Acid-base reactions inductive effects

Applying p/C Values in Organic Acid-Base Reactions 3. EFFECT QF STRUCTURE QN ACIDITY Effect of Periodic Trends on Acidity and Basicity Effect of Resonance on Acidity and Basicity Inductive Effects Effect of Hybridization on Acidity... 

Proton acid-base reactions are not particularly sensitive to stoic compression, and hence provide a good measure of inductive effects. For acid-base character, three sets of reference reactions can be used. The easiest of these to perform experimentally requires an analogy be drawn between the relative pK values of a series of protonated annelated pyridines and the pK values of the analogous isoelectronic benzene. The second is a direct measure of the kinetic acidity of the a- and P-sites on a soies of annelated benzenes. The third is a related direct assessment of kinetic acidity by protodetritiation. 

We begin our study of mechanisms in the context of acid-base chemistry in Chapter 3. Acid-base reactions are fundamental to organic reactions, and they lend themselves to introducing several important topics that students need early in the course (1) curved arrow notation for illustrating mechanisms, (2) the relationship between free-energy changes and equilibrium constants, and (3) the importance of inductive and resonance effects and of solvent effects. 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strong base, commonly an organometallic base such as butyllithium or phenyllithium. The hydrogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidic because the carbanion of the conjugate base (the ylide) is stabilized by the inductive effect of the positive phosphorus atom. In addition, a resonance structure with five bonds to phosphorus makes a minor contribution to the structure and provides some additional stabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reaction of triphenylphosphine with the appropriate alkyl halide (see Section 10.9). 

Comparison of the overall rate constants (when ionisation occurs along two competitive paths) or of the rate constants (when there is only one enolisation site) with that of a parent unsubstituted methyl ketone, e.g. acetone or acetophenone, shows that an alkyl group usually decreases ketone reactivity under conditions of base catalysis. This is in agreement with a small electron-repelling inductive effect which makes the carbanion ion less stable (e.g. the halogenation rate constant decreases by a factor of 6.5 on going from acetophenone to propiophenone when the reaction is catalysed by acetate ion [acetic acid-water 75 25 at 25°CI (Evans and Gordon, 1938). However, the factor is very small and could be explained by steric effects as well. 

The lower value of for dimers can be explained by simple arguments. The steric requirements of the bulky siloxane group should decrease the reaction rate constant of dimers compared with monomers. Inductive effects should decrease the rate as well. This latter conclusion is based on the mechanism proposed by Pohl and Osterholtz (5), which postulates that in acidic solutions (pH < 4.5), an equilibrium concentration of protonated silanol is rapidly established. The protonated silanol reacts with a neutral silicate species in the rate-determining step, and then, a hydronium ion is eliminated. The more electron-withdrawing siloxane group (compared with Si-OR and Si-OH) would decrease the stability of the protonated species and effectively shift the equilibrium toward the unprotonated form. Therefore, the observed reaction rate constant should decrease. 

It is difficult to stop this reaction after the addition of one Br atom because the electron-withdrawing inductive effect of Br. stabilizes the second enolate. As a result, the a H of a-bromopropiophenone is more acidic than the a H atoms of propiophenone, making it easier to remove with base. 

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 factor 2.48 corresponds to the average of the available p values of alkaline hydrolysis from the Hammett equation and attempts to place a on the same scale as the and 0p electronic constants. Since base-catalyzed hydrolysis involves both inductive and steric effects and acid-catalyzed hydrolysis involves only the steric effect, removing the steric effect leaves only the inductive effect, assuming that in both reactions the steric effects are the same. 

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