Substituent size, equilibrium distribution

These same authors (16) also compared experimental values as a function of x for a series of bulk equilibrates of the structure -+R(CH3)Si-0—J-x wherein R equalled H, CH3, CH3CH2, CH3CH2CH2, and CF3CH2CH2 in order to assess the effect of the substituent size on the equilibrium distribution. The values... 

The extent to which a given reaction responds to electronic perturbation constitutes a measure of the electronic demands of that reaction, which is determined by its mecha-, nism. The introduction of substituent groups into the framework and the subsequent alteration of reaction rates helps delineate the overall mechanism of reaction. Early work examining the electronic role of substituents on rate constants was first tackled by Burckhardt and firmly established by Hammett (13, 14, 77, 78). Hammett employed, as a model reaction, the ionization in water of substituted benzoic acids and determined their equilibrium constants K. See Equation 1.28. This led to an operational definition of o-, the substituent constant. It is a measure of the size of the electronic effect for a given substituent and represents a measure of electronic charge distribution in the benzene nucleus. 

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