A, Bronsted equation

The relative rates of N-alkylation by methyl iodide of 7 and 8 relative to 6 suggest a steric effect of a peri hydrogen a little larger than that of the comparable 2-methyl [Eq. (2)]. Later a more precise estimation, using a Bronsted equation to estimate the electronic effect of the benzo, shows that the... 

R. P. Bell and W. C. E. Higginson, Proc. Roy. Soc. (London), A197, 141 (1949). In this most extensively covered case it has been shown that 45 carboxylic acids and phenols fit a Bronsted equation (with a — A) with a mean deviation of only 0.1 log unit in Atha. The range in pK A is some 10 units and the maximum deviation is 0.3 log units. 

The Bronsted equation is a Class I free energy relationship and this may be shown by considering as an example the acid-catalysed dehydration of acetaldehyde hydrate (Equation 30). This reaction also provides a good example of an acid-catalysed reaction following a Bronsted equation (Figure 7). 

In the case of unsymmetrical reactions the equilibrium constant can be calculated for any donor provided it is known for any one donor. The equilibrium constant (Aeq = 0,37) for the reaction of imidazole with 4-nitrophenyl acetate (Equation 14, Ar = 4-nitrophenyl = 4NP) can easily be measured by use of forward and reverse rate constants (Agq = The equilibrium constant for the reaction is governed by a Bronsted Equation logA q = + C where P is the value As, the... 

Using a Bronsted equation in which the ionic strength term has been derived from an extended Debye-Huckel correlation, it has been shown that for the ions M+ = Li+, Na+, K+, Rb+, or Cs+ the first (Ati) pathway is operative but only in the case of Li+ to an appreciable extent. For the cations Li+ or Na+, the Arg route is operative whereas for K+, Rb+, or Cs+ the reaction proceeds via the kt mechanism, the rate constants extrapolated to zero ionic strength being remarkably similar. 

Classify a species as a Bronsted-Lowry acid or base and explain by a net ionic equation. (Examples 13.1,13.4,13.10 Problems 1-12) Questions and Problems assignable in OWL 2,4,12,34,64... 

The equations can be readily expressed in a somewhat more general form when applied to a Bronsted-Lowry acid A and its conjugate base B ... 

Write the balanced chemical equation for (a) the thermal decomposition of potassium chlorate without a catalyst (b) the reaction of bromine with water (c) the reaction between sodium chloride and concentrated sulfuric acid, (d) Identify each reaction as a Bronsted acid—base, Lewis acid—base, or redox reaction. 

Fig. 4 A Bronsted plot of log pKz phenol in methanol vs. log fcaa fn( OCH3-) for methanolysis of aryl acetates promoted by 9 Zn2 + ( OCH3), T = 25°C, data in Table 7. Dashed line corresponds to NLLSQ fit of data to Equation (15) encompassing all esters with Pi = -0.023 0.03 and f 2 = -0.690 0.005 with a breakpoint of pJTa H = 14.8. Reproduced from ref. 16f with permission.

The hexadecyltrimethylammonium cation causes a modest increase in rate constant for the anion-anion reaction [Fe(CN)5(4-CNpy)]3-+CN-. This can equally well be interpreted according to the pseudophase model developed from the Olson-Simonson treatment of kinetics in micellar systems or by the classical Bronsted equation (135). 

Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

The best way to spot Bronsted-Lowry acids and bases is to keep careful track of hydrogen ions in a chemical equation. Consider, for example, the dissociation of the base sodium carbonate in water. Note that although sodium carbonate is a base, it doesn t contain a hydroxide ion. 

A Bronstcd-I.owry aeid is defined as a proton donor, a Bronsted-Loory base as a proton acceptor. The definitions apply generally toprotic systems those in which proton transfers can occur. A general equation expressing proton transfer in aqueous solution is ... 

Carbonylation of Alcohols - Pd(tppts)3 catalyses the carbonylation of benzylic alcohols to the corresponding phenylacetic acids, in the presence of a Bronsted acid cocatalyst such as H2S04 or p-CH3C6H4S03H in biphasic aqueous/organic media (no organic solvent).305,451 For example, benzyl alcohol was converted to phenylacetic acid (Equation 6) and l-(4-isobutylphenyl)ethanol (IBPE) to ibuprofen (Figure 9). 

With the aid of the Bronsted equation [Eq. (15)] the reactivities of simple azoles or benzazoles may be predicted from a knowledge of their pKa values. While this equation was derived for N-methylation reactions using solvents such as DMSO and dimethyl sulfate, it can be applied to estimate nucleophilicity toward other alkylating agents in various solvents provided that (a) the Bronsted / values for the new systems are similar to the old one, and (b) that steric factors are unimportant. The foregoing discussion serves as a guide to indicate when such limitations apply. 

