Acid-base strength inductive 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). 

Generally, substituents located on saturated groups attached to nitrogen influence base strengths through their inductive effects in the same way that these substituents influence the strengths of carboxylic acids (see Section 18-2). 

The phosphonitrilic halides are weaker bases than are some related nonaromatic compounds, just as pyridine is a weaker base than piperidine. The bond type in the phosphonitrilic series is probably the same as in the phosphine-imines RaPrNR (80, 81) the stability of these compounds, too, increases with the electronegativity of the substituents on the phosphorus. Tetraphenylphosphine-imine is one of the most stable of these compounds, and forms a hydrochloride. Its base strength is therefore greater than those of the phosphonitrilic halides, presumably because the opportunities for electronic delocalization are less. The Af-ethyl ester of the trimeric phosphonitrilic acid, which has a cyclic structure, but in which resonance of the same type as in the phosphonitrilic halides cannot occur, also forms a hydrochloride ( 1). It is, therefore, a comparatively strong base, in spite of the inductive effect of the two oxygen atoms on the phosphorus. The extreme weakness of the phosphonitrilic halides as bases cannot therefore be wholly due to the inductive effect of the halogens. 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

The pXa value of oxazole itself has been determined240 by the chemical shifts of H-2 in acidic media and is 0.8 0.2 at 33°C. The feebly basic strength of oxazole relative to thiazole (pKa 2.53),297 pyridine (5.23),297 or 1-methylimidazole (7.33 )W8 is attributed primarily to the powerful inductive effect of the electronegative oxygen atom. This effect, evident in isoxazole (pX —2.03),240 299 is clearly more important than any base-strengthening effect from delocalization of the oxygen lone pair in oxazole. 

The add strength also acts upon the induction period. This is demonstrated by the study of the effect of the neutralisation of the acidity of silica-alumina (SA) by various sodium salts. An extension of the induction period is observed (Figure 4), which becomes more important when the base strength of the counter ion is increased. However, this effect is not accompanied by a significant alteration of the final yield, nor by a notable reduction of the rate at the inflexion point. 

It is apparent that acidity in organic molecules is influenced by several factors. Among these, inductive effects, solvent effects, and factors that influence the stability of both the acid and the anionic conjugate base are important. The base is obviously important since the strength of an acid is directly dependent on the strength of the base. 

The concept of resonance and another electronic effect called the inductive effect can be used to explain and predict the differences in strength between other organic acids and bases. 

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