Author Topic: microwave isomerization why not?  (Read 10981 times)

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  • Guest
microwave isomerization why not?
« on: September 21, 2003, 11:45:00 AM »
You know in Strike's book he refers to these Brazilian chemists who isomerised safrole in a conventional microwave oven with excellent results and in a very short period od time.
The drawback mentioned there was that they cut a hole in the top of the microwave to give space for a condenser, thereby puttingg themselves at risk of exposure to harmful high frequency raidiation.
But I digress, times have changed and techniches have improved a really good and much safer alternative for micro isomerization is to do it in a beaker with a "plugged" cone funell inveterted into the beaker, and packed with dry ice/ethanol slurry (_80 below} as a codensor so you can zap away at your propenylbenzene with little fear.
so what does the hive community think? why take the path of the most resistance when occam's razor always wins?


  • Guest
Could you please link to the method described?
« Reply #1 on: September 21, 2003, 08:19:00 PM »
Could you please link to the method described? Not everybee can guess that you probably mean:

Post 457915

(Rhodium: "Microwave Isomerization of Allylbenzenes", Methods Discourse)

Post 108497 (missing)

(dormouse: "Welcome to Snidely's research library!  -Snidely Whiplash", Novel Discourse)

Post 108390 (missing)

(dormouse: "Microwaves oven reactions.  -Labrat", Novel Discourse)

Post 162640 (missing)

(jimwig: "microwaves for honey", Methods Discourse)

Post 457915

(Rhodium: "Microwave Isomerization of Allylbenzenes", Methods Discourse)

Post 108400 (missing)

(dormouse: "Microwave Isomerization of Safrole  - Cherrie Baby", Novel Discourse)

Feeding TFSE with "microwave modifation" you will find an article. The authors state:

The modified oven was tested for microwave leakages. Levels were below the 1 mW/cm’ specified by legislation for new equipment ( < 5 mW/cm2 in use).

There is another excellent thread on the 'Microwave modification to monomode' in the results when using the above search string.

Lego thinks everybee with some elemental manual skills will bee able to modify a commercial microwave oven to an oven that can bee used for clandestine purposes. So your suggested modification is iLho too complicated. Attaching an jacketed coil condenser on top of the microwave connected to a flask inside the oven will do the same job more easily. The risk of exposition to microwave radiation is not too serious when the modification is done proper and one of these cheap tools is used to detect microwave leakages (the tools uses by professionals are only slightly more sophisticated).


  • Guest
A copper pipe would prevent microwave leakage
« Reply #2 on: September 21, 2003, 08:31:00 PM »
You suggest to place the condenser inside the oven? If so, and provided that the dry ice is transparent to microwaves (and thus wouldn't heat up itself), and have enough heat capacity to cool the boiling safrole, then I still would expect problems arising from 1) the fact that there is a 300°C temperature difference between dry ice and boiling safrole, and 2) that the dry ice at -78°C may cool some of the safrole below its solidification point and therefore clog the condenser contraption.

The drawback mentioned there was that they cut a hole in the top of the microwave to give space for a condenser, thereby puttingg themselves at risk of exposure to harmful high frequency raidiation.

There is no risk of microwaves escaping if you fasten a copper pipe to the exit hole on the top of the microwave and put the glass tube leading to the external condenser inside it. The copper pipe should fit relatively tight around the glass tube (no more than 1-2 mm slack), and it must be longer than 12 cm (15 cm is okay). As the wavelength of the 2.4 GHz microwave radiation is 12 cm, no radiation can escape through a metal tube longer than that.


  • Guest
« Reply #3 on: September 21, 2003, 11:06:00 PM »
that's why we call you the chief around here you really know your shit.
I'll provide a documented example of this simple contraption
It consist's of a inverted funnel placed atop a beaker. next comes the article....

I think it's in a back issue of CE Chemical & Engineering news (which btw) is a cool magazine.
So it may take me a while to find it.



  • Guest
The copper pipe should fit relatively tight...
« Reply #4 on: September 21, 2003, 11:44:00 PM »

The copper pipe should fit relatively tight around the glass tube (no more than 1-2 mm slack), and it must be longer than 12 cm (15 cm is okay). As the wavelength of the 2.4 GHz microwave radiation is 12 cm, no radiation can escape through a metal tube longer than that.

