Sciencemadness Discussion Board

Chlorination of dioxane, what's the mechanism?

kristofvagyok - 7-12-2012 at 18:12

Hi,

I do not often ask, but now I have a little question mark. Today I have done the chlorination of dioxane to 2,3-dichloro-dioxane with a little catalytic iodine after a recipe from Organic Syntheses, Coll. Vol. 8, p.161 (1993); Vol. 65, p.68 (1987).
Quote:

A. (1) Preparation of trans-2,3-dichloro-1,4-dioxane. (See (Note 1).) To a 2-L, three-necked, round-bottomed flask, equipped with two inlet tubes (with sintered-glass diffusers at the end) connected to a chlorine cylinder, and a reflux condenser connected to an outlet tube immersed in a potassium hydroxide solution, are added 1200 g (13.64 mol) of anhydrous dioxane (free of peroxides!) and 8 g (0.03 mol) of iodine. A stream of chlorine is passed through the dioxane/iodine solution heated at 90°C, and the reaction is monitored by NMR spectroscopy. After 9 hr, the conversion is 50% complete (Note 2); after 33 hr, about 90% complete. At this point, the stream of chlorine is interrupted. Reinitiation of the chlorine stream after some hours (next morning, for example) may be dangerous because it was observed in one case the mixture inflamed spontaneously! The reaction mixture is allowed to cool to room temperature, 500 mL of ether is added, and the solution is washed with aqueous sodium thiosulfate solution. The organic layer is separated, dried over sodium sulfate, the ether is evaporated under reduced pressure, and the residue is distilled through a 20-cm Vigreux column, to yield 1200–1300 g of trans-2,3-dichloro-1,4-dioxane, bp 89°C/16 mm (lit.1 bp 82.5°C/14 mm; mp 31°C) (Note 3).

I have worked with 1/10 of that amounts, freshly absolutized dioxane, ultrapure iodine ect. and passed through dry chlorine gas for 4 hours (from Cl cylinder), everything goes okay and currently the 40% of the dioxane have been converted to 2,3-dichloro-dioxane, but the interesting part comes here:

based on NMR data there is NO polychlorinated of monochlorinated compounds in the rxn mixture, just non reacted dioxane and the preferred compound. How? Do anyone knows a mechanism for this reaction?

UnintentionalChaos - 7-12-2012 at 23:23

This is quite an interesting reaction. Based on the trans geometry, I expect a halonium ion intermediate, but how it forms is beyond me.

Iodine monochloride is also probably implicated.

Nicodem - 12-12-2012 at 08:02

Ethers are easily alpha-halogenated, either electrophilically or radicaly (in the electrophilic attack the O-chloro product is the kinetic one, but this can probably rearrange to the C-chloro ether; radical chlorination of ethers proceeds via the usual radical chain mechanism).

The thus formed 2-chloro-1,4-dioxane can undergo easy reversible elimination, either E2 (close to impossible under the given reaction conditions) or E1 (soft acid catalysis by iodine?). The E1 elimination facilitated by the very high stability of the carbocation (-O-CH+- <=> -O+=CH-). The elimination/addition equilibria is fast and the elimination product is getting consumed all the time (thus explaining the absence of 2-chloro-1,4-dioxane in the product).

The so formed 2,3-dihydro-1,4-dioxine is an electron rich alkene and should be way more easily chlorinated than 1,4-dioxane itself (again explaining its absence in the product). By the electrophilic addition of Cl2 the trans-dichloro dioxane forms. The inductive effect of the additional chlorine makes the further E1 elimination much less likely (explaining lack of polychlorinated products), but I'm nevertheless surprised at the selectivity.

See also DOI: 10.1021/ja01315a009

kmno4 - 12-12-2012 at 13:54

At higher temperatures, maint product are tetrachlorodioxanes ( see references given at OS page, especially the 1-st one).
See also paper about chlorination of THF: 2,3 dichloro-THF is also main product. It turns out, 2-chloro-THF is unstable even below 0 C (doi:10.1135/cccc19712052)

Nicodem - 13-12-2012 at 11:01

The same 2,3-dichloro products from dioxane, THF and tetrahydropyran are obtained also by using trichloroisocyanuric acid, which is easily available and, for the average experimenter, more practical than using Cl2:
DOI: 10.1080/00304947109356057

kristofvagyok - 13-12-2012 at 14:53

Thanks for the refs and the ideas!

I would also recommend now the mechanism with monochlorination, the elimination, then the chlorine addition after doing a few reaction with the dichloro-dioxane (DCDO).

Summarizing the few results what I have obtained after doing a few reactions with DCDO: while DCDO is quite stable when it's pure, but when it is reacted with nucleophiles it (probably) goes through an elimination, what is followed by an isomerisation and usually ends up with a black tar in 80% yield.

Made 2 parallel reactions in benzene and in xylene with the same reagent. In benzene after 5h reflux nothing happened, while in the xylene after 2 hour nothing, then in 20 min the 90% of the material have changed to a black tar. Almost nothing was isolated.and what have came out from the reaction wasn't what I needed, NMR spectra is irrelevant, currently waiting for an MS.

Also thanks a lot for the idea with TCCA, gonna try out it soon.

chemrox - 13-12-2012 at 17:15

I suspect this is a free radical reaction where Cl2 homolysis is initiated by the iodine. @kristo: I enjoyed all your efforts to make money off your blog. It takes a lot of balls to ask all your visitors to give you a dollar and sell photos of your intermediates. I think I'll try that too. Where did you get the "donate" radio button?