Chlorides, second-order rate constants

The value of the second-order rate constant for nitration of benzene-sulphonic acid in anhydrous sulphuric acid varies with the concentration of the aromatic substrate and with that of additives such as nitromethane and sulphuryl chloride. The effect seems to depend on the total concentration of non-electrolyte, moderate values of which (up to about 0-5 mol 1 ) depress the rate constant. More substantial concentrations of non-electrolytes can cause marked rate enhancements in this medium. Added hydrogen sulphate salts or bases such as pyridine... 

The second-order rate constants for the reaction of a number of amines with benzyl chloride are tabulated below. Calculate A// and A5 from the data. Offer an explanation for the relative reactivity order for the amines. What trends do you observe in A// with reactivity ... 

The trans-effect can be used synthetically. In the reaction of Br- with Au(NH3)4+, the introduction of the first bromine weakens the Au—N bond trans to it so that the introduction of a second bromine is both sterospecifically trans and rapid. (A similar effect occurs in the corresponding chloride.) The third and fourth ammonia molecules are replaced with difficulty, permitting the isolation of AuBr2(NH3)2 (second-order rate constants at 25°C are k] = 3.40, k2 = 6.5, k2 = 9.3 x 10-5 and k4 — 2.68 x 10 2lmor s l at 25°C) [141]. 

In order to determine the efficiency of the polymers as reagents in nucleophilic catalysis, it was decided to study the rate of quaternization with benzyl chloride. Table I shows the second-order-rate constants for the benzylation reaction in ethanol. Comparison with DMAP indicates that poly(butadiene-co-pyrrolidinopyridine) is the most reactive of all the polymers examined and is even more reactive than the monomeric model. This enhanced reactivity is probably due to the enhanced hydrophobicity of the polymer chain in the vicinity of the reactive sites. 

On the other hand, the use of a-cyclodextrin decreased the rate of the reaction. This inhibition was explained by the fact that the relatively smaller cavity can only accommodate the binding of cyclopentadiene, leaving no room for the dienophile. Similar results were observed between the reaction of cyclopentadiene and acrylonitrile. The reaction between hydroxymethylanthracene and N-ethylmaleimide in water at 45°C has a second-order rate constant over 200 times larger than in acetonitrile (Eq. 12.2). In this case, the P-cyclodextrin became an inhibitor rather than an activator due to the even larger transition state, which cannot fit into its cavity. A slight deactivation was also observed with a salting-in salt solution (e.g., quanidinium chloride aqueous solution). 

Second-order rate constants (103fc2/M 1 s 1) for the homogeneous reaction of benzyl chloride with metal acetates in acetonitrile at room temperature0... 

Herriott and Picker (1975) have studied the reaction between sodium thiophenoxide and 1-bromobutane in benzene-water catalysed by various quaternary ammonium salts and by the dicyclohexyl-18-crown-6 isomers ([20] + [21]). The catalytic activities, as judged from the second-order rate constants, span a range of 104. The best catalyst appeared to be dicyclohexyl- 18-crown-6, directly followed by tetrabutylphosphonium chloride and tetrabutylammonium iodide. 

TABLE 6. Second-order rate constants for nucleophilic aromatic substitution reactions of 2,4-dinitrochlorobenzene and picryl chloride. Reprinted with permission from Reference 77. Copyright (1992) American Chemical Society... 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

When an associative mode of activation is indicated it is instructive to examine the ways in which reactivity, as measured by the second-order rate constant k2, depends upon the nature of the nucleophile and if a large number of substrates show the same pattern of preference it is useful to consider scales of nucleophilicity. A systematic study of the substitution reactions of fra/is-[Pt(py)2Cl2] (it appears that pyridine and piperidine were used interchangeably) in methanol at 30 °C (displacement of chloride) led to the establishment of an n scale, nm d= log,o(k2/kI) for the standard reaction,447 and later the more dimensionally correct Up,0 scale448-449 ( p,° = log,0(li2/ki)[MeOH]) so that npt° = npt+1-41 (unfortunately this distinction has not been strictly adhered to in the literature).449 A collection of n 0 values will be found in Table 14. 

