Induced Crystallization and Reproducibility of Preparative Reactions Reproducibility is one of the basic requirements of preparative reactions. Many such reactions are sensitive even to slight variations in the purity of the readion components and experimental conditions. I t is shown in this note that re~roducibilitv
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In successful experiments the starting material I dissolved in concentrated ammonia, and in a few minutes the product I1 began to separate as crystals. The reaction, then, was completed in 20-30 min. On the contrary, in unsuece~sfulexperiments no crystal formation could be observed even after extended reaction time. Instead, the homogeneous reaction mixture became dark and smell of HzS indicated opening of the thiapyrane ring which is notoriously susceptible t o nucleophiles. However, seeding of the ammonia solution with a few crystals of IImade the preparation fully reproducible. This is possible because the ammonolysis of the carbomethoxy group is faster than that of the ring.The latter reaction only becomescritical for the outcome of the preparation if a supersaturated solution of the product is formed. Then, the exposure of the primary reaction product to ammonia for an extended period of time results in virtually comolete destruction of the molecule. Ironicallv. s ~ ~ p c r s a u ~ r aichr~cms ted iomm more raiilv wnh extremely pure reaoenw, rcrlwnt- and in clean glassware. Sim~larly,induced rrvitallizatim guamntrrd high repndurihlc yields of the radiuartivr lat,clled phnrmarrutir~lK K P R O T E R O I . \ L ; ~ ~a ~ prvredurr which urhcrw~sewas unrelishlr. 'Horak, V..and K ~ ~ r h a r c qN.. k , ('hvm. I n d . W l (lD601.
' Snus, A , Klinwr. K. H.. and Hickel, E.. A r ~ n u i m i r ~Porsch rl , 27 (11, :35 ,1977, V. Horak Georgetown University Washington. D.C. 20057
580 / Journal of Chemical Education
N. Kucharczyk Wallace Laboratories Cranhury, New J e r s e y 08512
