Pi-inductive effect

The interaction moments directed along the major axis of the ring that are necessary to account for the observed moments of the meta derivatives of the trifluoromethylaniline compounds are appreciable. The values of 0.62 for the for the 3,5-ditrifluoromethyl derivative and 0.30 for the 3-trifluoromethyl derivative are comparable with the interaction moment, 0.66, assessed for N,N-dimethyl-4-trifluoromethylaniline. Inasmuch as resonance interactions are impossible in the meta derivatives, Ibbitson and his associates concluded that all the observed interaction moments for the trifluoromethyl groups are the consequence of pi inductive effects which are operative in both the meta and para positions. Unfortunately, no data are available for the wefa-perfluo-... 

Table IV. Effect of Damage on PI Induction in Tomato Foliage...

The above discussion assumes the usual inductive effect which halogens have on carbonium ions, but, of course, necessitates a reversal of the usual order for resonance effects. It is important to note that while fluorine can quite effectively form a pi bond with carbon, it cannot do so with the large mercury atom. Thus, structures such as 24 should be important only for the larger halogens. 

The magnetic fields induced by pi electrons are directional, i.e., unsymmetrical. A measurement which varies with the direction in which the measurement is taken is said to be anisotropic. Because the effects of molecular fields induced by 71 electrons are direction dependent, these are, therefore termed anisotropic effects. These effects are contrasted to inductive effects, which are symmetrical around the proton. 

Because inductive effects operate through halogen substitution decreases as the substituent moves farther from the carboxyl. For instance, 2-chlorobutanoic acid has pK = 2.86,3-chlorobutanoic acid has pi a = 4.05, and 4-chlorobutanoic acid has pKa = 4.52, similar to that of butanoic acid itself (Table 20.5). 

Substituents on carbon-2 of acetic acid can express only an inductive effect no resonance effect is possible because the CH2 is sp hybridized and no pi overlap is possible. 

We don t usually look to aromatic systems for examples of inductive effects, because the pi system of electrons is ripe for resonance effects. However, in analyzing the resonance forms of phenoxide on the next page, it becomes apparent that the negative charge is never distributed on the meta carbons. Meta substituents cannot exert any resonance stabilization or destabilization at the meta position, substituents can exert only an inductive effect. The series of phenols demonstrates this phenomenon, consistent with aliphatic carboxylic acids. 

The difference between sigma and pi complex stabilities of various methylbenzenes. Table 1, is dramatic. The positive inductive effect of the methyl groups helps stabihze the positive charge developed in the a complex, and accounts for the var-... 

Over the years we came up with methods to calculate bond dissociation energies, charge distribution in sigma-bonded systems, charge distribution in pi-bonded systems, the inductive effect, the resonance effect, the hyperconjugation effect and the polarisability effect. ... 

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