(Hive Bee)
03-09-03 13:53
No 415141
(Rated as: excellent)
Using the procedure posted here on the hive, for the preparation of 1,4-dimethoxybenzene, 2,5-dimethoxytoluene was prepared in the following manner:
30,5 g ( 0,3 moles ) of methylhydroquinone was placed in a 250 mL RBF. This was placed in a waterbath at room temperature. 110 mL 20% NaOH was added while stirring. The whole lot dissolved in a matter of seconds, and the mixture turned black/brown within seconds. After a minute 70 mL's of trimethylphosphate was added. The temperature remained constant during the next five minutes. A reflux condensor was attached and the waterbath was slowly heated to a gentle boil over the course of 30 min's. When the waterbath was boiling, the mixture refluxed slightly. This was continued for another hour.
After a hour, the mixture was removed from the waterbath and allowed to cool to almost room temperature. Next, the reaction mixture was added to a seperating funnel, containing 200 mL's of cold water. This was extracted three times with 3*75 mL's of DCM. The first extraction separated fine, but the following two caused heavy emulsion. A bit of NaCl was added, but it did not help a whole lot. Some carefull swirling got rid of the worst emulsion.
The DCM phase was very blackish looking, but two water washes took care of the worst color. The remaning DCM was deeply red in color. The DCM was placed in a large RBF and heated on an oil bath. The DCM was distilled off and the remaining viscious oil was distilled under vacuum. This yeilded a beautifull red-amber colored liquid, which boiled at 130-140 degrees C. The total yeild was 31 grammes(90% molar) of 2,5-dimethoxytoluene.
The reaction gave a very high yeild AND worked on the methyl substituted hydroquinone.
Regards
Bandil
(The Archetypical "Good Guy")
06-25-04 01:56
No 515313
(Rated as: good read)
After some more experience in the field of chemistry i've discovered that the former procedure is very flawed.
The methylation procedure is indeed correct, but the workup is very very sloppy.
The procedure is allright untill the point where it says: 200 mL's of cold water. This was extracted three times with.... Please use the following work up procedure instead:
This was extracted three times with 100 mL's DCM. The larger volume of DCM reduced the amount of emulsion. The emulsion layer was also carried over into the DCM phase, such that this was quite dirty looking. The water phase was discarded. The combined DCM extractions where washed with three times with 100 mL 10% aq. NaOH. This step is very crucial, as it removed any remaining non-di-methylated remains (toluhydroquinone and methyltolyhydroquinone). After the base washes it was washed once with dH2O, which gave a perfect separation with no emulsion remaining. This was dried preliminarily by shaking the DCM with brine. Finally a knife tip of MgSO4 was used to dry it completely(when predrying with brine the amount of drying agent can be cut down to almost nothing).
The final DCM phase was only faintly orange. The DCM was distilled off at atmospherical pressure, washed with brine, dried over a little MgSO4 and put in the "reused DCM bottle".
Some cooking oil was added to the residue, to avoid distilling to dryness. Aspirator vacuum was applied and the crude product distilled at 120°C-125°C to give a 65% molar yield of dimethoxytoluene as a water white oil. This crystallized to spectacular colourless crystals when placed in the freezer.
The morale is that the "old" procedure gave a VERY dirty product, because of the sloppy workup. TMP is also a weaker methylating agent than DMS, so it would be strange if it actually gave a higher yield.
Regards
Bandil
Now with 10% extra chemical pride!
(Hive Addict)
06-25-04 02:12
No 515314
It is known that DMS partially decomposes in aqueous solutions. How about TMP? Adding a little bit of acetone to the DMS reaction mixture boosts yields and keeps the amount of DMS needed (mol eq wise) low.
Post 507716 (GC_MS: "Permethylation of pyrogallol with DMS", Methods Discourse)
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