Minor correction, chief: if you don't want microwaves to leak to the outside, the diameter of the pipe should be smaller than 12 cm (or more precisely stated, the wavelength of the lowest mode possible in a cilindrical waveguide, which is IIRC a TE00). The length of the tube (or waveguide) can be any length.

And btw, for smaller diameters you still can have relatively much leakage to the outside, depending on the relation between wavelength of your radiation and the diameter of your hole...


  • Guest
Nice microwave articles, but no inverted funnels
« Reply #5 on: September 22, 2003, 01:02:00 AM »
This is the relevant section from Chemical & Engineering News, Vol. 80, No. 20, May 20 (2002), "Green Chemistry Gets Greener":

As the number of chemists practicing greener chemistry continues to grow, so do the opportunities to form collaborations drawing on the strengths of different research groups. One such example is a new collaborative effort between Li's group at Tulane and the groups of Rajender S. Varma of the Clean Processes Branch of EPA's National Risk Management Research Laboratory, Cincinnati, and Luc Moens of the National Renewable Energy Laboratory, Golden, Colo.

One area of focus for this collaboration is the development of new catalyst systems in ionic liquids. Graduate student Charlene C. K. Keh in Li's group discussed some of the initial results: a Prins-type cyclization of homoallylic alcohols with aldehydes in an ionic liquid for the direct preparation of a series of tetrahydropyranols.

The reaction includes 1-butyl-3-methylimidazolium hexafluorophosphate as the ionic liquid and cerium triflate as a catalyst. A small amount of benzoic acid is added to assist the Lewis acid catalysis, Keh noted, which optimizes yields at about 80%. The ionic liquids, supplied by the Varma and Moens groups, can be used up to three times without any loss in reaction yield, she said.

Varma's group has been exploring the use of ionic liquids as both a solvent and a cocatalyst. One example is the palladium-catalyzed oxidation of styrene to acetophenone (Wacker reaction) under solventless conditions using H2O2 as the oxidant [Green Chem., 4, 170 (2002)] DOI:


. The addition of a small amount of an ionic liquid, such as 1-butyl-3-methylimidazolium tetrafluoroborate, significantly enhances the reaction yield with high selectivity for acetophenone over benzaldehyde.

Postdoc Vasudevan V. Namboodiri in Varma's group described a microwave-assisted solventless synthesis of ionic liquids in high purity that should aid the collaborative research. Ionic liquids are generally prepared by refluxing an alkyl halide and an imidazole in an organic solvent for several hours, Namboodiri said.

In the new method, the ionic liquid can be prepared simply by heating the reactants together in a conventional microwave oven without solvent in a process that takes only a few minutes [Chem. Commun., 2001, 643] DOI:


. The method provides an opportunity to generate ionic liquids in situ and carry out a subsequent reaction all in one pot, Namboodiri said.

The above article included the following link, which contained a whole lot of interesting citations:

Solvent-Free Alternatives to Accelerated Organic Syntheses Using Microwaves, Ultrasound and Supported Reagents


Microwave irradiation, an unconventional energy source, has been used for a variety of organic transformations wherein chemical reactions are accelerated because of selective absorption of microwave (MW) energy by polar molecules, non-polar molecules being inert to the MW dielectric loss. The MW application under solventless conditions enables rapid synthetic transformations at ambient pressure thus providing unique chemical processes with special attributes such as ease of manipulation, enhanced reaction rates and higher yields. Recent results from our laboratory on this MW-expedited approach include the synthesis of a variety of industrially significant compounds and intermediates namely, imines, enamines, nitroalkenes, enones, oxidized sulfur compounds and ionic liquids. This solvent-free synthetic methodology involves microwave exposure of reactants in neat form or their reaction in presence of a catalyst or catalyzed by the surfaces of inexpensive and recyclable mineral supports namely silica, alumina, clay, or 'doped' surfaces, such as, Fe(NO3)3-clay (clayfen), Cu(NO3)2-clay (claycop), NH2OH-clay, PhI(OAc)2-alumina, NaIO4-silica and NaBH4-clay. A variety of condensation, cyclization, oxidation and reduction reactions include the convergent one-pot assembly of heterocyclic molecules from in situ generated intermediates such as a-tosyloxyketones and enamines.

