Mechanism proton acids

Mechanism of Acid-Catalyzed Hydration Three steps are involved m acid catalyzed hydration (Figure 17 7 on page 718) The first and last are rapid proton transfers between... 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

The mechanism for acid-catalyzed hydrolysis of amides involves attack by water on the protonated amide. An inqjortant feature of the chemistry of amides is that the most basic site in an amide is the carbonyl oxygen. Very little of the N-protonated form is present. The major factor that contributes to the stability of the O-protonated form is the... 

Two mechanisms for acid-catalyzed hydrolysis can be imagined depending on the initial site of protonation. One mechanism begins widi protonation at X this makes X a better leaving group. The alternative is protonation of the carbonyl oxygen. 

The reaction of a tertiary alcohol with HX takes place by an Sf l mechanism when acid protonates the hydroxyl oxygen atom, water is expelled to generate... 

Figure 22.1 MECHANISM Mechanism of acid-catalyzed enol formation. The protonated intermediate can lose H+, either from the oxygen atom to regenerate the kelo tautomer or from the a carbon atom to yield an enol.

It is not only in the field of kinetic relations that discrepancies exist. When the catalyst is a protonic acid and the reaction is carried out in dilute solution, the mechanisms describing the contribution of the catalyst are relatively well-known. But in most other cases and particularly when the catalyst is a metal derivative (see Chap. 4) none of the proposed mechanisms can be considered as definitive. 

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

Double-bond rearrangements can also take place on treatment with acids. Both proton and Lewis acids can be used. The mechanism in the case of proton acids is the reverse of the previous one first, a proton is gained, giving a carbocation, and then another is lost ... 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

In the acid-catalysis method, a proton or Lewis acid is used as the catalyst and the reaction is carried out at temperatures between -30 and 100°C. This is a Friedel-Crafts process with a carbocation mechanism" (illustrated for a proton acid... 

When proton acids catalyze alcohol dehydration, the mechanism is El. ° The principal process involves conversion of ROH to ROHj and cleavage of the latter to R and H2O, though with some acids a secondary process probably involves conversion of the alcohol to an inorganic ester and ionization of this (illustrated for H2SO4) ... 

In the first step of the mechanism, the OH group is converted by the reagent to a better leaving group (e.g., proton acids convert it to OHj). After that, the mecha-nism follows a course analogous to that for the Schmidt reaction of ketones (18-16) from the formation of 71 on ... 

Hydrogen ions accumulate in tissue damaged by inflammation and ischaemia and so pH is lowered. These protons may activate nociceptors directly via their own family of ion channels as well as sensitising them to mechanical stimulation. Acid-sensing ion channels (ASICS) are a family of sodium channels that are activated by protons — of special interest is one type found only in small dorsal root ganglion neurons that possibly are responsible for activation of nociceptors. Although the transduction of mechanical stimuli is poorly understood, ASICs are closely related to channels that respond to stretch. 

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

Fig. 11) would likely proceed by different mechanisms. Protonation of the diol (IV, Fig. 12) derived from theobromine would lead to ring opening at the C6— Cs position giving an imidazole isocyanate (XVI, Fig. 12). This could readily form XVII which after hydrolysis and loss of C02 would give dimethyl-allantoin (XVIII). On the other hand, the uric acid diol derived from caffeine (X, Fig. 12) cannot fragment by this mechanism. Accordingly, either or both of the processes could Occur via the form of the diol hydrated at the C6 carbonyl group (XIX, Fig. 12) which could readily lose C02 to give XX followed by rearrangement to trimethylallantoin (XXI).

A further measure of acidity is provided by rates of deuterium exchange between a labeled base such as DO and a proton acid. The mechanism involves exchange within weak ion-molecule encounter complexes as shown in equation 4. 

Before the publication of Colclough and Dainton s work, Gantmakher and Medvedev [11] had revived Hunter and Yohe s theory of direct initiation, but restricted it to solvents of moderately high dielectric constant. They maintained that in such solvents neither a protonic acid nor an alkyl halide co-catalyst is required. The experiments of Colclough and Dainton make this appear highly unlikely, although they do not disprove it completely. It is important to realise that several types of initiation could co-exist in the same system even if in certain systems co-catalysis by alkyl halides were proved, this does not exclude the existence of a concurrent direct initiation by the Hunter-Yohe, Gantmakher-Medvedev mechanism. 

The impact which was made by the writer s revival of the old ester mechanism in the context of polymerisations is attested by the number of polymer chemists who set about examining the validity of the theory experimentally. For example, Bywater in Canada confirmed that during the progress of a polymerisation of styrene by perchloric acid the acid could not be distilled out of the reaction mixture, but after exhaustion of the monomer it could be. This regeneration of the initiating acid after the consumption of the monomer is an often attested characteristic of pseudocationic polymerisations with many different protonic acids it is most simply explained by the decomposition of the ester to an alkene and the acid, i.e., a reversal of the initiation, when the monomer has been consumed. Enikolopian in the USSR found that the effect of pressure on the rate of polymerisation in the same system was not compatible with the propagation step involving an ion, and... 

Since the carboxonium ion has been eliminated as a possible propagating species, one is left with two alternatives which we may call the Keele and the Mainz theories. Plesch and Westermann [6, 8] have suggested that the cyclic formals polymerise by a ring-expansion mechanism, in which no free end is ever formed. This is illustrated in Reaction (B), where Y = H if the initiator is a protonic acid, and Y = Et if the initiator is a... 

To distinguish between the Mainz and Keele mechanisms it is necessary to do the following studies on strictly anhydrous polymerisation mixtures in which the initiator is a protonic acid ... 

The mechanism of acid hydrolysis is also different in acyclic amides and /1-lactams acid catalysis of acyclic amides proceeds via O-protonation (see Chapt. 4), whereas that of /1-lactams appears to be a unimolecular A1 type process, involving V-protonation (Fig. 5.6,b) [76], A-Protonation is not the result of reduced amide resonance but an intrinsic property of the /1-lactam structure, since bicyclic /1-lactams and monocyclic /1-lactams exhibit similar reactivity and behavior [76],... 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

The situation for hydrated Nafion in the acid form, or as containing aqueous acids or strong bases, is more complex because protons and defect protons (i.e., OH ions), migrate according to a somewhat different mechanism. Proton transfer in either case occurs throughout and between clusters of hydrogen bonded water molecules to a degree that depends on the relative water content. 

The question of protonation sites is one of the basic questions in the behaviour of complex organic molecules in solution, since protonated molecules are intermediates in synthetic organic chemistry, and the knowledge of protonation sites is important for the theory of reaction mechanisms of acid-catalysed reactions. It is also of fundamental importance for structural theory in general, since it is intimately connected with the concepts of mesomerism, electron density and bond polarization. 

All of the previous studies with cell suspension cultures of C. roseus have led to the conclusion that not all of the cells in suspension produce alkaloids, i.e., that some differentiation occurs. Neumann and co-workers at Halle 149) used fluorescence and electron microscopy to show that, like the intact plants, indole alkaloid accumulation occurs in the vacuoles of particular cells. Yet there appears to be no ultrastructural difference between these cells and those which do not produce alkaloids. It had been suggested earlier that basic alkaloids were accumulated by some kind of ion trap mechanism in acidic vacuoles (750). Indeed, a substantial pH difference was observed between those vacuoles which do accumulate alkaloids (pH 3) and those which do not (pH 5). It was concluded that the tonoplast of the alkaloid cells seemed to be highly permeable to the neutral form of the alkaloids, but only slightly permeable to the protonated forms. Cell lines which did not exhibit a difference in their vacuolar pH did not accumulate alkaloids. 

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