Hello,
I just dived into the topic of microwave assisted synthesis and found a literature I would like to reproduce. The problem is in the literature, a domestic microwave oven is used which we do not have for laboratory purposes. We have a dedicated instrument for microwave synthesis.
For whoever it may concern, I want to reproduce the synthesis of molecule 3 in https://www.tandfonline.com/doi/full/10.1080/00397910802138959?scroll=top&needAccess=true
The experimantel procedure calls for all reagents (1g 4-Nitrophenol, 0.6 mL 37% Formaldehyde solution, 1 mL conc. Hydrochloric acid) to be placed in a domestic microwave oven ( KenstarOM 20 DGQ ) and be irradiated with 20 % power for 180s. I already looked up the power output of this specific model, which is 800 W. 20 % of that means 160 W.
So my question is, how can I use this information to program our dedicated synthesis microwave to achieve the same conditions? Our machine calls for temperature and the vessels are sealed. I can program 160 W, but what temperature do I enter? The domestic microwave will give out its set power no matter what happens inside the cooking chamber, but the dedicated instrument will stop feeding power as soon as the set temperature is achieved (and only add power from time to time to keep it at that temperature).
I hope someone with expertise in this field could enlighten me.
Such a set up is extremely difficult to reproduce due to the many sources of variance and uncertainty. A modification of the circuit that drives the microwave transformer in such a way that it provides pulse width modulation (PWM) of the magnetron may be necessary. A triac circuit is required, together with a temperature sensor of some sort. All that requires deeper knowledge of high-voltage control electronics. Clearly, having metallic parts such as sensors in the vessel is risky business given the conductivity of microwave energy. The name of the game is repeatability. Can it be assured?
Hi Kevin Rudolph I think you should be able to carry out the experiment with a lab dedicated microwave setup. However a you may need to carryout a bit of temperature optimisation. Because the reaction in the publication you shared was carried out in a domestic microwave, I reckon it may not have been under super-heated conditions. This suggests you should be looking at temperatures within the range of 40-108 degrees (108 degrees is the boiling point of the HCl acid azeotrope). You are likely to obtain better results with a Lab dedicated system. If you have time to play further with optimisation, you could even try reducing the reaction time.
Precautions to take note of:
A teflon microwave vessel may be best for this experiment. Mars microwave digestion has very nice teflon vials.
HCl acid once heated produces HCl gas which is very corrosive and toxic. Ensure you allow the microwave vessel to cool down to room temperature before opening. Always open microwave vials in fume hood.
If your lab microwave oven has more than one sample slot, you may need to check the manual how to balance the samples therein to avoid overheating.
I think you can set both the power and the temperature to try to repeat that experiment without having to worry about that.
Good day!
I had a similar experience of reproducing the technique. Colleagues also used a household microwave oven. As far as I understand, there is no solvent in the technique? In a microwave reactor, I advise you to choose the temperature according to the solvent, according to the table in the manual attached to the delivery. In the reactor that we have in the laboratory, such reactions cannot be performed (check your and be careful). If no solvent is being used, I would advise using an autoclave or similar reactor to avoid damaging the septum in the microwave vial.
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