The negative p-value means that the reaction rate is increased by the substitution of groups which increase the electron density on the diazocarbon. It is interesting to note2 that the rate is also increased by the substitution of electron-withdrawing groups on the benzoic acid.In the five cases where the effect of a substituent on one ring was compared to the effect of two of the same substituents on different rings, it was found that the -values are satisfactorily additive.s" The good correlation for 3-nitro-4 '-methyldiphenyldiazomethane, where the nitro group with a strong deactivating effect and the methyl group with a moderate activating effect are both influencing the same reaction center, offers striking proof of the additive principle of substituent effects.The results show that an increase in the electron density on the diazocarbon increases the rate of the reaction. This fact is consistent with either of the mechanisms shown in Fig. 1. However, considera-