The oxanihc hydrazide and tetrazine fonned when /V-aryl-2-oxo-2-phenylamino-ethanehydrazonyl chloride (120) is treated with Et3 N in 1,4-dioxane-water (4 1, v/v) at 25 °C aiise from tlie intermediate nitrilium amide (121).101 A kinetic study has now established that (121) is fonned from (120) according to Scheme 15. The second-order rate constant kohs = 037<7X — 0.77, where pobs = 0.37 = pa + Pu since pa 1.18 [cf. that determined for acid dissociation of PhNHCON(CN)=NNHCgH4X], it has been possible to evaluate pl = —0.81, which compares favourably with the value p = —0.63 for C—Br heterolysis of PhC(Br)=NNHCgH4X in the same solvent. 

Combinations of product studies with kinetic data provide particularly powerful indications of reaction mechanisms. Either the presence or absence of a rate-product correlation may be of mechanistic significance. First, we have an explanation of rate-product correlations using the example of competing methanolysis (second-order rate constant, IcMeOH) and aminolysis (second-order rate constant, kam) of benzoyl chloride (59) in Scheme 2.21. The mechanism is initially assumed to involve independent competing pathways, as shown, so that the equations of correlation can be derived. 

Second-order rate constants for the reaction of mercury(II) chloride with tetraalkylstannanes according to equation 28 ... 

Presumably less nucleophilically assisted solvolyses could show higher a-deuterium isotope effects, and there is a linear relationship between the magnitude of nucleophilic solvent assistance (Table 2) and the a-deuterium isotope effect for solvolyses of 2-propyl sulpho-nates (Fig. 7). Another measure of nucleophilic assistance is the ratio k2 (OH )/, where k2 is the second-order rate constant for nucleophilic attack by OH and kx is the first-order rate constant for reaction with the solvent water, and a linear correlation was obtained by plotting the ratio versus the experimentally observed isotope effects for methyl and trideuteriomethyl sulphonates, chlorides, bromides and iodides (Hartman and Robertson, 1960). Using fractionation factors the latter correlation may also be explained by a leaving group effect on initial state vibrational frequencies (Hartshorn and Shiner, 1972), but there seems to be no sound evidence to support the view that Sn2 reactions must give a-deuterium isotope effects of 1-06 or less. 

Closely related to these investigations, Breslow and co-workers studied the Diels-Alder reaction of CP with methyl vinyl ketone (MVK) in water-like solvents, ethylene glycol and formamide, in the presence of lithium salts. They found clear differences and similarities between water and these two solvent systems. In the absence of Li salts, the second-order rate constant for the reaction at 20 °C increased in formamide ( 2 = 3184 X 10 m s" ), and even more in ethylene glycol (480 x 10 m" s" ), relative to a polar solvent such as methanol (75.5 x 10 m" s ) or non-polar solvent such as isooctane (5.940.3 x 10 m s ). The reactions in both polar solvents were faster in the presence of LiC104 than in the presence of LiCl, although the perchlorate ion has less salting-out effect than chloride ion in water [41]. 

Interesting publications of work undertaken in this field of reactivity by a research group in (what at that time was part of) the USSR, unfortunately, was published mainly in rather inaccessible journals. The rates of reaction of phenylacetylene with ethyl- and phenylmagnesium bromide were measured in diethyl ether and in tetrahydrofuran, respectively [32]. The results, presented in Table 8, clearly show the dramatic change in the second-order rate constants, when diethyl ether is replaced by tetrahydrofuran as the solvent. The same effect had been found in 1968 by others [33] for the reaction of benzylmagnesium chloride with phenylacetylene at 0°C the second-order rate constant (/c2 X 10 L mol sec ) was 0.008 in diethyl ether and 84 in tetrahydrofuran, a change by a factor of (more than) 10 thousand. 

Such sign reversal in px at an isokinetic point is also observed in the reactions of anilines with cumyl (PhCMe2-) arenesulfonates63 (4-as) and chlorides (4-C1). Structurally, these systems correspond to R1 = R2 = Me in the general a-substituted benzylic compounds so that the reactivity and the reaction mechanism are expected to be related to those of other a-substituted benzylic series discussed above57 63. The second-order rate constants for the reactions of aniline are 33.9 x 10 3 M 1 s 1 for cumyl arenesulfonate... 

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