Bronsted-type plot

The reactivity is compared with the equilibrium constant for dissociation of pyridinium molecules under the same conditions and the Bronsted-type plot (Figure 8) illustrates the extent of scatter usually obtained in such relationships. 

Figure 4 Bronsted-type plot for the alkaline hydrolysis of substituted phenyl 4-hydroxybenzoate esters - results diagnostic of parallel mechanisms...

Figure 4. Bronsted-type plot showing the dependence of log on pK, (RNHp for the reaction of the 4-methoxystyrene radical cation with a series of primary amines in acetonitrile ( ) and 4 1 aqueous acetonitrile Ml a, cyanomethylamine b, trifluoroethylamine c, cyanoethylamine d, methoxyethylamine e, n-butylamine.

A kinetic study and mechanistic investigation by UV spectrophotometry has been carried out on the reactions between triphenylphosphine, dialkyl acetylenedicarboxylates and NH-acids (such as Harman acids). A Bronsted-type plot of the derived second-order rate constants for the reactions of a series of primary amines (in water) with benzoyl methyl... 

From the limited data at present available, there appear to be interesting differences between proton transfer involving carbon acids and oxygen acids in aprotic solvents, but more systems must be studied before definite conclusions can be drawn. In particular, an attempt should be made to correlate the extent of proton transfer in the transition state (by means of a Bronsted-type plot) with the primary isotope effect, as has been done for aqueous media. 

Fig. 11.5 (A) Eigen-type plot for the general base-catalysed reactions of 4-chlorobenzaldehyde with a substituted hydrazine [8] the line is calculated from log kg = —0.341 log(1 + 106 09 pffa). (B) Eigen-type plot for general acid catalysis in the reaction of cyanic acid (HNCO) with aniline, and an alternative interpretation [9] the dashed line has a Bronsted slope of —0.19 and the break at p/fa near 10 in the solid line is consistent with proton transfer to an addition intermediate (PhNH2+CONH ).

Br0nsted type plot of log k against amine pifa has been performed for the aminolysis of NPBS with amines 3-6 (Fig. 17.2). The most notable feature is that the Bronsted plot is biphasie and two P values have to be eonsidered. The Pi value for the lower part of the plot is 0.7 and P2 is 0. The ehange oeeurs at approximately the pifa of NPBS (17.68). 

DR. DALE MARGERUM (Purdue University) You need to be careful because you may not obtain a true Bronsted plot from what you have just described. You don t want to make a plot where each system has a different rearrangement. You want to be certain to have a common type of acid or base with which you are reacting. The literature is full of inverse Bronsted relationships in which there has been a poor choice of reaction acid-base pairs. Much confusion has been generated as a result. 

The final step of the convolution analysis is the determination of the transfer coefficient a. This coefficient, sometimes called the symmetry factor, describes how variations in the reaction free energy affect the activation free energy (equation 26). The value of a does not depend on whether the reaction is a heterogeneous or a homogeneous ET (or even a different type of reaction such as a proton transfer, where a is better known as the Bronsted coefficient). Since the ET rate constant may be described by equation (4), the experimental determination of a is carried out by derivatization of the ln/Chet-AG° and thus of the experimental Inkhei- plots (AG° = F E — E°)) (equation 27). 

Buckingham and Engelhardt200 have studied the hydrolysis of propionic anhydride in the presence of kinetically inert complexes of the type [M(NH3)5OH]n+. These reactions occur by nucleophilic attack of coordinated hydroxide on the anhydride (Scheme 32). For reactions of M-OHl" l,+ with propionic anhydride, the Bronsted plot of log kMOH versus the p.Ka of M—OH2k+ is a smooth curve if values for reaction with HzO and OH- are included. Although Icmoh for [(NH3)5CoOH]2+ (3 M-1 s-1) is about 103-fold less than fcoH. its reaction will compete favourably at neutral pH with base hydrolysis. Such effects are considered in more detail in Section 61.4.2.2.3. 

