DUAL-PARAMETER SUBSTITUENT CONSTANTS, DELTA-E(X) AND DELTA-C(X), FOR THE CORRELATION OF PHYSICOCHEMICAL MEASUREMENTS

Since chemical bonds have independent covalent (soft or frontier controlled) and electrostatic (charged or charge controlled) contributions, fundamental problems arise when substituent-constant correlations are made with a single-parameter, Hammett-type reactivity scale. A method is offered and parameters are reported to carry out dual-parameter electrostatic DELTAE(x) and covalent DELTAC(x) substituent-constant analyses. The equation DELTA(chi)X -DELTAX(H) = d(A)(E)DELTAE(x) +d(A)(C)DELTAC(x) is offered as a substitute for current approaches to substituent-constant correlations for reactions of a family of donors or nucleophiles. For a family of electrophiles, the equation becomes DELTA(chi)X - DELTAX(H) = d(B)(E)DELTA(x) + d(B)(C)DELTAC(x). The DELTAE(x) and DELTAC(x) parameters are the dual-scale electrostatic and covalent substituent constants. These parameters are the counterparts of the single-scale Hammett sigma-parameters, while d(E) and d(C) are the electrostatic and covalent counterparts of rho. The A subscript on d(E) indicates that the reactions involve the same acceptor (electrophile) reacting with a family of donors. The B subscript indicates reactions of a donor with a family of acceptors. Values of DELTAE(x) and DELTAC(x) for a series of substituents are given, and a procedure is reported for estimating these quantities for other substituents. Data requiring different single-parameter scales (sigma+, sigma(I), sigma(R), and sigma(R)-, etc.) are correlated by the same set of DELTAE(x) and DELTAC(x) parameters with the two-term equation. The different scales of sigma-parameters are shown to correspond to reactions with different ratios of d(C)/d(E), i.e., different covalent and electrostatic contributions. The multitude of different one-parameter substituent-constant scales in the literature can be replaced by the single set of DELTAE(x) and DELTAC(x) parameters reported here, and more meaningful correlations and interpretations of data result. When a substituent-constant analysis is carried out using a probe molecule whose E and C values are known, the components of d(E) can be separated. This leads to a determination of the efficiency with which intervening atoms transmit the substituent effect to the reactive center. The DELTAE(x) and DELTAC(x) values can also be used to calculate E and C values for an entire family of donors and acceptors if the E and C values for four or more members of the family are known accurately.