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Communications to the Editor 7421 rium from Scheme I with an equation similar to eq 1 ;5b using the known values for K2K2 and K4 we may estimate K2 and K2 to be < 10"11 and > 1, respectively. The ionization pAla for the amidine {K2) is relatively low presumably owing to the powerfully electron-withdrawing ammonio group.Assuming equilibrium of the species II-VI and rate-limiting proton-catalyzed hydrolysis of the free carbodiimide VI, we may estimate the acid plateau rate constant (pH 0-4) from proton attack on the trimethylammonio model (&h = 3 X 102 M-1 s_1). The overall rate constant for reagent I becomes kt\K\K2K2/Kh5q in the acid plateau region which gives 3 X 10-2 s_l on substituting values for the equilibrium constants and & • The estimated value is much larger than that observed (10~3 s_1) indicating that one of the steps ]is rate limiting for the carbodiimide mechanism. The decomposition rate constant for VI increases with decrease in pH and it is likely that K2 becomes the rate-limiting step because this is pH in-

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