Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: owk9688 on May 01, 2017, 05:50:16 AM
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Can anyone explain to me why DMF purification protocols are more than just a simple distillation? From what I can tell, the major impurities are dimethylamine, water, carbon monoxide, and formic acid, all with substantially lower boiling points than DMF. If I just distill it at reduced pressure, why wont everything distill over and DMF be left over in the flask?
Thanks! I'd like to understand the reasoning behind having to pre-dry with CaH2 and then distill off the pure DMF at reduced pressure afterwards
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1). During distillation, the remaining impurity dimethylamine formate decomposes to formic acid and dimethylamine that absorbs high amounts of heat that delays distillation.
2). Formic acid forms a maximum azeotrope at 101oC and continues to decompose to carbon monoxide and water. That decomposition is catalyzed by acids. In other words, formic acid autocatalyzes its thermal decomposition.
3). As a general rule, thermal decompositions that involve gas formation (e.g. CO) may also occur at lower temperatures, under reduced pressure. As a consequence, DMF purification by simple vacuum distillation and without any pretreatment, does not help.
4) CaH2 reacts with water (being as preparation impurity) and forms Ca(OH)2 that finally forms calcium formate, which is more stable than dimethylamine formate, towards thermal decomposition.
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Wow...thank you! That was really insightful. I hadn't even begun to think about it on that level of understanding. Thanks so much!