Specific hydroxide ion catalysis

Finally, in the pH range of 9—10, the slope of the pH profile was unit positive, indicating specific hydroxide ion catalysis. It is thus apparent that the unprotonated N-terminal group imparts more reactively to the... 

The third test of the conjugate base mechanism that we put forward was based on the idea that the first step should be written as an equilibrium, and the reaction rate should show specific hydroxide ion catalysis. If this is indeed in equilibrium, and deuterium exchange studies say that it must be, then the rate of the reaction must depend on the hydroxide ion concentration, and on nothing else. 

A similar treatment for base-catalyzed reactions can be used to develop corresponding equations for specific base catalysis. Again, the terms specific hydroxide ion catalysis in water or specific lyate ion catalysis in another solvent (e.g., CHsO" in methanol) may be used for greater precision. (See reference 91.)... 

If one eliminates specific hydroxonium and hydroxide ion catalysis, which arise from effects on pre-equilibria, in the measured overall rate constants of diazo coupling reactions, one recognizes that there are some reactions which do and some which do not show general base catalysis. A base-catalysed combination, namely the reaction of p-chlorobenzene diazonium ion with 2-naphtholate-6,8-disulphonate anion has been investigated with respect to catalysis (Zollinger, 1955a, c). A non-linear dependence between rate and the concentration of pyridine, 2-, 3-and 4-picoline and 2,6-lutidine was found, corresponding to a ratio = 1 30 for pyridine (B = pyridine). 

A distinction is often made between specific catalysis by hydrogen and hydroxyl ions specific hydrogen and hydroxide-ion catalysis) and catalysis by acids and bases in general general acid and base catalysis). Actually such a distinction is somewhat artificial, as we shall see. As an example of general base catalysis, consider the simple mechanism... 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

The acid-base catalysis illustrated in these equations is termed general to distinguish it from specific acid or base catalysis in which the catalyst is the proton or hydroxide ion. 

The other kind of acid/base catalysis is called general rather than specific and abbreviated GAC or GBC. As the name implies this kind of catalysis depends not only on pH but also on the concentration of undissociated acids and bases other than hydroxide ion. It is a milder kind of catalysis and is used in living things. The proton transfer is not complete before the rate-determining step but occurs during it. A simple example is the catalysis by acetate ion of the formation of esters from alcohols and acetic anhydride. 

The mechanistic significance of the terms in Eq. (27) for pseudobase decomposition must, of course, be the microscopic reverse of the interpretations given for the pseudobase formation reactions. Thus, k,[H+] is the microscopic reverse of the kHl0 term, and may be formally interpreted as either the spontaneous loss of a molecule of water from the O-protonated pseudobase (i.e., specific-acid catalysis transition state C) or alternatively as elimination of hydroxide ion from the neutral pseudobase molecule with the aid of H30+ as a general-acid catalyst (transition state D). The k2 term is the microscopic reverse of fc0H[OH ], and so formally represents either the spontaneous decomposition of the pseudobase to heterocyclic cation and hydroxide ion (transition state A) or the kinetically equivalent general-acid catalysis of this reaction by a water molecule (transition state B). 

If the first step is rate-controlling (Case I), the rate is first order in the respective base, B, not in hydroxide ion. If several bases are present, their contributions to the rate are additive. This is called general base catalysis. In contrast, if the second step is rate-controlling (Case II), the rate is first order in hydroxide ion, regardless of what base or bases are used as catalyst. This is called specific base catalysis. Intriguingly, mathematics produces first order in OH" even though that ion is not a reactant in any of the kinetic steps. 

At low concentrations of BH+ such that k2 > k j [BH+], the observed first-order rate coefficient for conditions where buffer is present in excess over reactant is kx [B]. The rate of reaction is determined by a slow proton removal by base from carbon. At high concentrations of BH+, the observed first-order rate coefficient is (k1fe2/k-i)[B]/[BH+]. In this case, if the reaction is carried out in aqueous solution, the rate of reaction depends upon the hydroxide ion concentration and is independent of the buffer concentration at a fixed buffer ratio (specific base catalysis). The mechanism under these conditions consists of rapid pre-equilibrium formation of a carbanion followed by a slow step. Over the whole range of buffer concentration the first-order rate coefficient (M,hs) measured at fixed buffer ratio first increases (/ bs = kl [B]) with buffer concentration but reaches a limiting value (kohs = (ki k2 /k-i) [B] /[BH+]). This change in mechanism has been observed for a limited number of reactions [58]. Reactions (38) [58(a)] and (39) [58(b)] occurring in ethanol and reaction (40) [58(c)] in aqueous... 

The same distinctions may be drawn for base catalysed reactions. In the reverse of the aldol condensation reaction, the rate of the reaction is found to be dependent only upon the concentration of hydroxide ions, and so this is an example of specific base catalysis. This is indicative of rapid reversible deprotonation of the substrate before the rate limiting step. 

General acid catalysis occurs when the rate law includes a concentration term due to added acid (rate = AhaIHA]). Specific acid catalysis involves a rate law with only the oxonium ion (rate = itHlHjOq). Similar definitions apply to general base and specific base catalysis involving base and hydroxide ion respectively. 

Hydrous metal surface sites may act as general catalysts or as specific catalysts. Weak acidic sites and weak basic sites are found on surfaces that can promote reactions by donating protons (general acid catalysis) or hydroxide ions (general base catalysts). Surface sites may also exhibit nucleophilic or electrophilic character. 

As an alternative to the highly specific catalysis indicated by formulas I, II, and III, it is possible that the metal chelate compound merely participates in a generalized type of acid-base catalysis. Thus, the function of the metal would be to increase the acidity of the substrate through molecular association and thereby increase its susceptibility toward attack by other bases present such as hydroxide ion or water molecules. Under these conditions the diaquo chelate A would be an acid catalyst, the monohydroxy chelate Bi would be considered to be bifunctional in its effect, and the dihydroxy chelate B2 would probably be a weak basic catalyst. 

Stone suggests that catalysis occurs because the ionized carboxylate group of MPT is able to specifically sorb to the positively charged aluminum oxide surface where subsequent attack of hydroxide ions in the diffuse layer occurs. 

In Chapter 12 (p. 263) we pointed out that even weak bases—too weak to deprotonate a nucleophile by the mechanism we have just described for SBC—can still act as catalysts. Such catalysts are known as general base catalysts, and are the promoters of a parallel kind of acid-base catalysis called general rather than specific. General base catalysis, abbreviated GBC, depends not only on pH (i.e. the concentration of hydroxide ion) but also on the concentration of other bases too. General acid catalysis, abbreviated GAC, likewise depends not only on pH (i.e. the concentration of H3O+) but also on the concentration of other undissociated acids HA. General acid-base catalysis is a milder kind of catalysis and is characteristic of reactions catalysed by enzymes in the metabolism of living things. 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

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