Proton Transfer Mechanisms

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 -> prototropic conduction (see also Grotthus mechanism) in acid solutions can be interpreted in a similar way by assuming a proton exchange mechanism (proton transfer or jumping) [xii]. 

It has been proposed that, in analogy to Weller s mechanism, proton transfer In the first excited singlet state is responsible for the fast nonradiative decay and high photostability (7,8). However, it is doubtful whether proton transfer alone can account for these effects (8,71,73,74). As in the foregoing sections, only a few substances will be discussed for which interesting spectroscopic or photochemical evidence is available. 

Two species, the carbocation and the anion, react in this step, making it bimolecular. Note that molecnlarity refers only to individnal elementary steps in a mnltistep mechanism, not to the overall reaction itself. Step 1 of the mechanism (proton transfer) is bimolecnlar, step 2 (dissociation of the alkyloxonium ion) is nnimolecnlar, and step 3 (cation-anion combination) is bimolecnlar. 

Acids are preferentially adsorbed on alumina and magnesia, relative to adsorption on such nonbasic adsorbents as silica and Florisil. This is illustrated for alumina in Fig. 10-11, where the quantity Ag is plotted versus the pK value of the group i. Ag is equal to the experimental value of Q° (on alumina) minus the value calculated from a g value on silica (via Fig. 10-10). That is, Ag represents the preferential adsorption of the grewp i on alumina relative to silica. The dashed curve through the data of Fig. 10-11 is calculated [see Ref. (72)] on the basis of an ionization adsorption mechanism (proton transfer from sample to adsorbent) on... 

Mechanism 6.3 extends the general principles of electrophilic addition to acid-catalyzed hydration. In the first step of the mechanism, proton transfer to 2-methylpropene forms the tert-h xiy cation. This is followed in step 2 by reaction of the carbocation with a molecule of water acting as a nucleophile. The alkyloxonium ion formed in this step is simply the conjugate acid of tert-h xiy alcohol. Deprotonation of the alkyloxonium ion in step 3 yields the alcohol and regenerates the acid catalyst. 

Figure 5. BrSnsted Plot of Exchange Catalysis for the N-1 Proton of 2, 3 -cGMP. P mechanism proton transfer rate constants were obtained from catalytic broadening data (360 MHz, 3 + l C). Catalyst pK values were obtained by direct potentlometrlc titration of experimental mixtures and by extraction from the kinetic data (eq. 1). The pK of the HjO H2O + r reaction Is -1.7.

In the first step, the it bond of the alkene is protonated, generating a carbocation intermediate. In the second step, this intermediate is attacked by a bromide ion. Figure 9.1 shows an energy diagram for this two-step process. The observed r ioselectivity for this process can be attributed to the first step of the mechanism (proton transfer), which is the rate-determining step because it exhibits a higher transition state energy than the second step of the mechanism. 

The mechanism for this process (and all others like it) involves two core steps attack the carbonyl group, and then re-form the carbonyl group. That s it. Just two core steps. But very often, proton transfer steps are necessary when drawing a mechanism. Proton transfers can only occur at three different moments the beginning, the middle, or the end ... 

Kuipers E W, Vardi A, Danon A and Amirav A 1991 Surface-molecule proton transfer—a demonstration of the Eley-Ridel mechanism Phys.Rev. Lett. 66 116... 

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

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... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

Aromatic pyrazoles and indazoles, in the broad sense defined in Sections 4.04.1.1.1 and 4.04.1.1.2, will be discussed here. Tautomerism has already been discussed (Section 4.04.1.5) and acid-base equilibria will be considered in Section 4.04.2.1.3. These two topics are closely related (Scheme 10) as a common anion (156a) or a common cation (156b) is generally involved in the mechanism of proton transfer (e.g. 78T2259). For aromatic pyrazoles with exocyclic conjugation there is also a common anion (157) for the three tautomeric forms... 

Because proton-transfer reactions between oxygen atoms are usually very fast, step 3 can be assumed to be a rapid equilibrium. With the above mechanism assume4 let us examine the rate expression which would result, depending upon which of the steps is rate-determining. 

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

This mechanism explains the observed formation of the more highly substituted alcohol from unsymmetrical alkenes (Markownikoff s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water ... 

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Fig. 8.2. Contour plot showing a favOTed concerted mechanism for the first step in acetal hydrolysis, in which proton transfer is more complete in the transition state than C—O bond breaking.

This variation from the ester hydrolysis mechanism also reflects the poorer leaving ability of amide ions as compared to alkoxide ions. The evidence for the involvement of the dianion comes from kinetic studies and from solvent isotope effects, which suggest that a rate-limiting proton transfer is involved. The reaction is also higher than first-order in hydroxide ion under these circumstances, which is consistent with the dianion mechanism. 

The relative importance of the potential catalytic mechanisms depends on pH, which also determines the concentration of the other participating species such as water, hydronium ion, and hydroxide ion. At low pH, the general acid catalysis mechanism dominates, and comparison with analogous systems in which the intramolecular proton transfer is not available suggests that the intramolecular catalysis results in a 25- to 100-fold rate enhancement At neutral pH, the intramolecular general base catalysis mechanism begins to operate. It is estimated that the catalytic effect for this mechanism is a factor of about 10. Although the nucleophilic catalysis mechanism was not observed in the parent compound, it occurred in certain substituted derivatives. 

---