Empirical structure-reactivity correlations are developed for log k(298), the gas-phase rate constants for the reaction (Cl-center dot + HCR3 -> ClH + CR3 center dot). It has long been recognized that correlation with Delta H-r is weak. The poor performance of the linear Evans-Polanyi formulation is illustrated and was little improved by adding a quadratic term, for example, by making its slope smoothly dependent on Delta H-r [eta (Delta H-r - Delta H-r(min))/(Delta H-r(max) - Delta H-r(min))]. The "polar effect" (Cl delta----H---CR3 delta+)(double dagger) has also been long discussed, but there is no formalization of this dependence based on widely available independent variable(s). Using the sum of Hammett constants for the R substituents also gave at best modest correlations, either for sigma(para) or for its dissection into F (field/inductive) and R (resonance) effects. Much greater success was achieved by combining these approaches with the preferred independent variable set being either [(Delta H-r)(2), Delta H-r, Sigma F, and Sigma R] or [eta, Delta H-r, Sigma F and Sigma R]. For 64 rate constants that span 7 orders of magnitude, these correlation formulations give r(2) > 0.87 and a mean unsigned deviation of <0.5 log k units, with even better performance if primary, secondary, and tertiary reaction centers are treated separately.