Purification Methods
The cis-acid is prepared by catalytic reduction of phenylpropiolic acid and after distillation in a high vacuum at ~95o it gives the most stable allo-isomer m 68o. Recrystallisation from pet ether yields Liebermann's iso-cinnamic acid m 58o . When the allo-acid (m 68o) is heated at 20o above its melting point in a sealed capillary for 0.5hours and allowed to cool slowly, Erlenmyer's iso-cinnamic acid m 4 2o is formed. This form can also be obtained in larger amounts by heating the allo-acid at 80o for 3hours, and on cooling it remains liquid for several weeks but gives the m 42o acid on innoculation with the crystals from the capillary tube. This form is unchanged in 6 weeks when kept in a dark cupboard. All three forms have the same pK values and the same rate of bromination. There is also a very labile form with m 32o. [Liebermann, Chem Ber 26 1572 1893, Claisen & Crismer Justus Liebigs Ann Chem 218 135 1883, Robinson & James J Chem Soc 1453 1933, Berthoud & Urech Helv Chim Acta 13 437 1930, McCoy & McCoy J Org Chem 33 2354 1968, Beilstein 9 IV 2001.]
