Substituent effects on rate

To illustrate substituent effects on rate consider the nitration of benzene toluene and (trifluoromethyl)benzene... 

Substituent effects on rates (Nishida, 1967) and equilibria (Mindl and Vecera, 1972) of the heterolytic formation of benzhydryl cations, analogous to those obtained in the bromination of 1,1-diphenylethylenes, can be analysed in terms of selectivity relationships (50) and (51). Here ak is... 

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Kinetic and product isotope effect discrepancies 95 Oxidations by inorganic oxidants and quinones 95 Substituent effects on rates and equilibria 98 Miscellaneous reactions of dihydropyridines 101 Secondary hydrogen isotope effects 102 Theoretical studies 103 References 105... 

Substituent effects on rates and equilibria in hydride transfers between a range of NAD+ analogues have been examined. Anhydrous and aqueous acetonitrile and aqueous isopropyl alcohol have been used as reaction media, and earlier caveats as to possible complicating kinetic effects of nonproductive adduct formation apply, particularly where hydroxylic solvents are used. Data from hydride transfers have been compared with equilibria, kR +, and rates for pseudo-base formation, or for formation of cyanide adducts. The aqueous alcoholic solvent has an added disadvantage that p/fR+-values for the cations are necessarily composite. 

DFT has also proved useful for the calculation of substituent effects on rates. For example, studies of reactions of butadiene and cyclopentadiene with acrolein predict a large s-trans preference of the acrolein, and a 1 kcal/mol lower activation energy than with the ethylene reaction, but very similar energies of exo- and endo- transition states [38]. Jursic has also studied several heterocyclic cases [39]. 

Substituent effects on rate constants of base-promoted ionisation of ketones have led to the conclusion that an electron-withdrawing substituent increases the rate of ionisation, in agreement with the anionic character of the transition state. This is based chiefly on studies on halogenation and isotope exchange of aromatic ketones. Data on p-values observed by plotting ionisation rate constants versus Hammett -parameters (Table 3) for substituted... 

Substituent Effects on Rate Constants for Proton Transfer at Carbon... 

A number of kinetic isotope studies have been reported for gas-phase eliminations. Isopropyl bromide-dg decomposes more slowly than isopropyl bromide and the intramolecular isotope effects (A ,c2D4hx-c2D4)/ (C2D4hx-c2D.iH)) have been recorded for pyrolyses of ethyl acetate (2.0), chloride (2.20), and bromide (2.10) at 500°C. At the elevated reaction temperatures, these values correspond to the maximum predicted for complete loss of the C-H stretching vibration and they have been interpreted as indicating considerable weakening of the C-H bond in the transition state. Whether this is a homolytic or hetero-lytic bond fission, it is remarkably insensitive to beta substituent effects on rate. The intramolecular isotope effects in these cases could reflect predominantly a secondary isotope effect rather than the intended primary effect and dissection into an intermolecular and secondary isotope effect would prove more fruitful. (Section 2.2.1.)... 

SUBSTITUENT EFFECTS ON RATES OF ISOMERIZATION OF ALLYLIC ALCOHOLS IN AQUEOUS 60% DIOXaNE AT30°C... 

We have seen numerous examples of substituent effects on rates and equilibria of organic reactions and have developed a qualitative feel for various groups as electron-donating or electron-withdrawing. Beginning in the 1930s, Lewis P. Hammett of Columbia University developed a quantitative treatment of substituent effects represented in the equations ... 

It was relatively straightforward to prepare compounds whose structural and electronic characteristics could readily be varied. For example, substituted benzoic acids and substituted methyl benzoates are readily prepared. As a result it was relatively straightforward to study substituent effects on rates and equilibria by varying the nature of X (equations (1.1) and (1.2)). 

Scheme 11.24 Substituent effect on rate ofthe Claisen rearrangement in CCh [29]. The data were calculated from the reported half-life times according to f,2 = ln2/(c.

This kind of procedure is very common in organic chemistry. For example, the well-known Hammett equation, used for the estimation of substituent effects on rates and equilibria, is likewise based on an empirical reference process - the dissociation of substitued benzoic acid in water at 25°C(6). 

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