I need to remind that all of this needs to be done in a good hood? CO, Cl2, phosgene and methyl chloroformate are ALL toxic, the phosgene is the worst and most insidious of the lot. Org.Syn. gives directions on how to decontaminate glassware afterwards, Also how to get rid of unreacted phosgene from the generator and the toluene solution. Exercise extreme caution, pulmonary edema is no fun, death even less so.


Note: solubility of phosgene in toluene at 25 C is 67% w/w (1 g phosgene in 1.5 g toluene); the 12.5% w/w soln was mentioned by Vogel as commercially available in glass ampules. I would not recommend storing phosgene, I would advise reacting it in situ, and destroying any excess with 20% NaOH soln also useful for decontaminating all glassware involved, which otherwise remains coated with a film of phosgene for some time.

[Edited on 13-5-2007 by Sauron]
View user's profile View All Posts By User
smuv
International Hazard
*****




Posts: 842
Registered: 2-5-2007
Member Is Offline

Mood: Jingoistic

[*] posted on 11-5-2007 at 22:09


Have you checked aerosol degreasers/ brake cleaners?
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 11-5-2007 at 23:46


I'm not really desperate for a little trichloroethane.

I know how to nake CHCl3 and CCl4 and will do so when I have need of them.

The stoichiometry of the prep of phosgene from CCl4 and 100% sulfuric acid is:

3 CCl4 + 2 H2SO4 -> 3 C(=O)Cl2 + 4 HCl + S2O5Cl2

which is interesting because pyrosulfuryl chloride is a byproduct, the phosgene and HCl being gases pass over to the receiver. That's a nice little dividend.

I have not worked out the equation for chloroform and chromic acid yet.

[Edited on 13-5-2007 by Sauron]
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 19-4-2008 at 05:42


Breakthrough!

The problem I've been struggling with has apparently long ago been solved.

Let me restate it:

Methyl formate is hazardous to chlorinate under UV irradiation. Methyl chloroformate, less so.

Methyl formate is easy to make and easy to buy. Methyl chloroformate is made by reaction of phosgene with methanol, the phosgene being kept in excess to avoid side reaction forming dimethyl carbonate.

So I have been looking for ways to make methyl chloroformate without having to employ phosgene.

It turns out that methyl formate reacts readily in the dark with chlorine, to form methyl chloroformate. This is stated unequivocally in the paper "Superpallite" (J.Physical Chem., 23, 498-512 (1919). Thus we conclude that the formyl hydrogen is readily replaced by chlorine. Photolysis is not required; radical initiators are not required. The hazard of a chlorine-methyl formate vapor mixture exploding under UV irradiation is sidestepped, because no irradiation is employed. (Keep the reaction out of direct sunlight!)

No phosgene generator needed; no toluene solution of phosgene needed. No methanol needed. Just chlorinate methyl formate, and collect methyl chloroformate.

Before tossing any confetti, let me say this: methyl chloroformate is no picnic. Anyone contemplating making it, is advised to use a very efficient fume hood, wear protective apparel, etc and seriously consider the use of a SCBA. I would distill it directly into the flask in which it will be chlorinated under UV and follow all the precautions described in the Org.Syn. prep of trichloromethyl chloroformate. That ed product, too, is not a walk in the park with your favorite girl.

The article I cited is available in References.

[Edited on 19-4-2008 by Sauron]




Sic gorgeamus a los subjectatus nunc.
View user's profile View All Posts By User
Sauron
International Hazard
*****




Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline

Mood: metastable

[*] posted on 20-4-2008 at 08:36


Proposed Protocol for Chlorination of Methyl Formate "in the Dark"

MeOC(O)H + Cl2 -> MeOC(O)Cl + HCl

There are (at least) three ways to directly chlorinate methyl formate to methyl chloroformate.

1. Chlorination under UV as described in JACS 55 p.214 (1933)
2. Chlorination with radical initiator acetyl peroxide (Nippon Kagaku Kaishi (1921-47) (1948), 51(Ind. Chem. Sect.), 157-8).
3. Chlorination in absence of strong light (mentioned in J.Phys.Chem 23, 498 (1919).)

