A general limiting law is proposed which can be used to systematize the various laws of additivity of molecular properties. This can be stated as follows: ``If φ is a molecular property, then for the disproportionation reaction: RNR+SNS⇄2RNS, Δφ→0 as the separation between R and S becomes large compared to their dimensions. It is shown that the zero-order approximation is equivalent to the law of additivity of atomic properties, the first-order approximation to the law of additivity of bond properties, the second-order approximation to the law of additivity of group properties, and so forth. It is shown that for Cp and S° (ideal gases), the additivity of atomic properties works to about ±2 cal/mole-°K, while the additivity of bond properties is usually good to about ±1 cal/mole-°K. The latter also estimates ΔHf° to about ±3 kcal/mole. The group additivity relation is generally obeyed to within ±0.5 cal/mole-°K for Cp and S° and about ±0.6 kcal/mole for ΔHf°. Tables are presented for each of the partial properties at 25°C. The agreements found for the various additivity rules is examined from a molecular point of view and certain extensions and limitations indicated. The application and utility of the rules in estimation of thermodynamic properties is discussed. The estimation of bond dissociation energies is possible with the additivity rules as are the thermodynamic properties of free radicals. An application of the rules to species NR2 and NS2 show that R and S may be ordered according to their general bond-weakening or bond-strengthening properties. For systems where N is C2H4 it is shown that the first and second bond dissociation energies for RC2H4R is very likely a constant = 59.5 kcal, the excitation energy of C2H4.

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Rate constants for the gas-phase reactions of OH radicals with ethylene and halogenated ethylene compoundsMaking use of the independently measured rate constants for the reactions X+X+M~X.+M (where X=Br, I, etc.) it is possible to calculate the times needed to approach the stationary state for X atoms. On applying these results to the kinetic studies which have been made of halogenation reactions, it is found that for most thermal brominations, the times are in excess of 100 seconds. For thermal chlorinations, the times are in excess of 1000 seconds while for more strongly bonded diatomic molecules they are still higher. In photochemical systems the times are of the order of 0.1 to 1.0 second for absorbed intensities of 10 12 quanta/cc sec which are quite usual. The implications of these values for many of the systems studied, are that heterogeneous initiation and termination of chains are contributing significantly to the kinetics of thermal halogenations at temperatures below 500°C.An examination of the brominations of isobutane, toluene, and neopentane shows that the mechanisms proposed in each case may *