Acid-base strength steric effects

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

Base strength is relatively unaffected by steric effects because a base removes a relatively unhindered proton. The strength of a base depends only on how well the base shares its electrons with a proton. Thus, ferf-butoxide ion is a stronger base than ethoxide ion since ferf-butanol = 18) is a weaker acid than ethanol (piSfa = 15.9). 

Bhattacharya and Dhar have recently described adducts of bases like pyridine, 2-, 3- and 4-methylpyridine, 2,4- and 2,6-butadienes, quinoline, aniline and piperidine with hydroxamic acids (R CONROH, where R = Ph, o-MeQH4, 0-NO2QH4, 0-CIQH4 R = Ph, o-MeQH4), investigating the effect of steric factors, electronic effects, base strength and concentration in the stabilization of the adducts. 

Ortho and / flra-substituted acids give different straight lines owing to chelation and steric hindrance. To account for slopes less than —1, we must assume that the formation constant of the ion pair, the medium effects, or both depend on the strength of the acid. Mead found linear correlations between heats of neutralisation of amines with trichloroacetic add in benzene and base strength in water primary, secondary and tertiary amines fell on parallel lines. 

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

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