The application of this solventless MW approach to multi-component reactions has been highlighted which can be adapted for high-speed parallel synthesis of the library of biologically active molecules. The distinct advantages of this strategy, that reduces or prevents pollution ‘at-source,’ has been exemplified by efficient functional group transformations involving non-metallic hypervalent iodine oxidants and substitution of conventional metal-based reducing agents as demonstrated in crossed Cannizzaro reaction using paraformalehyde. As an example, recently discovered solventless oxidation of alcohols using neat reagent, iron(III) nitrate, would save 3L/mol of solvent usage and prevent generation of 40-50g/mol of solid waste when compared to the traditional protocols.

More recent examples include the identification and synthesis of environmentally friendlier reaction media (ionic liquids) and reaction conditions for well known C-C bond-forming reactions in relatively benign solvents such as polyethylene glycols (PEG) where recycling of catalysts have been demonstrated in our laboratory.

Recent Publications:

Microwave Technology: Chemical Applications.
R.S. Varma: Kik-Othmer On-line Encyclopedia of Chemical Technology, in press (2002).

Feature article: Environmentally Friendlier Organic Transformations on Mineral Supports Under Non-traditional Conditions.
U.R. Pillai, E. Sahle-Demessie, and R.S. Varma: J. Material Chem., 12, 3199-3207 (2002).

Organic Synthesis Using Microwaves and Supported Reagents.
R.S. Varma in "Microwaves in Organic Synthesis," A. Loupy (Ed.), Wiley-VCH, Weinheim, Chapter 6, pp 181-218 (2002).

An Efficient and Ecofriendly Oxidation of Alkenes Using Iron Nitrate and Molecular Oxygen.
U.R. Pillai, E. Sahle-Demessie, V.V. Namboodiri and R.S. Varma: Green Chemistry, 4, 495 (2002).

Nafion-catalyzed Preparation of benzhydryl Ethers.
M.A. Stanescu and R.S. Varma: Tetrahedron Lett., 43, 7307 (2002).

An Improved Preparation of 1,3-Dialkylimidazolium Tetrafluroborate Ionic Liquids Using Microwaves.
V.V. Namboodiri and R.S. Varma: Tetrahedron Lett., 43, 5381 (2002).

Solvent-free Sonochemical Preparation of Ionic Liquids.
V.V. Namboodiri and R.S. Varma: Organic Letters, 4, 3161 (2002).

Direct Formation of Tetrahydropyranols via Catalysis in Ionic Liquid.
C.C.K. Keh, V.V. Namboodiri, R.S. Varma, and C.-J. Li: Tetrahedron Lett., 43, 4993 (2002).

Iron-catalyzed Solvent-free of Alcohols and Phenols into Diphenylmethyl (DPM) Ethers.
V.V. Namboodiri and R.S. Varma: Tetrahedron Lett., 43, 4593 (2002).

Microwave-Expedited Olefin Epoxidation Over Hydrotalcites Using Hydrogen Peroxide and Acetonitrile.
U.R. Pillai, E. Sahle-Demessie, and R.S. Varma: Tetrahedron Lett., 43, 2909 (2002).

Selective Oxidation of Styrene to Acetophenone in Presence of Ionic Liquids.
V.V. Namboodiri, R. S. Varma, E. Sahle-Demessie and U. R. Pillai: Green Chemistry, 4, 170 (2002).


Clay and Clay-supported Reagents in Organic Synthesis.
R.S. Varma: Tetrahedron Report Number 598, Tetrahedron, 58, 1235-55 (2002).

Microwave-assisted Preparation of Dialkylimidazolium Tetrachloroaluminates and Their Use as Catalysts in the Solvent-free Tetrahydropyranylation of Alcohols and Phenols.
V. V. Namboodiri and R.S. Varma: Chem. Commun., 342 (2002).