The first method is hazardous. The second is complex and relatively low yielding. And in both cases the reaction can proceed beyond methyl chloroformate to the three chloromethyl chloroformates.

For these reasons methyl chloroformate per se is usually prepared from action of methanol on a toluene solution of phosgene, the phosgene being maintained in excess.

However, laboratory manipulation of phosgene is inherently very dangerous.

Therefore I propose to employ direct chlorination without photolysis or radical initiation (methos 3).

In order to minimize the risk of forming explosive mixtures of Cl2 and MeOC(=O)H, the chlorination of methyl formate is conducted in absence of strong light or UV irradiation, that is, out of direct sunlight. In order to avoid exposure to the lachrymatory nature of methyl chloroformate and the more toxic contaminant chloromethyl chloroformate, formed under these conditions to only a small extent, the preparation is carried out in a large glove box and on a relatively small scale (here, 2 mols, 120 g methyl formate, 132 ml.)

A 250 ml jacketed three necked, round bottom flask is equipped with a gas inlet tube, magnetic stirring bar and a dry ice-acetone refluc ondenser closed with a drying tube. A thermocouple probe is set below liquid level and connected to a digital thermometer. The apparatus is situated atop a digital balance so that the added chlorine can be weighed conveniently. The flask's cooling jacket is connected through the glove box tap to a recirculating chiller thermostatically regulated at 30 C.

The flask is charged with 120 g methyl formate, cooling and stirring commenced, and chlorine is passed in from a gas generator charged with TCCA and a dropping funnel with half-strength concentrated HCl prepared by mixing conc HCl with an equal volume of water. Chlorine exiting the generator is scrubbed through a wash bottle loaded with anhydrous calcium carbonate. The rate of chlorine generation and addition is kept slow at first so that chlorine does not accumulate in the flask headspace but is efficiently absorbed by the methyl formate. When approximately one mol Cl2 (70 g) has been added, cooling is discontinued and the temperature is allowed to rise to 70-80 C gradually, the mixture refluxing the while. When approx. 140 g Cl2 (2 mols) has been added, as evidenced by c.69 g weight gain, the reflux temperature should be 77 C corresponding to methyl chloroformate. The chlorination is continued until a definite persistent yellow-green color persists, indicative of no more chlorine absorption. The chlorine generation can be temporarily halted by bypassing the gas into the scrubber, while stirring is continued. If the color diminishes, chlorine addition is resumed until the color becomes permanent.

The gas generator is then shut down, stirring discontinued, etc.

The contents of the flask consisting of methyl chloroformate, a small amount of chloromethyl chloroformate, unreacted chlorine and hydrogen chloride, are subjected to aspirator vacuum to remove Cl2 and HCl as much as possible and then are fractionated at normal pressure. After residual gases pass off, methyl chloroformate us collected at 77 C leaving a small residue of chloromethyl chloroformate, which is left in pot for a subsequent run.

To reiterate, methyl chloroformate is a commercially available reagent widely used in academic and commercial laboratories. It is a lachrymator/irritant. The phosgene-related effects of chloroformates only appear when the ester methyl group is partially chlorinated or perchlorinated. In the course of chlorination from -OCH3 -> -OCH2Cl -> -OCHCl2 -> -OCCl3, the irritant and lachrymatory properties diminish while the phosgene-like toxicity increases. However, even the trichloromethyl chloroformate is a far safer reagent in the lab than phosgene itself.

It is my view that methyl chloroformate prepared and purified in a glove box as described, is very effectively contained.

The subsequent utilization of methyl chloroformate in the Org.Syn. procedure for UV chlorination to the perchloromethyl chloroformate when carried out in a good fume hood is also a safe method of obtaining this useful reagent.

An alternative is the relatively longer chlorination of dimethyl carbonate to hexachlorodimethyl carbonate (triphosgene) which is a nonvolatile solid.

As mentioned upthread by leu, this can be decomposed to diphosgene (trichloromethyl chloroformate) easily enough but leu failed to mention that the other decomposition fragment is phosgene itself.

Which is what we are trying to avoid even using in situ.

[Edited on 21-4-2008 by Sauron]




Sic gorgeamus a los subjectatus nunc.
View user's profile View All Posts By User

  Go To Top