Correlations Hammett

Good to excellent Hammett plots were obtained using substituent constants (see Figure 2.6). Surprisingly, literature examples of good Hammett correlations of stability constants are rare The p-values are shown in Table 2.7. 

The reaction of substituted 1-arylethyl chlorides with K OC(CH3)3 in DMSO does not follow a Hammett correlation. Instead, the reactivity order is />-N02 > p-MeO > P-CF3 > P-CH3 > H > p-Cl. What explanation can you offer for the failure to observe a Hammett relationship ... 

The Hammett correlation of the acid-catalyzed dehydration of 1,2-diarylethanols has been studied. 

The acidity of various substituted acetophenones has been measured in DMSO. Would you expect the p value for a Hammett correlation to be positive or negative Would you expect the best correlation with a, or [Pg.442]

If the two interaction mechanisms maintain the same relative importance throughout the reaction series, that is, if the ratio pj/pi is constant, Eq. (7-26) becomes identical in form with Eq. (7-24). This in fact appears to be the usual situation in Hammett correlations, PP- and the two interaction mechanisms are commonly discussed in terms of the inductive and resonance effects of electronic displacement. 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Few other reactions of series of substituted pyridines have been investigated extensively. Dondoni, Modena, and Todesco have measured the rate of N-oxidation of a limited series of pyridines and found a good correlation with normal u-values with a p-value of — 2.23. The A-alkylation of pyridines with alkyl iodides in nitrobenzene has been studied by Brown and Cahn and by Clarke and Rothwell. Unfortunately, the only data available are for the parent compound and for alkyl derivatives, and, since the a-values for the various alkyl groups in a given position are substantially constant, this leaves a correlation of only three independent points. However, the rates of A-alkylation of the j8- and y-alkyl derivatives are so nearly equal that it appears as if no correlation existed. Clarke and Rothwell have also studied the alkylation with allyl bromide in nitromethane at various temperatures, and in this case a more extensive series is available. The authors state that no overall Hammett correlation is obtained however, the j8-substituted derivatives fall on one straight line and the y-derivatives on another one with a different slope. The data are shown in Fig. 2. The line for the j8-compounds, p = — 2.53 0.31, r = 0.95, is seen not to be very good the line for the y-derivatives, p = — 1.42 0.06, r = 0.99, is much more satisfactory. It does not seem likely that the discrepancy is due to the intervention of resonance effects, since in this case one would expect the correlation for the y-derivatives to be poorer than that for the j8-analogs. More extensive studies with a wider variety of substituents would seem very desirable. 

Illuminati et al. have also investigated the methoxydechlorination of 4-substituted-2- and 2-substituted-4-chloroquinolines. The relation between the reaction site, the 2- or 4-position, and the substituent in the 4- or 2-position, respectively, is always meta. The authors found the two reaction series well correlated with one another, but diverging quite seriously from the Hammett correlation. They concluded that mesomerically electron-donating substituents, because of the importance of resonance structures like 12 and 13, are more deactivating than expected, while electron-withdrawing substituents, and even the methyl group, seem to follow normal a correlation. 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

There are two principal reasons why an attempted Hammett correlation might fail. The mechanism may change over the range of substituents chosen, or the choice of a as the correlating parameter may be a poor one for a particular reaction series. 

Hammett correlation. Carry out a Hammett correlation for the reaction of substituted benzoic acids with diazophenylamine. Values in ethanol at 303 K are as follows ... 

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. 

Hammett correlation, 225-228 Hammond s postulate, 242 Heat capacity of activation, 160-161 Hydrated electrons, 266-267 Hydroxyl radicals, 266-268... 

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222... 

Rough Hammett correlations of with the resonance constants R for para-substituted isomers (ppar = —0.25) and with om+ for meto-substituted... 

The role of substituents X on the mononuclear heterocyclic rearrangement (MHR) of 20 phenylhydrazones 54 of 3-benzoyl-5-phenyl-l,2,4-oxadiazole into the triazoles 55 (Equation 2) has been investigated, allowing the influence of X on the product distribution to be evaluated and first-order rate constants and Hammett correlations to be determined <1999T12885>. 

Electronic effects. Nucleophilic attack is favoured by electron-withdrawing groups on the amide and the acyloxyl side chains. Interpolated bimolecular rate constants at 308 K for the series of para-substituted /V-acetoxy-/V-butoxybenzamides 25c, 26b-g and 26i (Table 5) gave a weak but positive Hammett correlation with a constants ip = 0.13, r = 0.86).42,43 These Sn2 reactions are analogous to those of aniline and substituted pyridines with phenacyl bromides, which have similar Arrhenius activation energies and entropies of activation in methanol (EA= 14-16 kcal mol-1, AS = — 27 to —31 calK-1 mol-1) and 4-substituted phenacyl halides afforded a similar Hammett correlation with pyridine in methanol (cr, p — 0.25).175... 

In reactivity, the same series of mutagens exhibited positive Hammett correlations for AaiI acid catalysed solvolysis (p — + 0.32) and SN2 reactivity with A-methyl-aniline (p — + 1.7), hydroxide (p — +0.55) and L-cysteine ethyl ester (p — + 1.1), all reactions in which the benzoyloxyl group leaves with electrons and which are therefore accelerated by electron-withdrawing groups. The negative Hammet p-value for... 

Fig. 29 Hammett correlation of mutagenicities of 28 at 1 pmol per plate without S9.

Experiments were carried out using a series of reagents, P(C6H4R)3. A Hammett correlation of kcat against 3a gave the reaction constant Peat = +1-03. The positive value is consistent with the chemistry proposed at this stage, in that the most weakly-coordinated phosphine would be expected to be the most reactive catalyst. 

Both equilibrium and kinetic data have been obtained. The most extensive set of equilibrium constants was determined for pyridines, RC5H4N. Values of K24 lie in the range 2xl0-2 (R = 2-Me) to > 5xl04 (R = 4-Me2N) according to a Hammett correlation, p-—7.5 (35). The... 

Brown-Hammett correlation of 1-aryl-1-cyclobutylethyl p-nitrobenzoate gave a p+ value of -3.94 which is more positive than that of a model compound, l-aryl-l,2-dimethylpropyl p-nitrobenzoate (p+ = -4.65), which indicates substantial neighboring group participation of the cyclobutyl ring even in tertiary carbocationic systems.12 Solvent effects on the kinetics of the solvolysis... 

At constant pH, the parameter p is dependent on the electronic effects of the nitrosoarene substituent(s). Using only definite acceptor substituted nitrosoarenes, a Hammett correlation on the a scale was obtained25. Separate investigation of fc2RSH and fc2(rearr) for a wider selection of nitrosoarenes reacting with GSH revealed reasonable correlation with Hammett a constants only for fc2RSH (p = +1.4). However, Utrearr) was found to fit better on the Hammett a+ scale (p+ = —3.5)30 [for further discussion of Utrearr) see Section II.D.l.b]. 

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