Two-step protonation

The conclusions reached about proton transfer from phenylazoresorcinol monoanions are quite different from the behaviour which has been described for other hydrogen-bonded acids. For phenylazoresorcinol monoanions, it appears that direct attack by base on the hydrogen-bonded proton is an important process and can compete with two-step proton removal. For two-step proton transfer through an open form of the phenylazoresorcinol monoanion it is found that the rate of proton transfer from the open form is... 

The first evidence that two step proton transfer from a hydrogen-bonded acid could occur consisted of Eigen plots for proton removal by buffer bases. The demonstration of a change in the rate-limiting step as in (30) and (31) provides even more clear evidence and permits the calculation of the rate coefficients and equilibrium constant for opening of the hydrogen bond. 

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... 

This chemical way to enrich the isotope-containing mixtures is easier and more effective than many other methods currently in use. The formation of dihydro derivatives in the Birch reaction can, obviously, occur not only via protonation of an anion radical but also that of a dianion. For example, the anion radical CioHg. can easily acquire another electron to give the dianion Ci0Hs. Then a two-step protonation gives dihydronaphtalene ... 

YouwHiseea few mechanisms in this chapter wherewe have written an intramolecular deprotonation. This saves writing two steps—protonation of the enoiate and deprotonation of N (here)—but quite possibly this is not the actual mechanism by which the proton transfer takes place. Any proton wiii do, as will any base—do not take the arrows here too literally. 

The reaction apparently takes place in two steps protonation followed by isomerisation ... 

These two steps—protonation followed by nucleophilic attack—are the exact rever.se of the opening of epoxide rings with strong nucleophiles, where nucleophilic attack precedes protonation. 

Tautomerization, the process of converting one tautomer into another, is catalyzed by both acid and base. Tautomerization always requires two steps (protonation and deprotonation), but the order of these steps depends on whether the reaction takes place in acid or base. In Mechanisms 23.1 and 23.2 for tautomerization, the keto form is converted to the enol form. All of the steps are reversible, though, so they equally apply to the conversion of the enol form to the keto form. 

Also in the case of Cu.lO", where a [2]catenand-type moiety is present, a two-step protonation reaction occurs, but in this case the luminescence of such moiety is quenched by the Cu-based one [66]. Unfortunately, it is not possible to determine the quenching rate and mechanism for a variety of reasons, including (i) the instability of the protonated forms under laser irradiation in CH2CI2 [56] (ii) the lack of electrochemical potentials of the protonated forms and (iii) the weakness of the MLCT emission band of the Cu-based moiety, which is partially masked by the tail of the emission of the protonated subunit, albeit quenched. However, the energy transfer mechanism is thermodynamically allowed and probably active. 

The most widely used concept to describe the basic charging behaviour of metal (hydr)oxides is the two step protonation reaction which leads to a so called V o-pK model for the description of the basic charging ... 

The other simplest combination of the non-equivalent D-A members is a 1. 2-donor/acceptor compound, l,r-bis(l-(anthraquinonyl)ethynyl)ferrocene (1,1 -FcAq2). To this complex, two-step protonation occurs, while only the first protonation causes the intramolecular electron transfer reaction. 

Which site-binding model is best suited to describe the adsorption of H and 0 H ions to specific sites at the oxide/water interface is still a matter of controversy. In the classical approach [50] the surface is assumed to be monofunctional and all surface oxygen ions are considered equivalent. Since most inorganic oxides show amphoteric acid-base properties, it was postulated that the charge buildup mechanism may be represented by the following two-step protonation process ( two-pA model ) ... 

Equations (l)-(4) are the foundations of electrical double layer theory and are often used in modeling the adsorption of metal ions at interfaces of charged solid and electrolyte solutions. In a typieal TLM, the outer layer capacitance is often fixed at a lower value (i.e., C2 = 0.2 F/m ), whereas iimer layer capacitance (Ci) can be adjusted to between 1.0 and 1.4 F/m [25]. It should be noted that the three-plane model (TPM) is a variation of the classical triple-layer model, in which the outer layer eapaeitanee is not fixed. Although the physical presentations of the TLM and TPM are identical as shown in Fig. 2, i.e., both involve a surface layer (0), an inner Helmholtz plane (p), and an outer Helmholtz plane d) where the diffuse double layer starts, a one-step protonation process (i.e., 1 piC approach) is, in general, assumed in the TPM, in eontrast to a two-step protonation process (i.e., 2 p/C approach) in the TLM. Another distinct difference is that pair-forming ions are assumed to be on the outer Helmholtz plane in the TPM but on the inner Helmholtz plane in the TLM. In our study, the outer layer capacitance is allowed to vary while the pair-forming ions are placed on the iimer Helmholtz plane with a complete set of surface eomplexation reactions being considered. Therefore, our approach represents a hybrid of the TPM and TLM. 

Tile formation of the first alkyl halide begins with protonation of the ether to form a good leaving group, followed by an S],j2 process in which a hafide ion functions as a nucleophile and attacks the protonated ether. The second alkyl halide is then formed with the same two steps— protonation followed by an Sn2 attack. If either R group is tertiary, then substitution is more hkely to proceed via an S],jl process rather than S],j2. 

The mechanisms for acid-catalyzed and base-catalyzed tautomerization involve the same two steps (protonation at the carbonyl group and deprotonation of the a position). The difference between these mechanisms is the order of events. In acid conditions, the first step is protonation of the carbonyl group, giving a positively charged intermediate. In basic conditions, the first step is deprotonation of the a position, giving a negatively charged intermediate. 

To convert the enol into a ketone, our mechanism will have two steps protonate and deprotonate. But we must decide what order to use. Do we first protonate Or do we first deprotonate To answer this question, we look at the conditions. Since we are in acidic conditions, we should first protonate (forming a positively charged intermediate), and only then do we remove the other proton. 

Mechanism of the electrophilic addition of HBr to 2-methylpropene. The reaction occurs in two steps, protonation and bromide addition, and involves a carbocation intermediate. 

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