Polar substituent effects

When the reaction is well-established as a radical one it is still possible to find explanations of polar substituent effects, usually in terms of ion-radical intermediates, dipole-dipole repulsion, solvation, or charge-transfer structures like those postulated for -complexes.488 Reac-... 

Taft also defined the Es parameter for aromatic ortho substituents in a fashion similar to Eq. 3 4). Charton advanced evidence that the original value is linked rather with the polar substituent effect despite the original definition51. We have shown that the set of Es parameters defined by Eq. 3 and 4 can also be used to rationalize the steric effect of aromatic ortho substituents 7). In general, the ortho substituent effect in biological activity is analyzable as linear combination of substituent parameters 8) (Eq. 5),... 

Chemical shifts of 4-methyne protons in condensed 4//-pyran derivative 630 correlate well with Hammett a constants for substituents X polar substituent effects operating in molecules 630 were discussed.462 The spectra of some 13C-IabeIed 2//-pyrans were also measured.309... 

Azo compound decomposition is much less susceptible to polar substituent effects, and so probably has less charge separation in the transition state,75 but is more sensitive to geometrical restrictions. Bridgehead azo compounds decompose at rates lower than expected on the basis of their tertiary nature, whereas peresters are much less strongly affected.70 The difference can be rationalized by the proposal that the transition state comes farther along the reaction coordinate in azo decomposition, so that the nonplanarity forced on the incipient radical by the ring system is felt more strongly there. 

Transmission of Polar Substituent Effects in Bridgehead Fluorides. There is a considerable literature, mainly from the group of Adcock, dealing with the transmission of polar substituent effects with... 

Generally, the rate of alkaline hydrolysis of a series of substituted phenyl benzoates was decreased in the presence of 0.5 m BiuNBr, the retardation being larger for esters with electron-donating substituents. The data from 22 esters were fitted to a multiparameter equation, the results showing that solvent electrophilicity was the main factor responsible for changes in the ortho, para and meta polar substituent effects with medium.15... 

Rates of the alkaline hydrolysis of 12 ortho-, meta- and ) ara-X-substituted phenyl tosylates, 4-MeC6H4S02C6H4X, in aqueous 0.5 m Bu4NBr over a wide temperature range have been analysed using the modified Fujita-Nishioka multi-parameter equation. It was concluded from both these and previously reported data by the same group that solvent electrophilicity was the main factor responsible for changes in the ortho, meta and para polar substituent effects with medium.60... 

Adcock, W. and Kok, G.B., Polar substituent effects on 19F chemical shifts of aryl and vinyl fluorides a fluorine-19 nuclear magnetic resonance study of some l,l-difluoro-2-(4-substituted-bicyclo[2,2,2]oct-l-yl)ethenes, J. Org. Chem., 50, 1079, 1985. 

The mechanisms for the transmission of substituent effects through a saturated framework to an encapsulated ion are discussed in Ref. 352. Inductive (or polar) substituent effects in clathrochelates were considered to be a combination of induction via cr bond and through-space field effects. For transition metal complexes, a induction should be reflected in both the ligand field parameters of... 

Reynolds, W.F. (1983). Polar Substituent Effects. Prog.Phys.Org.Chem., 14,165-203. 

Much of the discussion which follows is related to Sjf2 reactions and more specifically to bimolecular nucleophilic substitution at a saturated carbon atom, (Ingold, 1953 Bunton, 1963). Many branches of chemistry have profited from the detailed studies made on this deceptively simple reaction (2), which has attracted the attention of physical organic chemists for many years. Especially notable contributions have been made by Hughes and Ingold (Ingold, 1953). These have led to important advances in our understanding of mechanisms, steric effects, polar substituent effects, salt effects and solvent effects. 

Polar substituent effects arise from changes in electronic charge in a reaction brought about by bond order and solvation changes. 

The linear free energy equations (Chapter 2) do not directly give the charge characteristics of the reaction. The following discussion shows how the polar substituent effects can be treated to obtain a quantitative measure of relative charge. 

Equations (9a) and (9b) can be assumed to govern the resonance and inductive components of the polar substituent effect. 

Another model of transmission of polar substituent effects (often described in introductory organic chemistry texts) is the so-called inductive model, in which it is proposed that a charged or dipolar group modifies the pKa of an acidic functional group by successive polarization of all the intervening a-... 

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