Dual Substituent Parameters

A more ambitious goal is to separate completely resonance effects from polar effects. This involves using separate substituent constants to account for resonance and polar effects. The modified equation, called a dual-substituent-parameter equation, takes... 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

Examples of the successful application of such dual substituent parameter (DSP) treatments with field and resonance parameters will be discussed in Sections 8.3 (hydroxy-de-diazoniation) and 10.5 (Sandmeyer reaction). 

Before we close this section we make reference to an extended form of the Hammett equation in which the substituent constant and the reaction constant are separated into contributions from the field effect (F) and the mesomeric effect (R). This procedure was suggested by Taft in 1957 for 4-substituted benzene derivatives. It is called a dual substituent parameter (DSP) equation (Scheme 7-2). 

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

Gilbert et al. (1992, see also Galli, 1988) showed that a dual substituent parameter treatment (DSP, see Sec. 7.2) gave the best fit to the influence of substituents as measured by Doyle et al. (1987 a) with Fe(CN)6, as well as for reactions with the ferro complex of ethenediaminetetraacetic acid (Fen-EDTA). 

Kuokkanen evaluated a series of constants. Kpeg for substituted diazonium ions with PEG 1000 and found a reaction constant (p = 1.12) comparable to those for complexation with the three crown ethers investigated by Nakazumi et al. (1983), p = 1.18-1.38). It is therefore likely that the host-guest interaction of diazonium ions with acyclic polyethers is basically similar to that with crown ethers. A dual substituent parameter analysis (DSP, see Sec. 8.3) for (Nakazumi et al., 1987)... 

More accurate information on k3 is obtainable if the equilibrium constant K is also determined at various crown ether concentrations, as shown by Nakazumi et al. (1981, 1983). The results with benzenediazonium tetrafluoroborate and 3- and 4-substituted derivatives demonstrate that k3 is not unmeasurably small, but that ky-values are generally 1-2% of k2 for complexation with 18-crown-6, 0.1-0.5% of k2 with 21-crown-7, and 2-10% of k2 with dicyclohexano-24-crown-8. A dual substituent parameter (DSP) analysis of A 3-values (Nakazumi et al., 1987) showed that the dediazoniation mechanism of the complexed diazonium ions does not differ appreciably from that of the free diazonium ions. 

Dodecahydrododecaborate dianion 380 Dual substituent parameter (equations) 50,... 

Hammett equation(s) 78, 93, 148ff., 151 f., 153ff., 167f., 190, 193, 196, 297, 299, 308, 312, 375, 381, 392, see also Dual substituent parameter, and Quantitative structure-reactivity relationships Hammond postulate, in additions of nucleophiles to diazonium ions 157 Hard and soft acids/bases principle (Pearson) 49, 54, 109... 

Linear free energy relationships, see Bronsted equation, Dual substituent parameter (equations), Hammett equation(s), Quantitative structure-activity relationships, Ritchie nucleophilicity equation... 

Multiple regression on ar and parameters employs the dual substituent-parameter equation, which may be written as in equation 964. 

However, in more recent years it has become usual to employ ar or crR-type constants, either together in the dual substituent-parameter equation or individually in special linear regression equations which hold for particular infrared magnitudes. In this connection a long series of papers by Katritzky, Topsom and their colleagues on Infrared intensities as a quantitative measure of intramolecular interactions is of particular importance. We will sample this series of papers, insofar as they help to elucidate the electronic effects of sulfinyl and sulfonyl groups. 

Furthermore, solvent, temperature and substituent effects on pXHB are also presented and dual substituent parameters have been introduced into two carbonyl series (CH3)2NC(0)X (X = CF3, CH2C1, H, CH3, NMe2) and CH3C(0)X (X = CH3CO, CF3, H, F, CH3, OCH3, NMe2) and related to the following equation ... 

A Generalized Treatment of Substituent Effects in Benzene Series. A Statistical Analysis by the Dual Substituent Parameter Equation. By S. Eh REN SON, Department of Chemistry, Brookhaven National Laboratory, Upton, Long Island, New York R. T. C. BROWNLEE, Department of Chemistry, La Trobe University, Bundorra, Victoria, Australia R. W. Taet, Department of Chemistry, University of California, Irvine, California. I... 

A Generalized Treatment of Substituent Effects in the Benzene Series. A Statistical Analysis by the Dual Substituent Parameter Equation (1)... 

The dual substituent parameter treatment attributes substituent effects to an additive blend of polar (/) and pi delocalization (R) effects, each of which may be represented as a op product (3) ... 

Swain and Lupton (2q) have recently presented a modified form of the dual substituent parameter treatment. In this treatment, the p/parameters were... 

The previous analysis by the dual substituent parameter equation of substituent effects in the naphthalene series provided support for the scale, especially for sets involving nonconjugating positions (2p). The available data yield six basis sets which presumably give a critical analysis and, in particular, provide distinctions between conjugative (three sets) and nonconjugative positions (three sets). The results (using the earlier symbolism (2p)) are given in Table X. 

Charton, M., Prog. Phys. Org. Chem., 8, 235 (1971) has reported extensively on correlations of rate data for ortho substituted benzene derivatives using the dual substituent parameter treatment in the form with an additional (intercept) parameter, and in our opinion, too limited substituent data sets. For these and related reasons which we have discussed, we question the significance of many of Charton s correlations. 

It is important to note that the fitting according to eq. (1) requires zero intercept behavior i.e., F =. 00 for H (for which Oj = Or =. 00). While we recognize that the data for the unsubstituted (H) member of a set may be as subject to experimental error as any other member, such error is generally relatively small for a set of reliable data. Any constant error from this source will be distributed among all of the substituents in such a manner as to achieve best fit. Any loss in precision of fitting of the set which may result by such a procedure we believe is a small price to pay compared to the violence done by introduction in eq. (I) of a completely variable constant parameter. The latter procedure has been utilized by other authors both in treatments by the simple Hammett equation and by the dual substituent parameter equation. 

It is apparent from these examples (and the previous ones) that the dual substituent parameter treatment appears to provide an important potential means of characterization of aromatic substitution (and other) transition states. 

Diparametric equation A relationship in which the effect of structure on a property is represented by two parameters, one of which is generally composite. Examples discussed in this work include the LD, CR and MYT equations. Other examples are the Taft, Ehrenson and Brownlee DSP (dual substituent parameter), Yukawa-Tsuno YT and the Swain, Unger, Rosenquist and Swain SURS equations. The DSP equation is a special case of the LDR equation with the intercept set equal to zero. It is inconvenient to use and has no advantages. The SURS equation uses composite parameters which are of poorer quality than... 

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