Hammett diagram

Fig. 1—2 shows a Hammett diagram for 14 different imidazolides of benzoic acids with a wide range of substituents upon which the reactivity is strongly dependent for example, the difference in rate constants between jV-(4-nitrobenzoyl)imidazole and iV-(4-dimethylaminobenzoyl)imidazole under the same reaction conditions amounts to a factor of about 3000. The Hammett reaction constant p = + 1.85 for the series shown in Fig. 1—2 indicates clearly that the hydrolysis is following a nucleophilic addition-elimination reaction path. 

Fig. 1—2. Hammett diagram for the hydrolysis of imidazolides of aromatic carboxylic acids in water/ tetrahydrofuran (3 1) at 21 °C.[91...

It will be obvious that the treatment thus far includes only substituents that are meta or para to the reaction site. When Hammett plots are made with data for ortho-substituted reactants, scatter diagrams usually result. This failure might be attributed to steric effects, but this is not very helpful, and many attempts have been made... 

A number of authors measured the influence of ring substituents on the rate of catalytic reduction of aromatic nitro compounds by hydrogen (5-7,114,128). The series have been composed in such a way as to allow the Hammett correlations, but, with a single exception, scatter diagrams resulted. The successful case by Rflzicka and Santrochova 128) (series 102, 12 points, slopes for three different platinum catalysts 0.24, 0.34, and 0.92, respectively) differs from the others in the use of platinum catalysts, whereas the other authors worked with rhodium (4,5), palladium (5,114), ruthenium... 

It was not a perfect correlation but Hammett had removed the examples where steric hindrance was important. Aliphatic compounds can adopt a variety of conformations (Chapter 18) and the substituent in some of them will interfere with the reaction. Similarly, in ortho-substituted aromatic compounds the nearby substituent might exert steric hindrance on the reaction. Only with meta-and para-substituted compounds was the substituent held out of the way, on a rigid framework, and in electronic communication with the reaction site through the flat but conjugated benzene ring. The diagrams show the para substituent. 

Fig. 3.17. Reaction potential energy diagram illustrating the Curtin-Hammett principle.

Catalytic Reactions diving Scatter Diagrams in Coordinates of the Hammett Equation... 

The Hammett equation gave satisfactory results only in half of the tested reaction series. The reason for the failures is not certain but there is strong suspicion that they are connected with solvent-catalyst interactions. Only further work can substantiate this because homogeneous reactions also give scatter diagrams sometimes for not too clear causes. On the other hand, even unsuccessful correlations may be valuable as they indicate possible changes in mechanisms or intervention of some factors which have not been accounted for and provoke new, more detailed investigations. 

Reaction coordinate diagrams illustrating reactions of diastereomeric olefin complexes. In scenario A, olefin binding is enantio-determining. In B, the diastereomeric olefin complexes are in rapid equilibrium and enantio-determination is the conversion of the olefin adducts to products. B is an example of Curtin-Hammett conditions. 

Energy diagram for the asymmetric hydrogenation reaction under Curtin-Hammett conditions. The enantioselectivity-determining step is the oxidative addition (OA) of hydrogen. 

Diagram demonstrating the Curtin-Hammett principle, where the product ratio is determined by the relative heights of the barriers and not the relative stabilities of the intermediates. 

Hammett plots have been constructed for the acid- and base-catalysed decomposition of methyl hemiacetals of benzaldehydes in aqueous solution. The data are analysed in terms of three-dimensional More O Ferrall-Jencks diagrams and of Cordes interaction effects. 

The log k. vs. e value plot is useful, since the variation in range of the electron density of the double bond is wider than that of the Hammett constants. From potential energy diagrams (Figures 4 and 6), a smaller change in the activation... 

This potential diagram can explain the Hammett plots for both sides of the reaction equation examples are shown in Figure 12 for the addition reactions of p-X-Cf l4-S with the substituted phenylacetylenes (CH=C-Cf,H4-Y-/7) [11]. If the stabilities of P-X-C6H4-S are not much influenced by the substituent X, the slopes (p kJx) the Hammett plots in Figure 12(a) should be almost mirror images of the slopes p k.Jy) in Figure 12(b). However, the observed slopes indicate that p (fca)xl>lP ( a)Yl which implies contribution of the y term to the variations of k., and k, . 

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