Protolytic reactions

This realization led me to study related possible intermolecular electrophilic reactions of saturated hydrocarbons, Not only protolytic reactions but also a broad scope of reactions with varied electrophiles (alkylation, formylation, nitration, halogenation, oxygenation, etc.) were found to be feasible when using snperacidic, low-nucleophilicity reaction conditions. 

Not only protolytic reactions but also a whole range of varied elee-trophilic reactions can be carried out on alkanes under superacidic conditions. 

On the above basis it is, in principle, unnecessary to treat the strength of bases separately from acids, since any protolytic reaction involving an acid must also involve its conjugate base. The basic properties of ammonia and various amines in water are readily understood on the Bronsted-Lowry concept. 

Protic, which are analogous to water, have polar character, and are involved in protolytic reactions... 

An important experimental rule for protolytic reactions was estabhshed by Johannes Nicolaus Brpnsted in 1918 (it was later extended to other reactions). He showed that for a series of reactions of the same type, the rate constants and the equilibrium constants are related simply as... 

This equation corresponds to today s general convention of expressing base strength also be means of pKa, where K is considered in the sense of the Bronsted acid-base theory as a protolysis constant of the following protolytic reactions for acids ... 

Aprotic solvents are not protogenic, but can be protophilic, e.g. acetone, 1,4-dioxan, tetrahy drof uran, dimethy If ormamide, hexamethylphos-phortriamide, propylene carbonate and sulpholane. Solvents that do not participate in protolytic reactions, i.e. do not donate or accept a proton, are usually chemically inert, such as benzene, chlorobenzene, chloroform, tetrachloromethane, etc. 

If an amphiprotic solvent contains an acid and base that are neither mutually conjugate nor are conjugated with the solvent, a protolytic reaction occurs between these dissolved components. Four possible situations can arise. If both the acid and base are strong, then neutralization occurs between the lyonium ions and the lyate ions. If the acid is weak and the base strong, the acid reacts with the lyate ions produced by the strong base. The opposite case is analogous. A weak acid and a weak base exchange protons ... 

Reaction (II) could be the neutralization of acetic acid by potassium hydroxide, yielding potassium acetate which can be isolated in the crystalline state. On dissolution in water the K+ cation is only hydrated in solution but does not participate in a protolytic reaction. In this way, the weak base CH3COO is quantitatively introduced into solution in the absence of an equilibrium amount of the conjugate weak acid CH3COOH. Thus... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

The protolytic reaction between two molecules must be formulated as the formation of the pair... 

Protolytic reactions can also occur in fused salts. The solvent participates in these reactions provided that at least one of its ions has protogenic and/or protophilic character. An example of a solvent in which the cation is aprotic and the anion protophilic is ethylpyridinium bromide (m.p. 114°C). The acid HA is protolysed in this solvent (HA -I- Br HBr + A"). Hydrogen bromide acts as a solvated proton and the acidity is expressed as... 

This reaction sequence is completed by protolytic reactions of the dianionic radicals. 

The above models imply that the proton loss of the OH group of the coordinated substrate shifts the mechanism from oxidation to epoxidation with Ru(III). Such a straightforward interpretation of the pH effect was not presented for reactions of the other substrates, i.e. the protolytic reaction, which would act as a switch between the two mechanisms, cannot be identified. 

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers of solvated protons and solvated hydroxide ions unless we add an acid or base to it. A solution containing more solvated protons than hydroxide ions is said to be an acid within the Lowry-Brpnsted theory, and a solution comprising more hydroxide ions than solvated protons is said to be a base. 

In short, the species which essentially differ from each other by one proton only, are known as conjugate base and acid respectively. Sometimes, such a reaction is termed as protolytic reaction or protolysis, where Ax and Bx make the first conjugate acid-base pair and A2 and B2 the other pair. 

