Dried with MgSO₄ or CaSO₄, or refluxed with fresh Ca turnings, then fractionally distd under reduced pressure, collecting the middle fraction and storing with CaH₂ or P₂O₅. Has also been purified by passage through a column of alumina. Alternatively it is dried over MgSO₄ and distd in a vacuum. The middle fraction is degassed by several freeze-thaw cycles and then fractionated in an isolated fractionating column (which has been evacuated and sealed off at ~10-6 mm) over a steam bath. The middle fraction is retained. The final samples were vacuum distd from this sample and again retaining the middle fraction. The purity is > 99.9 % (no other peaks are visible on GLC and the NMR spectrum is consistent with the structure).