in this series are Hellerman et al. (1965) . The authors are glad to acknowledge the skilled technical assistance of M. Kathleen Garrity.

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ESTIMATION OF CYSTEINE WITH O-IODOBENZOIC ACID255 1 t-butyl hypochlorite, 2,3-epoxybutane with hydro-cyclic compound, in other cases the attack is chloric acid, 2,3-diacetoxybutane with hydro-intermolecular. All evidence is opposed to the chloric acid, 3-chloro-2-butanol with thionyl chlo-concept of a carbonium ion as an intermediate. ride, with thionyl chloride and pyridine, with When a carbon-to-chlorine bond possesses conphosphorus trichloride, and with potassium hy-siderable ionic character, replacement of the chlodroxide, is best accounted for on the basis of re-rine atom by an electron donating reagent takes moval of one atom (or radical) with simultaneous place by attack on the opposite side of the carbon attack at the back side of the carbon atom by a atom with inversion, rather than a t the front withnucleophilic reagent. In some cases the attack out inversion. is intramolecular, leading to the formation of a PASADENA, CALIFORNIA