Structure-reactivity relationships in these reactions have been studied by several authors. Bruice and Schmir299 found that a series of substituted imidazoles react with / -nitrophenyl acetate at rates which depend on their basicity, and are correlated by the Bronsted equation with a/3-value of 0.8 although the simple relationship is complicated by the increasing importance of reaction with the anion of more acidic imidazoles. The reaction of imidazole itself with a series of substituted phenyl acetates is correlated by the Hammett equation withp = 1.90296. 

The Bronsted relationships. The effectiveness of a specific base as a general base catalyst can usually be related to its basicity (pfCa) via the Bronsted equation (Eq. 9-90). Here, kB is defined by Eq. 9-88 and G is a... 

SAMPLE SOLUTION (a) Always start with an equation for an acid-base reaction. Ammonia is a Bronsted base and accepts a proton from the —OH group of acetic acid. Ammonia is converted to its conjugate acid, and acetic acid to its conjugate base. 

Write a chemical equation showing in what respect a Sc3+ ion is a Bronsted acid in aqueous solution. 

Cleavage of a carbon-hydrogen bond to yield a carbanion and proton is a Bronsted acid-base reaction (Equations 5.24 and 5.25). The mechanism is not... 

When the catalyst is a Bronsted acid, migration is not stereospecific. Under these conditions, syn and anti forms are readily interconverted, presumably via the pathway shown in Equation 6.55. 

It follows from this equation that the products of a Bronsted-Lowry acid-base reaction, BH+ and A-, are themselves acids and bases. The species BH+ produced when the base B accepts a proton from HA can itself donate a proton back to A-, meaning that it is a Bronsted-Lowry acid. Similarly, the species A- produced when HA loses a proton can itself accept a proton back from BH+, meaning that it is a Bronsted-Lowry base. Chemical species whose formulas differ only by one... 

PROBLEM 15.1 Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water ... 

The above two equations describing the behavior of Bronsted acids and bases are not strictly correct because a Bronsted acid does not just dissociate, it donates a proton to something which accepts a proton. The proton does not just dissociate and Boat around in solution but is always attached to something. Furthermore a Bronsted base does not just find a proton to accept it accepts a proton from a Bronsted acid. Thus acidity and basicity are paired behavior— you can t have one without the other. This is the most common misconception about acids and bases and leads to die greatest amount of difficulties when trying to apply die principles of acidity and basicity to real reactions. 

Second-order rate constants for the reactions of phenacyl bromide with a number of anionic or neutral nucleophiles in 3 2 (v/v) acetone-water have been measured at several temperatures.141 Correlation analysis with the Bronsted equation or Swain-Scott equation is not satisfactory. Better results were obtained with the two-parameter Edwards equation. 

Solvolysis of the R,R and R,S isomers of 2-bromo-9-(l-X-ethyl)fluorenes, X = Cl, Br, I, or OBs, in 25% (v/v) acetonitrile in water has been studied with respect to rates of formation of elimination products and of substitution products (X = OH or NHCOMe).142 The parent 9-(l-X-ethyl)fluorenes and the 2,2/-dibromo-9-(l-X-ethyl)-fluorenes were also studied. Various effects of leaving group and of the presence of nucleophiles on the competition between the reactions were observed and the Bronsted equation was applied to the results for the elimination reactions. A related study of solvolysis of 9-(X-methyl)fluorenes, X = I, Br, or Bs, was also carried out, in which the Swain-Scott equation was applied to nucleophilic selectivities in the S 2 reactions.143... 

Rate constants of ketonization of acetophenone enol along paths (a) and (b) have been be estimated using the Bronsted Equation (16).20 Those predicted for path (a) are orders of magnitude below the observed rate constant k c = 0.18 s 1, while those for path (b) were found to be in reasonable agreement with experiment. The concerted mechanism (c) does not satisfactorily account for structure-reactivity relationships observed in aqueous solution. It may, however, well be the dominant mechanism in aprotic solvents containing small amounts of water. 

It is interesting that eqn. (80) can be written in a form that is similar to the Bronsted equation... 

It is possible that this relationship between the thermal effect of the complex reaction and its observed activation energy is responsible for the fact that the Bronsted equation (or its analogs) appear to be valid for some elementary reactions as well. A question arises whether the observed activation energy can be higher than the activation energy of elementary steps. 

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