Since 1924, the Bronsted relation has been applied to many general acid and base catalysed reactions, such as those discussed in Sect. 2.2, as well as to proton transfer equilibria like (43)—(45). Over limited ranges of acid strength and for variation within a similar catalyst type, G and a or j3 are constant and the relation holds well. Different catalyst types in a reaction often do not fit on a single Bronsted plot, but give different Bronsted lines. This was observed for the decomposition of nitramide [68]. It has also been observed in proton transfer from l,4-dicyano-2-butene(51)... 

Similar Bronsted exponents, 0.94 0.02 for phenolate ions and 0.98 0.08 for secondary amines, were observed but the Bronsted plots for these two types of catalyst were separated by about half a unit in log 0 k. The values of the Bronsted exponents are close to the limiting values of unity expected for normal proton transfer. Reaction (78) is thermodynamically favourable in the reverse direction and for fully normal proton transfer the rate coefficients for recombination of the carbanions with phenols and ammonium ions should be around 101 0 1 mole"1 sec"1. Calculations using the approximate pif 21 for this acid measured [69] in dimethyl... 

The mechanisms of two other reactions described in Sect. 2.2 involve slow proton transfer to unsaturated carbon. The general acid catalysed cleavage of vinyl mercuric halides [42, 50] for example, allyl mercuric iodide, CH2=CHCH2HgI (XXII), gives Bronsted exponents around 0.7. Linear Bronsted plots are obtained with carboxylic acid catalysts but, as observed in other A—SE 2 reactions, general acids of different structural types (for example, hydronium ion or bisulphate ion) show substantial deviations. Bronsted catalysis of the hydrolysis of diazo compounds (N2 =CR X) has been studied by the groups of Albery and Kreevoy. With... 

The catalytic coefficients for all acids do not fall on the same Bronsted plot and Bronsted exponents from 0.56—0.71 are obtained depending upon the catalyst type. Observations like these may mean that the Bronsted exponent is not a precise index of transition state structure [183],... 

Acidities as expressed by the absorbances of the infrared bands are plotted in Figure 4. Data were recorded under 2 mm of pressure and after evacuation for 30 minutes to remove reversibly adsorbed species. The 3230 cm" band, like the 1538 cm" band, is caused by pyridinium ions and can be useful for estimating acidity when the latter band becomes too intense. Although some scatter exists, the Bronsted acidity tends to increase with ionic radius. Nearly 30-40% of this acidity is reversible. In the 1450 cm" region, 2 types of adsorbed pyridine can be distinguished, 1 species absorbing at 1452 cm" and the other at 1438 cm" The latter band is more intense and removed easily by evacuation. The remaining 1452 cm" band arises from Lewis acidity. Its intensity is the same for all the rare earth faujasites. 

For the hydrolysis of bis-4-nitrophenylphosphate hydrolysis promoted by zinc complexes of L5-L14, kinetic studies were carried out at pH = 11 to ensure full deprotonation of the Zn-OH2 moiety of the complexes. Rate constants obtained at 25 °C were used in the preparation of a Bronsted plot. In this plot, reactivity differences as a function of the type of chelate ligand present are clearly evident. Zinc complexes supported by L5-L7 and L9 exhibit the highest reactivity and the... 

There are indications that free energies of formation of hydrogen bonds vary substantially with the structural type of the partners in ways that are not correlated with acid-base dissociation constants. If such a variation is incorporated in it would be sufficient to account for the observation that acids covering a variety of structural types do not fit on a single Bronsted plot. ... 

Williams and his coworkers studied the hydrolyses of a large number of nitrogen nucleophiles of different structural types and observed both general base-catalysed and nucleophilic pathways178. The differing behaviour showed up clearly on the Bronsted plot, primary amines (ft = 0.66) following a nucleophilic catalysis pathway whilst a general base line included imidazole, N-methylimidazole, pyridine and 4-picoline. 

Interpretation of the slightly curved Bronsted correlations in terms of Marcus type theory is clouded by the curvature not being accurately defined. This is quite apart from any reservations about the validity of the energy-profile equations. The data can often be force fitted to a linear plot with little difference in correlation coefficient from that to the quadratic (Fig. 7). 

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