A likely impurity is monoethyl malonate; check IR for OH bands at 3330 br cm-1 . To ca 50g of ester add ice cold NaOH (50g in 200mL of H2O and 200g of ice). Swirl a few times (filter off ice if necessary), place it in a separating funnel and extract with 2 x 75mL of Et2O. Dry the extract (MgSO4) (since traces of acid decompose the t-Bu group of the ester, the distillation flask has to be washed with aqueous NaOH, rinsed with H2O and allowed to dry). Addition of some K2CO3 or MgO before distilling is recommended to inhibit decomposition. Distil it under reduced pressure through a 10cm Vigreux column (p 11). Decomposition is evidenced by severe foaming due to autocatalytic decomposition and cannot be prevented from accelerating except by stopping the distillation and rewashing the distillation flask with alkali again. [Breslow et al. J Am Chem Soc 66 1287 1944, Hauser et al. J Am Chem Soc 64 2714 1942, Strube Org Synth Coll Vol IV 417 1963, Stube Org Synth 37 35 1957, Beilstein 2 IV 1884.]
