- Purification of N,N-Dimethylformamide (DMF)
Decomposes slightly at its normal boiling point to give small amounts of dimethylamine and carbon monoxide. The decomposition is catalysed by acidic or basic materials, so that even at room temperature N,N-Dimethylformamide (CAS NO. 68-12-2) is appreciably decomposed if allowed to stand for several hours with solid KOH, NaOH or CaH2. If these reagents are used as dehydrating agents, therefore, they should not be refluxed with the DMF. Use of CaSO4, MgSO4, silica gel or Linde type 4A molecular sieves is preferable, followed by distn under reduced pressure. This procedure is adequate for most laboratory purposes. Larger amounts of water can be removed by azeotropic distn with *benzene (10% v/v, previously dried over CaH2), at atmospheric pressure: water and *benzene distil below 80 °C. The liquid remaining in the distn flask is further dried by adding MgSO4 (previously ignited overnight at 300-400 °C) to give 25gL. After shaking for one day, a further quantity of MgSO4 is added, and the DMF distd at 15-20mm pressure through a 3-ft vacuum-jacketed column packed with steel helices. However, MgSO4 is an inefficient drying agent, leaving about 0.01M water in the final DMF. More efficient drying (to around 0.001-0.007M water) is achieved by standing with powdered BaO, followed by decanting before distn, with alumina powder (50gL; previously heated overnight to 500-600 °C), and distilling from more of the alumina; or by refluxing at 120-140 °C for 24h with triphenylchlorosilane (5-10g/L), then distilling at ca 5mm pressure. Free amine in DMF can be detected by colour reaction with l-fluoro-2,4-dinitrobenzene. It has also been purified by drying overnight over KOH pellets and then distd from BaO through a 10 cm Vigreux column.
It has been purified by distilling from K2CO3 under high vac and fractionated in an all-glass apparatus. The middle fraction is collected, degassed (seven or eight freeze-thaw cycles) and redistd under as high a vacuum a possible.
Rapid purification: Stir over CaH2 (5% w/v) overnight, filter, then distil at 20mmHg. Store the distd DMF over 3A or 4A molecular sieves. For solid phase synthesis, the DMF used must be of high quality and free from amines.
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