Using MICROWAVE REACTOR
Krystyna R. Brzezinska
Materials 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
Mechanism of Microwave Heating
Dipolar oscillation
Ionic conduction
Molecular Orientation = Molecular Friction = Heat
Speed Up Chemical Reaction?
Temp. up to 250 oC and pressure up to 20 bars
 Heat much faster than the conventional reaction.
Vials: 0.2-0.5 ml, 0.5-2 ml, 2-5 ml, 10-20 ml
Polar Solvents
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!
• Decrease initial power use:
‘Sample Absorption’ - High or Very High.
• Dilute reaction mixture.
Less Polar Solvents
Less polar solvents (e.g. toluene, dioxane, THF)
 Heat poorly
• Concentrate the reaction mixture
- speeds workup
- lower cost of ‘waste’ solvent removal
• Add polar co-solvent
• Increase ionic concentration
Solvent polarity
Effect of a co-solvent
Set temp: 200 °C
4.5 mL Toluene
4.0 Toluene + 0.5 mL DMF
Reaction rate?
Arrhenius Equation:
K = A e–DG/RT
Reaction rate ~ doubles for every 10 oC temperature increase
o
Using the 10 Rule to determine a starting point
K = A e–DG/RT
Prepare Three Vials
o
140 C
5 min
o
160 C
5 min
First Choice
o
180 C
5 min
Temperatures and pressures measured*
with some common organic solvents
_______________________________________________________________________________________________
Solvent
BP
Temp
Pressure**
(°C)
(°C)
(bar)
_______________________________________________________________________________________________
N,N-Dimethylformamide (DMF)
153
250
5
Water
100
220
16
Ethanol
78
180
16
Methanol
65
160
17
N-Methylpyrrolidinone (NMP)
202
220
5
Ammonium hydroxide (28%)
150
19
Dimethylsulfoxide (DMSO)
189
250
5
Pyridine
115
220
8
1,2-Dimethoxyethane (DME)
85
200
6
Dichloromethane
40
140
15
Acetonitrile
86
200
10
o-Dichlorobenzene
190
250
2
1,4-Dioxane
101
200
4
Acetone
56
150
5
Tetrahydrofuran (THF)
65- 67
180
12
Xylene
137-138
150
2
1,2-Dichloroethane
83
170
2
Toluene
110
170
4
Diethyl 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 using
different reaction conditions and compositions.
Red indicates good microwave absorption, while Blue indicates less good **Volume = 2.5 mL
Reaction is not proceeding at all or not
going to completion:
• 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
Decomposition:
• 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.
Questions?
Please contact Krystyna (kbrzez@mrl.ucsb.edu)
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