Rate-determining proton transfer steps

For mechanism (6) the observed rate coefficient (feBH + ) is the rate coefficient for a proton transfer step, whereas in (7) the observed rate coefficient does not refer to a single proton transfer step. The following examples illustrate the ways in which rate coefficients for a simple proton transfer to or from carbon have been obtained by measuring the rate of an overall reaction composed of a number of steps. 

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According to the mechanism suggested by Huisgen and Ruchardt [217] and by O Ferrall et al. [216], an ion pair is formed in a rate-determining proton transfer step. It subsequently splits off N2 to form an ion pair containing the benzhydryl cation. The carbonium ion either undergoes covalent bond formation with the anion or diffuses out of its solvent cage and then reacts with ethanol, viz. 

For another contrast between SAC and GAC we need only refer you back to the two Z/E isomer-izations earlier in the chapter. Isomerization of the diene is GAC—protonation at carbon is the slow step—and isomerization of the allylic alcohol is SAC. What we didn t tell you earlier was that the GAC reaction has a normal kinetic isotope effect of k(H)lk(D) = 2.5 and a negative entropy of activation AS = -36 J mol-1 K-1—just what we should expect for a bimolecular reaction involving rate-determining proton transfer from oxygen to carbon. Notice that the intermediate cation is tile same whichever the route only the ways of getting there, including the rate-determining steps, are different. 

For rate-determining proton transfer (mechanism A-SK2), the determined AV value directly refers to the slow step. For the mechanisms with pre-equilibrium proton transfer, A1 and A2, the experimental AV is the sum of the volume changes of the two steps... 

It may be concluded that rate-determining proton transfer in the first step (mechanism A-Se2) is indicated if kH/kD > 1 [2, 4, 99]. On the other hand, pre-equilibrium proton transfer (mechanisms A1 or A2) is indicated if kH/kD < 0.6. 

It is concluded that all available experimental information fully supports the mechanism suggested by Bell and McTigue [229]. Similar changes of the rate-determining step at high pH may occur for many other reactions with rate-determining proton transfer in acidic solution. They have not been detected because no measurements have been done at high pH as the rates are too slow in many systems. 

One further reaction of this type studied by Kreevoy and his group [50] is the acid catalysed cleavage of unsaturated compounds containing a carbon—mercury bond, for example, allyl mercuric iodide (29). The important conclusion is that rate-determining proton transfer to unsaturated carbon occurs as a first step (A—SE2 mechanism) and as expected the reactions are catalysed by general acids. 

The first mechanism (a) occurs if fe t < k2 and the observed rate coefficient is given by feobs = k1. The second mechanism (b) applies if fe i > fe2 and then kohs = k2 x K where K = fe1 /fe j. The two mechanisms which correspond, respectively, to a rate-determining proton transfer and a pre-equilibrium followed by a subsequent step have been discussed in detail for isotope exchange reactions in Sect. 2.2.1. The second possibility (b) is apparently favoured by Cram [120] for racemization of 2-methyl-3-phenylpropionitrile whereas Melander [119] has interpreted his results in terms of the first (a). From the variation of the rate coefficient for racemization in different solvent mixtures of methanol/ dimethylsulphoxide a Bronsted exponent j8 = 1.1 was calculated [119] using an acidity function method which will be described fully in Sect. 4.6. 

Bakke, Bethell and Parker also found that diphenyldiazomethane decomposed by a cation mechanism when the reaction was carried out by anodic oxidation in acetonitrile-water (deuterium oxide) mixtures. The primary hydrogen-deuterium kinetic isotope effect of 3.3 at 17 °C found in this reaction was identical to that found for the acid catalysed reaction above, and confirmed that proton transfer to the diazo carbon occurs in the ratedetermining step of the reaction. This is obviously consistent with the formation of a carbocation intermediate see equation 31. The rate-determining proton transfer reaction has been confirmed by the observation that the reaction ceases when 2,6-lutidine is added. 

Knowing rate data in pure H2O and D2O, having available thermodynamic data on lyonium ion acidities, and by assuming the Bronsted catalysis law, it is an easy matter to calculate rates in mixed solvents based on (a) proton transfer as a rate-determining step, or (b) pre-equilibrium proton transfer. The present system adheres to rate-determining proton transfer, and thus A-2 mechanisms are eliminated. The agreement between theory and experimental values suggests that special features... 

STEP 1 Add a proton. A rate-determining proton transfer from HI to the carbon-carbon double bond gives a 3° carbocation intermediate ... 

The latter process is known to involve rate-determining proton transfer to carbon, and since the two reactions are kinetically very similar they probably have similar rate-determining steps. 

The rate-determining step (rds) consists of a pre-equilibrium involving the protonation followed by a rate-determining electron transfer step. 

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