Solvent-free tetrahydropyranylation (THP) of alcohols and phenols and their regeneration by catalytic aluminum chloride hexahydrate.
V. V. Namboodiri and R.S. Varma: Tetrahedron Lett., 43, 1143 (2002).

Solid-state Synthesis of Heterocyclic Hydrazones Using Microwaves Under Catalyst-free Conditions.
M. Ješelnik, R.S. Varma, S. Polanc, and M. Kocevar: Green Chemistry, 4, 35 (2002).

Solvent-Free Preparation of Ionic Liquids Using a Household Microwave Oven.
R.S. Varma and V. V. Namboodiri: Pure Applied Chem., 73, 1309 (2001).

Catalyst-free Reactions Under Solvent-free Conditions: Microwave-assisted Synthesis of Heterocyclic Hydrazones Below the Melting Points of Neat Reactants.
M. Ješelnik, R.S. Varma, S. Polanc, and M. Kocevar: Chem. Commun., 1716 (2001).

Solvent-free Reduction of Aromatic Nitro Compounds with Alumina-supported Hydrazine Under Microwave Irradiation
A. Vass, J. Dudás, J. Toth and R.S. Varma: Tetrahedron Lett., 42, 5347 (2001).

Microwave Organic Synthesis.
R.S. Varma in "McGraw-Hill Yearbook of Science and Technology 2002", pp 223-225, McGraw-Hill, New York, NY (2001).

Microwave Accelerated Solvent-Free Chemical Reactions.
R.S. Varma: AMPERE (Association for Microwave Power in Europe for Research and Education), p 3 (2001).

Microwave-Accelerated Suzuki Cross-coupling Reaction in Polyethylene Glycol (PEG).
V.V. Namboodiri and R.S. Varma: Green Chemistry, 3, 146 (2001).

Highly Diastereoselective Michael Reaction Under Solvent-free Conditions using Microwaves: Conjugate Addition of Flavanone to its Chalcone Precursor
T. Patonay, R.S. Varma, A. Vass, A. Lévai and J. Dudás: Tetrahedron Lett., 42, 1403 (2001).

An Expeditious Solvent-Free Route to Ionic Liquids Using Microwaves.
R.S. Varma and V.V. Namboodiri: Chem. Commun., 643 (2001).

Solvent-Free Accelerated Organic Syntheses Using Microwaves.
R.S. Varma: Pure Applied Chem., 73, 193 (2001).

Environmentally Benign Organic Transformations Using Microwave Irradiation Under Solvent-Free Conditions.
R.S. Varma in "Green Chemistry: Challenging Perspectives," P.T. Anastas and P. Tundo (Eds.), pp 221-244, Oxford University Press, Oxford (2000).

Solid State Oxidation of Thiols to Disulfides Using Ammonium Persulfate
R.S. Varma, H.M. Meshram and R. Dahiya: Synth. Commun., 30, 1249 (2000).

Expeditious Solvent-Free Organic Syntheses Using Microwave Irradiation.
R.S. Varma in "ACS Symposium Series No. 767/ Green Chemical Syntheses and Processes" P.T. Anastas, L. Heine and T. Williamson (Eds.), Chapter 23, pp 292-312, American Chemical Society, Washington DC (2000).

Microwave-accelerated Three-component Condensation Reaction on Clay: Solvent-free Synthesis of Imidazo[1,2-a] annulated Pyridines, Pyrazines and Pyrimidines.
R.S. Varma and D. Kumar: Tetrahedron Lett., 40, 7665 (1999).


  • Guest
Mass qty's of KOH?
« Reply #6 on: September 22, 2003, 02:39:00 AM »
I think this was pointed out in an earlier post that the amounts of Potassium Hydroxide were really prohibitive when it came to doing that reaction.. and not everyone has N-Butanol lying around..


  • Guest
Any alcohol is OK, but the base may be a problem
« Reply #7 on: September 22, 2003, 09:15:00 PM »
If you follow the links given above to the

original article

(, you'll see that any lower alcohol can be used, but you are correct in that their suggested amount of base is pretty high, between a 1:1 and 2:1 ratio between KOH:Safrole is used. Hopefully that can be adjusted downwards somehow.


