The Field Effect

Sections 4.9-4.13 were concerned with the effects of substituents on chemical equilibria in cases where the reactant and product contained conjugated systems that differed in some respect Substituents can, however, also affect equilibria in cases where neither reactant nor product is conjugated or in which both contain identical conjugated systems. 

Suppose, for example, that the substituent has an electronic charge (e.g.. 

Consider, for example, the successive ionization of the two carboxyl groups in malonic acid, 

Therefore malonic acid (pX 2.77) is not only a stronger acid than the semi-malonate ion, but is also a stronger acid than acetic acid (pX 4.76). 

Suppose now that the charge r at the reaction center changes during the reaction by an amount 5 r. The electrostatic energy of interaction Ep in equation (4.131)] will then change by an amount dEp given by 

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Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

In contrast, equilibrium properties have been successfully discussed in terms of the field effect. Notable instances are those of the ionisation constants of saturated dibasic acids, - and of carboxyl groups held in... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

In addition to the steric effects shown especially by alkyl and aryl groups, the field effect of strongly polar groups must also be considered. For example, 4-hydroxypyrid-2-one-6-carboxylic acid (22)... 

Figure 14-13. Evolution of the field-effect mobility of OFETs for five organic materials polythio-phenc (PT) and its derivatives, qualerthiophcne (4T), scxithio-phenc (6T), dihcxyl-sexithiophene (DH6T). and pcntaecnc.

Another significant feature found recently is that the effect of the chain length on the field-effect mobility is much less pronounced than indicated in earlier reports [68, 74]. The increase from 4T to 6T corresponds to about a factor of ten, while that from 6T to 8T is only two (the low mobility measured for the dihexyl-substituted 8T must be ascribed to the difficulty in synthesizing and purifying this compound 75 J). Representative data arc gathered in Table 14-1. Also note that the effect of alkyl end substitution is reduced by a factor of two to three (as compared to up to 1000 in earlier reports 68 ). 

Figure 14-21. Varialion of the field-effect mobility as a functior of the conductivity of various) doped polyfdodccyloxy-terlhienyl) (PDOT), a polylhiophene derivative (adapted from Ref. II3 ).

An alternative method to estimate the field-effect mobility consists of using the transconduclanee in the linear regime, given by Eq. (14.32). We noie that the... 

Figure 14-23. Variation of the field-effect mobility, as deduced by differentiating the drain current at Vt,=-i V, as a function of the gale voltage, for the same device as in Figure 14-22.

As suggested by Roberts and Moreland many years ago (1953), the acidity constants of 4-substituted bicyclooctane-l-carboxylic acids provide a very suitable system for defining a field/induction parameter. In this rigid system the substituent X is held firmly in place and there is little possibility for mesomeric delocalization or polarization interactions between X and COOH (or COO-). Therefore, it can be assumed that X influences the deprotonation of COOH only through space (the field effect) and through intervening o-bonds. On this basis Taft (1956, p. 595) and Swain and Lupton (1968) were able to calculate values for o and crR. 

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). 

This order is opposite to that expected from the field effect (p. 16). It is an example of the Baker-Nathan order (p. 71). 

For a review of the field effects of alkyl groups, see Levitt, L.S. Widing, H.F. Prog. Phys. 

The concept of hyperconjugation arose from the discovery of apparently anomalous electron-release patterns for alkyl groups. By the field effect alone, the order of electron release for simple alkyl groups connected to an unsaturated system is fert-butyl > isopropyl > ethyl > methyl, and this order is observed in many phenomena. Thus, the dipole moments in the gas phase of PhCHa, PhC2Hs, PhCH(CHa)2, and PhC(CHa)a are, respectively, 0.37, 0.58, 0.65 and 0.700. ... 

However, Baker and Nathan ° observed that the rates of reaction with pyridine of p-substituted benzyl bromides (see Reaction 10-44) were about opposite that expected from electron release by the field effect. That is, the methyl-substituted compound reacted fastest and the tert-butyl-substituted compounded reacted slowest. 

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

If only the field effect were operating, 1 would be more stable than 2, since deuterium is electron-donating with respect to hydrogen (p. 18), assuming that the field effect of deuterium could be felt two bonds away. 

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