Substitution rate constants for

Ten years ago Rorabacher (13) observed the substitution rate constants for aquonickel(II) ion with different amines (Table II). There is a decrease in the rate constants by a factor of 14 in going from ammonia to dimethylamine. If nickel-(II) substitution reactions are dissociative, then why is the effect this large Is this a steric effect with some associative contribution or is it an outer-sphere effect There has been surprisingly little investigation of the nature of the entering ligand so far as its bulk or its nucleophilicity is concerned even for what have been generally considered as simple substitution reactions. 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

The results in table 2.6 show that the rates of reaction of compounds such as phenol and i-napthol are equal to the encounter rate. This observation is noteworthy because it shows that despite their potentially very high reactivity these compounds do not draw into reaction other electrophiles, and the nitronium ion remains solely effective. These particular instances illustrate an important general principle if by increasing the reactivity of the aromatic reactant in a substitution reaction, a plateau in rate constant for the reaction is achieved which can be identified as the rate constant for encounter of the reacting species, and if further structural modifications of the aromatic in the direction of further increasing its potential reactivity ultimately raise the rate constant above this plateau, then the incursion of a new electrophile must be admitted. 

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

Ratio of second order rate constant for substitution in indicated solvent to that for substitution in methanol at 25 C... 

The value of the rate constant can be determined by substituting the rate, the [C3H5O], and the [H+] for an experiment into the rate law and solving for k. Using the data from experiment 1, for example, gives a rate constant of 3.31 X 10 s h The average rate constant for the eight experiments is 3.49 X 10-5 M-i s-i ... 

Complexes of Ir(III) are kineticaHy inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of prBr(NH3)3] " [35884-02-7] at 298 K has been measured at 2 x 10 ° (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid... 

Example 4.3. The p value for alkaline saponification of methyl esters of substituted benzoic acids is 2.38, and the rate constant for saponification of methyl benzoate under the conditions of interest is 2 x 10 s . Calculate the rate constant for the hydrolysis... 

Table 5.13. Rate Constants for Nuclei hilic Substitution in Primary Alkyl Substrates ...

The second-order rate constants for hydration and the kinetic solvent isotope effect for hydration of several 2-substituted 1,3-butadienes ate given below. Discuss the information these data provide about the hydration mechanism. 

Fig. 8.4. Logarithm of the first-order rate constants for the hydrolysis of substituted benzylidene-l,l-dimethyl-ethylamines as a fiinction of pH. [Reproduced fiom J. Am. Chem. Soc. 85 2843 (1963) by permission of the American Chemical Society.]...

The table below gives first-order rate constants for reaction of substituted benzenes with w-nitrobenzenesulfonyl peroxide. From these data, calculate the overall relative reactivity and partial rate factors. Does this reaction fit the pattern of an electrophilic aromatic substitution If so, does the active electrophile exhibit low, moderate, or high substrate and position selectivity ... 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

The total rate constant for the reaction comprises a 1.9 1 ratio of 4- to 2-mono-substitution. 

Quantitative structure-reactivity analysis is one of the most powerful tools for elucidating the mechanisms of organic reactions. In the earliest study, Van Etten et al. 71) analyzed the pseudo-first-order rate constants for the alkaline hydrolysis of a variety of substituted phenyl acetates in the absence and in the presence of cyclodextrin. The... 

A single chlorine atom can bring about the decomposition of tens of thousands of ozone molecules. Bromine atoms can substitute for chlorine indeed the rate constant for the Br-catalyzed reaction is larger than that tor the reaction just cited. 

Tabic 1.3 Relative Rate Constants for Reactions of Radicals with Alkyl-Substituted Acrylate Esters CHR CFEcOaCHs"... 

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05... 

The reader can easily show by substitutions from the equilibrium conditions for Eqs. (6-21) and (6-22) that the rate constant for Eq. (6-23) can be calculated from the experimental value in Eq. (6-20) by the equation... 

Hammett correlation. The rate constants for reactions of a group of para-substituted 2-nitrophenyl bromides with the base piperidine are given.40 Estimate the reaction constants p and a for COj. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

Yukawa and coworkers (1972)84 determined a0 values from the rate constants for alkaline hydrolysis of m- and p-substituted-benzyl benzoates in 70% (v/v) aqueous acetone at 25 °C. ap° values for SOMe and S02Me were found to be 0.573 and 0.749 respectively. These were compared with 0.564 and 0.721, respectively, for values determined from the rate constants of alkaline hydrolysis of substituted ethyl benzoates in 85% aqueous ethanol. From these values there is no evidence for any — R cross-conjugative effect of SOMe as a substituent in the benzoate moiety, which is eliminated when it is in the benzyl. However, both the values for SOMe are substantially higher than most of the ap values for SOMe which we have surveyed previously. For S02Me the order ap° > if significant,... 

Miller226 applied the Hammett equation to the rate constants for the reaction of 4-substituted l-chloro-2-nitrobenzenes with OMe in methanol at 50°C. a values (denoted ct in accordance with the practice briefly in vogue at that time, 1956) were used for + R substituents, and S02Me conformed well at a a value of 1.04952. Act value of 1.117 for S02Ph was derived from the Hammett plot, intermediate between the values based on phenol and anilinium ionizations by Szmant and Suld88 at about the same time. 

A5-hexenyl substituent, extensive cyclization occurs to yield the cyclopentylcarbinyl product from the yields of uncyclized and cyclized products for A5-hexenylmercury chloride, the rate constants for equation 50 have been estimated (vide supra). The SH2 reaction 49 has also been invoked to be the key step in the alkylation of -substituted styrenes by a free-radical addition-elimination sequence, namely96... 

More recently, the reaction advancement of resole syntheses (pH = 8 and 60°C) was monitored using high-performance liquid chromatography (HPLC), 13C NMR, and chemical assays.55,56 The disappearance of phenol and the appearances of various hydroxymethyl-substituted phenolic monomers and dimers have been measured. By assessing the residual monomer as a function of reaction time, this work also demonstrated the unusually high reactivity of 2,6-dihydroxymethyl-phenol. The rate constants for phenolic monomers toward formaldehyde substitution have been measured (Table 7.6). 

Rate constants for the substitution reactions of square-planar dithio-phosphates and dithiocarbonate complexes of Ni(II), Pd(II), and Pt(II), with ethylenediamine and cyanide ion as nucleophiles, have been measured in methanol. The results were compared with those obtained in previous investigations, and interpreted in terms of the stabilities of 5-coordinate species that are formed prior to substitution (377). 

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