## Abstract

The photolysis of 3-pentanone-d$_{10}$ has been studied over a temperature range from 25 to 314 degrees C, and the kinetic results agree closely with those observed with 2,2,4,4-(3-pentanone)-d$_{4}$. The reaction Et$\cdot $ + Me $\cdot $CD$_{2}$$\cdot $CO$\cdot $CD$_{2}\cdot $ Me$\rightarrow $Et$\cdot $D + Me$\cdot \dot{{\rm C}}$D$\cdot $CO$\cdot $CD$_{2}$$\cdot $Me is common to both systems and no significant difference was found between the two possible values, either of the energy of activation or of the frequency factor. A value of 1$\cdot $6 kcal/mole was found for the mean difference in activation energy for the abstraction of D or H, respectively, by the ethyl radical from the $\alpha $-methylene group of 3-pentanone (-d$_{10}$, -d$_{4}$ or -d$_{0}$). This agrees well with the corresponding value of 1$\cdot $6 kcal/mole for the abstraction of D or H by the methyl radical from acetone (-d$_{6}$ or -d$_{0}$). All five reactions share the same value of the frequency factor within the limits of experimental error. We may conclude that an appreciable isotopic effect is observed only when the substitution of D for H has increased the strength of the bond to be broken, and that this effect is an increase in the activation energy alone, the frequency factor remaining unaltered.