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

The correlation equation logk(25 degrees C) = s(f)(N-f + E-f), where s(f) and N-f are nucleofuge-specific parameters referring to leaving group/solvent combinations and Er 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 E-f parameters of reference electrofuges and the s(f) and N-f parameters of reference nucleofuges reported in the preceding article for determining further s(f), N-f, and E-f parameters. Since s(f) is usually close to 1.0, one comes to the semiquantitative rule that at 25 degrees C, compounds R-X for which N-f + E-f > -2 will solvolyze with half-lives of less than a minute, while the solvolysis half-lives will exceed I month if N-f + E-f < -6.5.