sprocket
September 22nd, 2006, 09:13 PM
This one has been nagging me for a while. It seems well-known that nitromethane forms nitroacetaldoxime (methazonic acid) in basic aqueous solution, but never have I seen a mechanism for it. In various papers they mention a certain mechanism being similiar to that of methazonic acid, but no references are given (Ex: http://www.iupac.org/publications/pac/1989/pdf/6107x1217.pdf).
There is also a short article about nitroacetaldoxime in PATR 2700 on page A16 with references. However I have been unable to get hold of those papers. It also seems our dear Federoff either made a typo or forgot to enter one of the references in the table, because there are four references in the text and only three in the table.
Refs: 1) Beil 1,627 2)O. Schultze, Ber 29, 2289(1896) 3) W. R. Dunstan & E. Goulding, Ber 42, 2030-1(1909)
It is clear that nitromethane reacts with hydroxides to form the enolate (nitronate) H2C=NO2-. This must be the first step in the reaction, followed by a condensation of some sort (this is what bugs me) and finally an E1cB elimination. The product here should be the conjugate base of methazonic acid, which must then be acidified to obtain the actual nitroacetaldoxime.
Now to the weird part. I prepared a small batch of nitroacetaldoxime sodium salt by treating nitromethane with aqueous sodium hydroxide following this procedure:
To a 250 ml Erlenmeyer flask 25 ml nitromethane (technical grade) and 25 ml 5M NaOH solution was added. The reaction progressed slowly at first; the liquids were in separate phases. After about 30 seconds of swirling the flask the liquids had turned red and completely mixed with significant evolution of heat. The flask was then swirled in a stream of 20°C water to control the temperature. After about 2 minutes the solution instantly solidified into a brown moist mass. Swirling then stopped and the reaction vessel was allowed to cool to RT. The brown precipitate was transferred to a filter paper and washed with 50 ml cold distilled water. After 12 hours of drying a small amount of the sample was dissolved in D2O and the 1H-NMR chemical shifts were determined in a Bruker 500 MHz NMR.
Results: 1H NMR (500 MHz, D2O) δ ppm 6.74 (d, J = 8.93 Hz), 7.13 (d, J = 8.84 Hz), 7.62 (d, J = 8.84 Hz), 8.10 (d, J = 8.93 Hz)
Clearly this is not nitroacetaldoxime, seeing as it has four doublets. If this was nitroacetaldoxime there would be two singlet peaks and one doublet. I realize now that I forgot to acidify the solution, but I should still have one single and one doublet peak if it was the nitroacetaldoxime sodium salt I obtained. What happened?
If the nitroacetaldoxime could form an enolate that would have two doublet peaks, but is this possible?
Attached is the NMR-spectrum.
There is also a short article about nitroacetaldoxime in PATR 2700 on page A16 with references. However I have been unable to get hold of those papers. It also seems our dear Federoff either made a typo or forgot to enter one of the references in the table, because there are four references in the text and only three in the table.
Refs: 1) Beil 1,627 2)O. Schultze, Ber 29, 2289(1896) 3) W. R. Dunstan & E. Goulding, Ber 42, 2030-1(1909)
It is clear that nitromethane reacts with hydroxides to form the enolate (nitronate) H2C=NO2-. This must be the first step in the reaction, followed by a condensation of some sort (this is what bugs me) and finally an E1cB elimination. The product here should be the conjugate base of methazonic acid, which must then be acidified to obtain the actual nitroacetaldoxime.
Now to the weird part. I prepared a small batch of nitroacetaldoxime sodium salt by treating nitromethane with aqueous sodium hydroxide following this procedure:
To a 250 ml Erlenmeyer flask 25 ml nitromethane (technical grade) and 25 ml 5M NaOH solution was added. The reaction progressed slowly at first; the liquids were in separate phases. After about 30 seconds of swirling the flask the liquids had turned red and completely mixed with significant evolution of heat. The flask was then swirled in a stream of 20°C water to control the temperature. After about 2 minutes the solution instantly solidified into a brown moist mass. Swirling then stopped and the reaction vessel was allowed to cool to RT. The brown precipitate was transferred to a filter paper and washed with 50 ml cold distilled water. After 12 hours of drying a small amount of the sample was dissolved in D2O and the 1H-NMR chemical shifts were determined in a Bruker 500 MHz NMR.
Results: 1H NMR (500 MHz, D2O) δ ppm 6.74 (d, J = 8.93 Hz), 7.13 (d, J = 8.84 Hz), 7.62 (d, J = 8.84 Hz), 8.10 (d, J = 8.93 Hz)
Clearly this is not nitroacetaldoxime, seeing as it has four doublets. If this was nitroacetaldoxime there would be two singlet peaks and one doublet. I realize now that I forgot to acidify the solution, but I should still have one single and one doublet peak if it was the nitroacetaldoxime sodium salt I obtained. What happened?
If the nitroacetaldoxime could form an enolate that would have two doublet peaks, but is this possible?
Attached is the NMR-spectrum.