Hammett o,p correlations

Systematic Hammett o/p correlation with m- and p-substituted benzenes ... 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilicity of radicals is by way of a Hammett o p correlation. Thus, the reactions of radicals with substituted. styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character " while haloalkyl radicals" and oxygcn-ccntcrcd radicals" have electrophilic character (Table 1.4). It is anticipated that electron-withdrawing substituents (e g. Cl, F, CO R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

Louis P. Hammett (1894-1987) invented physical organic chemistry and at Columbia University in 1935 derived the Hammett o/p relationship. The impact was enormous and in the 1960s chemists were still working out more such correlations. 

The efficient enantioselective alkylation of the P/y-unsaturated ester 39 was achieved1351 by use of the N-anthracenylmethyl catalyst 12 (R=benzyl, X=Br) together with CsOH-H20 under phase transfer conditions analogous to those in the alkylation of the O Donnell imine 23, as shown in Scheme 13. The enantioselectivity of the alkylation correlates with Hammett o constants, and the N,N-dimethylamino substituents in 39 showed the most effective enantioselectivity. The tight ion pair in which the enolate... 

FIGURE 1. Hammett plots [o (a) and cr+ (b), respectively] for the formation of semimercaptals from GSH and substituted nitrosobenzenes. Log k /k values were calculated from the rate constants reported for pH 7.4, 37 °C18 22 35 and pH 7.49, 25 °C, ionic strength 1 M30. Correction for pH is unnecessary as k and k ° are affected in the same manner. Correction for the different temperatures has been revealed to be insignificant. (The Hammett parameter p appears to vary with 1 IT while a seems to be independent of temperature54. Hence, log k /k 0 has been assumed to correlate with 1 IT.) For want of a for the complex nitrosochloramphenicol substituent (20), a of 4-CHOHMe was used55... 

The electrophilic character of n-perfluoroalkyl radicals was confirmed by a correlation of the rates of addition of the n-C8F17 radical to a series of para-substituted styrenes with Hammett o values, as shown in Fig. 3. The p value is negative (-0.53), as would be expected for an electrophilic reactant [115]. 

Substituent effects have been reported previously and work on this topic continues. The half wave potentials of a series of substituted benzyl chlorides and bromides gave excellent correlations with Hammett o- substituent constants48. The positive p values from these Hammett LFERs (p = 5.0 and 2.8, respectively for chlorides and bromides) suggest that the potential-determining electrochemical process involves the formation of radical anion intermediates. 

Fig. 21, Correlation of A.P. —I.P. for the formation of acetyl ion in the mass spectra of substituted acetophenones with Hammett o and constants.

Hansch constant A measure of the capability of a solute for hydrophobic (lipophilic) interaction based on the partition coefficient P for distribution of the solute between octan-l-ol and water. The most general way of applying P in correlation analysis, qsar, etc., is as log P, but the behavior of substituted benzene derivatives may be quantified by a substituent constant scale, 7t, which is defined in a way analogous to the Hammett o scale. There are various 7t scales, depending on the substrate series used as reference. 

O- F distance proton is transferred towards fluoride and the hydrofluoric acid is formed. The O F distance for the proton transfer for variously substituted phenols correlate nicely with the Hammett constants o-p, as shown in Fig. 22. 

The interfacial hypothesis has been also proposed independently and very recently by Smyth et al. (ref. 41) who correlate the rate constants with the Hammett parameters (p and o ) and found that the solubilization of KF cannot be the rate-limiting step since p is as large as + 6.4. 

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