Relative rate of substitution

Another interesting idea that can be drawn from Table 6 is with regard to the relative rate of substitution of the last reactive ring position in dimethylol derivatives. It is clear that there is no favorable change in the activation enthalpy... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Unfortunately, several less direct observations indicate that the kinetic conclusions found in the substitution of Ir4(C0)i2 are not easily generalized. For instance, it is known that progressive substitution in other clusters, such as Ru3(CO)jo(NO)2205 and Co3(CO)9CR44 always exhibit predominantly SN1 kinetics. Moreover, it is also known that there is no large increase in the relative rates of substitution in either Co4(CO)j2 48> or Rh4(CO)i2 61 25° since both these clusters react with a stoichiometric amount of triphenylphosphine to give essentially the monosubstituted cluster. 

The reaction proceeds almost exclusively by direct substitution (ipso), as shown by reactions of isomeric chlorotoluene complexes (Scheme 4).so The relative rates of substitution of the isomeric ligands are similar, in the order 1.0 1.4 2.4 for o m p.50 A version of cine substitution is possible under special conditions, as discussed below. 

In further studies, Matteson et a/.10,11 reported relative rates of substitution of various boronic esters by mercuric chloride details are given in Table 3. The solvent used was a mixture of ethanol (88 %), water (8 %), and glycerol (4 %), buffered with sodium acetate and acetic acid, and reactions were run in the presence of added sodium chloride. Under these conditions, the kinetics of reaction of benzylboronic ester with mercuric chloride were found to obey the rate law... 

Relative rates of substitution of tetraalkyltins by mercuric halides are in Table 10. The similarity of the four sets of relative rates led Abraham and Johnston21 to conclude that all four series of substitutions proceed by the same mechanism, SE2(open). The pronounced steric sequences of relative rates (Table 10) may be considered in terms of the analysis of Abraham and Spalding (see p. 71) there is a very slight extra steric effect when mercuric chloride is the electrophile, but there is no obvious reason why this should be so. 

The relative rates of substitution para to the chloro and bromo substituents agree with the predictions of the Extended Selectivity Relationship to about the same extent as the meta-halogens. The... 

Fig. 53. The relationship between the calculated and observed relative rates of acetylation of the polymethylbenzenes. Crossed circles indicate relative rates of substitution at positions not adjacent to a methyl group.

Scheme4.30. Relative rates of substitution at a-substituted methyl derivatives [112, 128].

Relative Rates of Substitution of the Fundamental 5-Membebed Rings... 

Relative rates of substitution of the unsubstituted rings at 75°C. See dementi and Marino.143... 

Relative rates of substitution at 25°C at position 5 of the 2-methoxycarbonyl derivatives, obtained from the rate constants of Table IV, corrected for isomer distributions. Linda and Marino.84... 

Overall Relative Rates of Substitutions in Benzofuban and Benzothiophene" b... 

Relative rates of substitution for a number of substituted thiophenes sufficient to test the applicability of po+ relationships are available for seven reactions bromination by molecular bromine, chlorination by molecular chlorine, protodetritiation, proto-dedeuteriation, acetoxymercuration, tin tetrachloride-catalyzed acetylation, and trifluoroacetylation. The relevant data are assembled in Table XXIII. 

Relative Rates of Substitution of Thiophenes RC4H,S in Base-Catalyzed Deuteration... 

Partial rate factors for trifluoroacetylation at the 5-position of 2-substi-tuted furans in 1,2-dichloroethane at 75°C [Table 6.7, [72JCS(P2)71]] correlate approximately with u+ values, with p = -10. The effects of para substituents on the rate of acetylation and trifluoroacetylation of 2-aryl-5-methylfurans have been studied. In the latter reaction, the relative rates of substitution at the 3-position (to give 6.10) were H, 1.0 Cl, 0.43 Me, 5.3 and OMe, 35.4, which correlated with the Yukawa-Tsuno equation, p = -2.6, r = 0.74. For acetylation, the corresponding relative rates for... 

These data are in accord with an associative mechanism of substitution with the ligands that bond preferentially to Pt(II), giving faster substitution reactions. From these and similar studies, it is possible to arrange a series of ligands on a scale of relative rates of substitution, with the order being... 

Two further examples may be used to illustrate the dependence of the rate of SN1 substitution on the stability of the resultant carbonium ion intermediate. First, suggest what are the relative rates of substitution between CH3CH2Br and PhCH2Br and second, the relative rates of substitution between... 

Electrophilic substitution of aromatic compounds proceeds via a two-step sequence - addition (of then elimination (of H ), of which the former is usually the slower (rate-determining) step. Qualitative predictions of relative rates of substitution at different ring positions can be made by inspecting the strac-tures of the o-complexes (Wheland intermediates) formed in the first step, on the assumption that their relative stabilities reflect the relative energies of the transition states that lead to them. 

Relative rates of substitution Electrophilic reagent Reference... 

Table 2.9. Relative Rates of Substitution vs. Elimination for Alkyl Bromides...

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