Protolytic mechanisms

All three methods for generating pH gradients depend on electrolytic, or protolytic, mechanisms to form pH gradients. The pH gradients obtained with carrier ampholytes and the buffer pairs are linear, but IPGs can be constructed that are nonlinear.2... 

Cracking of small saturated hydrocarbons, catalyzed by zeolites, can proceed via two mechanisms, both involving carbocations the bimolecular chain reaction, which involves carbenium ions that are further transformed by / -scission, and the unimolecular protolytic mechanism, involving alkanium ions that are formed by the direct protonation of the alkane by the Br0nsted acid OH groups of the catalyst. This latter mechanism, originally proposed by Haag and Dassau, is the predominant one at about 800 K in medium-pore zeolites, like HZSM-5, which favor monomolecular reactions. While rela-... 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

The following non-radical chain mechanism was proposed for the reaction in aqueous solution (for sake of simplicity, fast protolytic and complex-formation reactions are not shown) (36) ... 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

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. 

One thing is clear there can be no acid-base equilibrium between states of different multiplicities thus it is correct to consider only the pK of the singlet state , or the pA of the triplet state . However, the question of the protolytic equilibrium between an mr singlet and a tttt or charge transfer (CT) singlet remains open. This problem is illustrated in Figure 4.48 for the case of 4-hydroxybenzophenone, in which there is a reversal in the order of mr and CT states between the acid and base forms. Excitation of the protonated molecule in ethanol for example leads to the ground state deprotonated form, but the detailed mechanism of this process is not known. 

On the other hand, above 20mol% SbF5, a small but increasing amount of unionized SbF5 can be observed, which may rationalize the change in the mechanism of alkane activation from the protolytic to the oxidative pathway, when the concentration of SbF5 increases over 20mol% (see Section 5.1.1). 

The synthesis of hydroxycarbamates from secondary aliphatic amines, C02 and epoxides has been found to be catalyzed by (5,10,15,20-tetraphenylporphinato) aluminum(III) acetate, AI(TPP)(02CCH3) [80], Scheme 6.14 illustrates the mechanism proposed for the catalytic process, which can be carried out under not severe conditions (293-343 K 0.1-5 MPa C02 pressure). The key step here is the insertion of epoxide into the Al-O bond of the A1-carbamate A (Scheme 6.14), which preliminarily forms by the reaction of A1(TPP)(02CCH3) with the amine and C02. Protolytic cleavage of the Al-alkoxide bond in the insertion product, C, by dialkylcarbamic acid regenerates the catalytkally active carbamato-species A and... 

This mechanism presents the closest analogy to protolytic equilibrium. Combination of equations (37) and (44) yields... 

Retention of configuration of the alkyl group is consistent with the concerted cyclic mechanism used to explain why carboxylic acids alone protolytically cleave carbon-boron bonds (Equation B2.13). 

Figure 1-20. Mechanism of the protolytic dissociation in the tetramer (H20)3HC1...

As for the dehydrogenation reaction, IRC calculations [67] indicate that the protolytic cracking of linear and branched alkanes follows different mechanisms. For ethane and propane the products of the reaction are methane and the proper alkoxide. For isobutane, as one follows the reaction path towards the products, the t-butyl cation decomposes into propene and a proton which restores the acid site of the zeolite. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

Abstract The ab initio pseudopotential plane wave DPT simulation of the structure and properties of zeolite active sites and elementary catalytic reactions are discussed through the example of the protonation of water and the first step in the protolytic cracking mechanism of saturated hydrocarbons. 

Among the numerous theoretical approaches applied to the zeolite reactivity problem, we focus our attention mainly on calculations, which use recently developed ab initio molecular dynamics techniques. After a very brief overview of the main features of this methodology, we discuss some applications taken from the modeling of zeolite chemistry the characterization of the catalytic sites, the protonation of a water molecule and the mechanism of the protolytic reactions of alkanes. 

Pedersen (39) has presented the theory of protolytic reactions and considers that the mechanisms are usually bimolecular. He makes the seemingly justifiable assumption that a proton cannot move spontaneously from one place in the molecule to another without a catalyst, while valency electrons can move spontaneously within the molecule. For example, in the triad systems,... 

It can be shown that the mechanism expressed in equation (41) always appears as a first-order reaction catalyzed by bases (proton acceptors) in general, when the hydrogen ion concentration is so great that [RH] [ ]. If 1 takes up protons to form mainly SH, we measure the velocity of the protolytic reaction between RH and B, i.e., the velocity of dissociation of the very weak acid RH. If I takes up protons to form RH, the apparent basic catalysis is a disguised acid catalysis of the ion I. If neither of these possibilities is especially favored, we get an apparent basic catalysis which is the result of both a protolytic reaction between RH and the bases and between the acids and I. From the kinetic experiments, it is impossible to decide which of the three possible cases we have in a given reaction. 

Kinetics and mechanisms of dissociation of manganese complexes with porphyrins in mixed protolytic solvents 04MI14. 

The mechanism and scope of rare-earth metal-catalyzed intramolecular hydrophosphination has been studied in detail by Marks and coworkers [147,178-181]. The hydrophosphination of phosphinoalkenes is believed to proceed through a mechanism analogous to that of hydroamination. The rate-determining alkene insertion into the Ln-P bond is nearly thermoneutral, while the faster protolytic o-bond metathesis step is exothermic (Fig. 22) [179,181]. The experimental observation of a first-order rate dependence on catalyst concentration and zero-order rate dependence on substrate concentration are supportive of this mechanism. A notable feature is a significant product inhibition observed after the first half-life of the reaction. This is apparently caused by a competitive binding of a cyclic phosphine to the metal center that impedes coordination of the phosphinoalkene substrate and, therefore, diminishes catalytic performance [179]. 

---