  • Guest
« Reply #8 on: September 22, 2003, 11:07:00 PM »
Maybee a co-catalyst could bee used to cut down on the amount of base needed?


  • Guest
Suggestion for cutting down on the base
« Reply #9 on: September 23, 2003, 02:09:00 AM »
The easiest way is probably to reduce the amount of solvent as well as the base (it seems like the concentration of the base in the solvent is more important than the absolute amount). Also, dissolve 5 mole% of a PTC like TBAB (tetrabutylammoniumbromide, or Bu4N+Br-) or Aliquat 336 in the alcoholic solvent before adding the safrole, then mix it all vigorously (maybe by shaking it in a stoppered flask, as the liquids probably aren't miscible)


  • Guest
Modifying a domestic microwave oven for reflux
« Reply #10 on: October 02, 2003, 03:16:00 AM »
Design and application of a reflux modification for the synthesis of organometallic compounds using microwave dielectric loss heating effects
David R. Baghurst and D. Michael P. Mingos Abstract

Journal of Organometallic Chemistry, 384, C57-C60 (1990)



A commercially available microwave oven has been modified so that syntheses involving the refluxing of organic solvents can be safely and conveniently undertaken. The applications of this technique for accelerating the rates of reactions leading to the synthesis of some commonly used organometallic and coordination compounds are described.


  • Guest
Alternative Microwave Reflux Design
« Reply #11 on: November 28, 2003, 06:27:00 PM »


  • Guest
Chemical Society Reviews: Microwave Chemistry
« Reply #12 on: September 21, 2004, 02:48:00 AM »
Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry
D. Michael P. Mingos and David. R. Baghurst

Chem. Soc. Rev. 20, 1-47 (1991)



  • Guest
Has anyone tried making 4M KOH in BuOH
« Reply #13 on: September 21, 2004, 04:41:00 AM »
SWIM tried MW isomerization a while ago, but had trouble making the 4M soln of KOH in BuOH. Had it stirring with gentle heating for several days. The solution went an orange colour but there was quite a lot that still hadn't dissolved. The time to do all this was prohibitive and so he went back to conventional methods.
So unless the concentration doesn't really matter and just the molar amount than it could work... but I'd be interested to see if just using Ethanol and KOH would work.


  • Guest
regarding safety
« Reply #14 on: September 21, 2004, 11:45:00 AM »

There is no risk of microwaves escaping if you fasten a copper pipe to the exit hole on the top of the microwave and put the glass tube leading to the external condenser inside it. The copper pipe should fit relatively tight around the glass tube (no more than 1-2 mm slack), and it must be longer than 12 cm (15 cm is okay). As the wavelength of the 2.4 GHz microwave radiation is 12 cm, no radiation can escape through a metal tube longer than that.

This is shortened and by this misleading, sorry.
- The coppertube has to have good electrical contact to the housing of the microwave (the INNER housing on household MWs). Just popping a tube into a hole or even fixing it with epoxy or whatever insulating will result in a nice "slit-antenna" with a gain of 15-25db (directed energy) - this will make it really dangerous.
The radiation of a open microwave oven is not dangerous anymore in a distance of more than 1m - not DANGEROUS, I didnt say HEALTHY in the long run, ok? Except you direct the energy by an antenna or similar of course.
- The main point is that the coppertube has to have an inner diameter of less than 6cm (half-wavelength), thats mucho more important than the length of 12 cm.
True is that out of  a metaltube with less than 6cm dia. and more than 12 cm lentgh VIRTUALLY no more radiation escapes.
Tubes of unlimited length with a diameter of 6cm+ are called "waveguides" and are the usual way to transport microwave energy.

Why the coppertube should fit to the glasstube with narrow space, is not understandable to me, actually you risk an surface effect so the tube gets wet by condensation. The risk is small though and the energy would be minimal then so this doesnt really matter IMHO.

In short:
- holes have to be smaller than 6cm in diameter (smaller is better)
- avoid any "slits" of a length more than 6cm max. - 0,5cm is ok.
- electrical contact of metallic parts adapted to the microwave is a must have - screw/rivet AND solder!