Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: nj_bartel on October 25, 2009, 08:19:10 PM
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I took the melting point on some recrystallized lidocaine for a lab recently and got a range (55-56 oC) 13 oC below the lit value (68-69 oC). This is, I think, the second time in a couple years of lab I've gotten a product with a substantially lower MP than lit values, but with a very tight melting range (almost instantaneous, and the bulb was not turned up very hot). Is the only logical explanation for this that I obtained a different product? It would be confusing if this is the case because I obtained a decent MP range for my previous product, and there's essentially only one type of reaction that could take place in the following step.
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maybe the reaction wasn't complete ??? happened to me before :)
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Are you sure you got rid of all the solvent from your solid?
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I am not. It would, in fact, be very likely that there was residual solvent in there. Seeing as how this was lab (not research), we only pressed dry with filter paper over vacuum instead of leaving it connected to a pump in full vac for a few hours. Is that a common reason for this to occur?
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Yes ... i work in a lab ... and thats how we dry a compound completely. ... first like half an hour on the vacuum pump .. then in a vacuum oven ... and then in a normal oven if you;re planning to use it the next day
if the solvent system was an acid .. we make it acid free by washing it in 10% sodium bicarbonate solution ... which also helps... !!!
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I think that could be a reasonable explanation for the depressed but narrow melting point range. I guess I would still expect a bit wider MP range, but depending on how much solvent was there it could be that the trace solvent adsorbed onto your product freed up upon heating and then dissolved your sample.