Rate constant for substitution reactions

The rate constants for substitution reactions of labile complexes are frequently in the order of 101 to 106 M-1 s whereas those for inert complexes are as low as 10-5 to 10-8 s-1. Certainly the difference in electronic structures is one factor contributing to this enormous variation in rates of substitution. Other reasons will be discussed later in this chapter. 

Table 8 Rate constants for substitution reactions of metal porphyrin complexes at 25 in aqueous solution...

Table 10 Rate constants for substitution reactions of haemoglobin model complexes at 20 °C in aqueous solution at pH 7.3...

Table 11 Rate constants for substitution reactions of metal ions with apocarb-oxypeptidase A in aqueous solution at 25 °C, pH 7.0, and 1.0 M ionic strength ...

Figure 12 A plot of nucleophile concentration against pseudo first-order rate constant for substitution reactions of various ligands with trans- [Pt(py)2Cl2].

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. 

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

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. 

We have also examined the effect of substituents on orientation in the addition of BH3 to the carbon-carbon double bond. Consider the substituted ethylene XCH=CH2. The boron may become bonded either to carbon 1 or to carbon 2. The overall rate constant for the reaction is given by... 

For these transition states, equations analogous to eqs. (92) and (93) may be written. For case (c), we may obtain equations analogous to eqs. (85) and (88) which are not capable of further simplification. From the above discussion, we see that in the event of the concerted mechanism, the rate constants for the reaction of the disubstituted dienophiles with symmetric dienes should be successfully correlated by eq. (30), which should result in values of a and /3 equal to those obtained from the correlation of the rate constants of substituted dienophiles (with the same diene under the same reaction conditions) with eq. (2). In the event of the two-step mechanism, the rate constants for the reaction of the disubstituted dienophiles with symmetric dienes should not be correlated by eq. (30) unless the two substituents are identical. In addition, the values of a and (3 obtained from the correlation of rate constants for disubstituted dienophiles with eq. (30) should not be equal to the values of a and 3 obtained for the correlation of the rate constants of substituted dienophiles with eq. (2). 

The Ir(III,IV,IV) complex [Ir3N(S04)6(0H2)3]4 undergoes ligand substitution of the three H20 ligands with azide, Cl, Br, and NCS-. The three H20 ligands, one on each metal, are all equivalent towards substitution.103 The rate constant for the reaction with azide is 17.3 x 10-4 M 1s 1. 

Ohta, T. (1983) Rate constants for the reactions of OH radicals with alkyl substituted olefins. Int. J. Chem. Kinet. 16, 879-886. 

Sauer, Sustmann and coworkers59 have reported second-order rate constants for the reaction of trans-1 -substituted 1,3-butadienes with tetracyanoethylene (TCNE) in dichloro-methane at 20 °C their values are X, log 6 OMe, 7.935, vinyl, 5.456 Ph, 5.814 Me, 5.243 H, 3.228. The data were correlated with the CR equation the best regression equation is ... 

Craig and coworkers62 have reported rate constants for the reaction of 2-substituted... 

We have also correlated rate constants for the reaction of 3- or 4-substituted N,N-dimethylanilines with methyl iodide, 3- or 4-substituted benzoic acids with diphenyldia-zomethane and 3- or 4-substituted benzoyl chlorides with aniline with the MYT equation. The best regression equations obtained are ... 

Rate constants for the reaction of a series of substituted anilines 70-72 with A-acetoxy-A-butoxybenzamide 25c are presented in Table 7. 

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

The introduction of a substituent in an organic compound may affect its reactivity in a given reaction. A number of quantitative relationships have been suggested in connection with the effect of substituents on the rate constant of the reaction. Such structure-reactivity co-relations are helpful in predicting the reactivity of organic compounds in various reactions and also in verifying the reaction mechanism. One such useful relationship was proposed by Hemmett, which relates the equilibrium and rate constants for the reaction of meta and para substituted benzene derivatives. 

Howard CJ, Evenson KM. 1976. Rate constants for the reactions of OH with CH4and fluorine, chlorine, and bromine substituted methanes at 296K. J Chem Phys 64 197-202. 

Soil A reversible equilibrium is quickly established when aniline covalently bonds with humates in soils forming imine linkages. These quinoidal structures may oxidize to give nitrogen-substituted quinoid rings. The average second-order rate constant for this reaction in a pH 7 buffer at 30 °C is 9.47 x 10 L/g-h (Parris, 1980). In sterile soil, aniline partially degraded to azobenzene, phenazine, formanilide, and acetanilide and the tentatively identified compounds nitrobenzene and jD-benzoquinone (Pillai et ah, 1982). 

Howard, C.J. and Evenson, K.M. Rate constants for the reaction of OH with ethane and some halogen substituted ethanes at 296 °K,/ Chem. Phys., 64(ll) 4303-4306, 1976. 

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