If two conjugate acid-base pairs are coupled with one another, then, depending on the magnitude of the constants (2a, b) a proton exchange can take place and this is described as a protolytic reaction. In general, the solvent itself is one acid-base pair. 

K thus represents the quotient of two acidity constants. If water is the solvent the protolytic reaction is described as the dissociation of an acid or base. 

Eigen, M W. Kruse, G. Maass, and L. De Maeyer, Rate Constants of Protolytic Reactions in Aqueous Solution, in Progress in Reaction Kinetics (G. Porter, Ed.), Vol. 2, Chap 6, Macmillan, New York, 1964. 

To answer the above question new results have been obtained by the study of very fast protolytic reactions in aqueous solution. These were carried out during the last few years by means of relaxation methods (sound absorption, dispersion of the dissociation field effect, temperature jump method) (for a survey cf. [3]). The neutralization reaction HgO+ -j- OH- - (Ha0)8 is the most characteristic example. It was possible to determine the rate constant of this reaction by measuring the time dependence of the dissociation field effect of very pure water of specific conductivity of 6 7 10-8 (at 25°C). 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the <7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion ... 

A comparison of C—C and C—H bond reactivities in protolytic reactions indicated that under the conditions of alkylation C—H cleavage is much more pronounced (in the reaction of ethane, the relative ratio of C—C C—H cleavages is 9 1 in the absence of benzene and 1 10 in the presence of benzene). This indicates that overall alkylation can also involve the attack of protonated benzene (benzenium ion) on the C—H bond of the alkane [Eq. (5.48)] or interaction of benzene with the five-coordinate alkonium ion prior to its cleavage to the trivalent cation ... 

Eigen M, Kruse W, Maass G, de Maeyer L (1964) Rate constants of protolytic reactions in aqueous solution. Prog Reaction Kin 2 285-318... 

According to Bronsted (1928), hydrocarbons cannot participate in protolytic reactions. The incorrectness of this conclusion (which we have already indicated above) and the many departures from the quantitative theory of Bronsted (Izmailov, 1959) show that the theory is inadequate for the description of protolytic reactions. 

The er-alkyl complex [Me2Si(OrBuXNtBu)]2YCH(SiMe3)2 (69b) is highly reactive and undergoes protolytic reactions with various reagents as well as... 

Owing to the longer lifetime of the triplet state it is expected that the protolytic reaction will usually reach equilibrium within the lifetime of the state. Unlike the fluorescence titration method for pAr(S1) described above, the triplet-triplet absorption technique leads directly to pA"(T ]) without the necessity for a knowledge of lifetimes. Phosphorescence titration studies, on the other hand, will involve the lifetime term log t0/To just as for fluorescence. 

The pH at the half-way point on full titration curves of the Fig. 3 type, or on the similar curves approaching the plateau in Fig. 8, gives a simple indication of the p-K Sj) in favourable cases. For compounds of either Case I or Case II type, the pH at the half-way point is log o [k2T 0/(l + 6iT0)], which is only the same as p.K(Si) if t 0 = t0 and k1T0 1. Apart from the complication of the lifetime ratio, the half-way point on an emission titration curve can only be expected to correspond to the excited state pK-value if equilibrium is achieved in the protolytic reaction i.e. k1 and k2 must be large compared with the rates of the decay processes, which sum to 1/t0 and 1/tq respectively, whence r0 > 1 and k 2T 0 > 1. [The 1 in the denominator, or numerator, of equations like (25), (26), (27), or (29) always takes care of the extent to which failure to achieve equilibrium is important.]... 

Rate constants of excited state protolytic reactions... 

These equations represent a transfer of a proton from A, (Acid,) to B2 (Base2). Reactions between acids and bases are hence termed protolytic reactions. All these reactions lead to equilibrium, in some cases the equilibrium may be shifted almost completely in one or another direction. The overall direction of these reactions depends on the relative strengths of acids and bases involved in these systems. 

Some examples of protolytic reactions are collected below ... 

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