Using MICROWAVE REACTOR

Krystyna R. BrzezinskaMaterials Research Laboratory

Microwave

• Collective name for electromagnetic irradiation with frequencies in the range of 0.3-300 GHz.• To avoid interferences between applications - 2.45 GHz corresponding to wavelength 12.2 cm.• temp: 60-250 °C, temp increase: 2-5 °C/sec• Power: 0-300 W at 2.45 MHz Pressure: 0-20 bar• Pressure: 0-20 bar 20 bar = 290 psi, 250 psi = 17 bar

Dipolar oscillation Ionic conduction

Mechanism of Microwave Heating

Molecular Orientation = Molecular Friction = Heat

Temp. up to 250 oC and pressure up to 20 bars Heat much faster than the conventional reaction.

Speed Up Chemical Reaction?

Vials: 0.2-0.5 ml, 0.5-2 ml, 2-5 ml, 10-20 ml

Polar solvents (e.g. DMF, DMSO, ROH etc.) Heat rapidly. Same concentration as conventional reaction.

Polar solvent with high ionic content (e.g. DMF + K2CO3 etc)Heat VERY rapidly and may overshoot set temperature.

Temp too high!

Polar Solvents

• Dilute reaction mixture.

• Decrease initial power use: ‘Sample Absorption’ - High or Very High.

Less polar solvents (e.g. toluene, dioxane, THF)

Less Polar Solvents

• Concentrate the reaction mixture- speeds workup- lower cost of ‘waste’ solvent removal

Heat poorly

• Add polar co-solvent

• Increase ionic concentration

4.5 mL Toluene 4.0 Toluene + 0.5 mL DMF

Set temp: 200 °C

Solvent polarity

Effect of a co-solvent

Reaction rate?

Arrhenius Equation: K = A e–G/RT

Reaction rate ~ doubles for every 10 oC temperature increase

Using the 10o Rule to determine a starting point

K = A e–G/RT

Prepare Three Vials

160oC

5 min140

oC

5 min

180oC

5 min

First Choice

Temperatures and pressures measured* with some common organic solvents

 ______________________________________________________________________________________________

_Solvent BP Temp Pressure**

(°C) (°C) (bar)______________________________________________________________________________________________

_

N,N-Dimethylformamide (DMF) 153 250 5Water 100 220 16Ethanol 78 180 16Methanol 65 160 17N-Methylpyrrolidinone (NMP) 202 220 5Ammonium hydroxide (28%) - 150 19Dimethylsulfoxide (DMSO) 189 250 5 Pyridine 115 220 8 1,2-Dimethoxyethane (DME) 85 200 6Dichloromethane 40 140 15Acetonitrile 86 200 10o-Dichlorobenzene 190 250 21,4-Dioxane 101 200 4Acetone 56 150 5Tetrahydrofuran (THF) 65- 67 180 12Xylene 137-138 150 21,2-Dichloroethane 83 170 2Toluene 110 170 4Diethyl ether 35 135 4*This is a general table with data generated under various reaction conditions. The actual temperature and pressure achieved with a given solvent might differ significantly usingdifferent reaction conditions and compositions. Red indicates good microwave absorption, while Blue indicates less good **Volume = 2.5 mL

• Increase temp. (limit is pressure buil-up in the vial and security max = 250 oC).

• Extend reaction time.

• Increase initial power ‘Sample Absorption’ – Normal, High or Very High

• Increase concentration.

• Use more polar solvent (water at high temp become less polar).

•Add polar co-solvent

Reaction is not proceeding at all or not going to completion:

• Lower temperature.

• Increase initial power ‘Sample Absorption’ – Normal, High or Very High

• Shorten reaction time. Product is actually formed, but then decomposes rapidly at elevated temperatures.

• Decrease concentration.

• Change reagents.

Decomposition:

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Questions?