How fast do R-X bonds ionize? A semiquantitative approach.

The correlation equation log k(25 degrees C) = sf(Nf + Ef), where sf and Nf are nucleofuge-specific parameters referring to leaving group/solvent combinations and Ef are electrofuge-specific parameters referring to the incipient carbocation R+, are used to predict ionization rate constants of alkyl derivatives R--X. We show how to employ the Ef parameters of reference electrofuges and the sf and Nf parameters of reference nucleofuges reported in the preceding article for determining further sf, Nf, and Ef parameters. Since sf is usually close to 1.0, one comes to the semiquantitative rule that at 25 degrees C, compounds R--X for which Nf + Ef>-2 will solvolyze with half-lives of less than a minute, while the solvolysis half-lives will exceed 1 month if Nf + Ef<-6.5.