Hughes-Ingold

Hughes, Ingold and Reed discussed the relative merits of the and e3 schemes as mechanisms for nitration by considering the properties of acetic acid, nitromethane, nitric acid and sulphuric acid as media for the reaction. The facts have already been discussed ( 2.2.3, 2.2.4, 2.3.2, 2.4.2, 2.4.3, 3-2). 

Table 8-1 shows the application of the Hughes-lngold hypothesis to aliphatic nucleophilic reactions of various charge types. These predictions are borne out by observations on many reactions. It should be noted - that the Hughes—Ingold rule... 

We have two cases to consider here. If the reactants are nonpolar and the products are nonpolar, it is a reasonable hypothesis that the transition state also is nonpolar. In this case, the Hughes-Ingold hypothesis (Section 8.1) leads us to expect that the solvent will have little effect on the rate. The decomposition of azo compounds provides an example. The reaction is... 

Summarizing the discussion in this section, it is obvious that the various forms of diazotization methods became understandable only after the long historical development leading to the breakthrough by Hughes, Ingold, and Ridd in 1950 to 1958. 

The names for these mechanisms vary throughout the literature. For example, the Sgi mechanism has also been called the Sp2, the Se2 (closed), and the Se2 (cyclic) mechanism. The original designations, Se 1, Se2, and so on, were devised by the Hughes-Ingold school. It has been contended that the SeI mechanism violates the principle of conservation of orbital symmetry (p. 1068), and that the Se2 (back) mechanism partially violates it Slack, D.A. Baird, M.C. J. Am. Chem. Soc., 1976, 98, 5539. 

Spontaneous decarboxylations of carboxylate ions and hydrolyses of aryl phosphate dianions and aryl sulfate monoanions are much faster in organic solvents than in water (Thomson, 1970 Kemp and Paul, 1970 Bunton et al., 1967 Kirby and Varvoglis, 1967). This solvent effect is consistent with the Hughes-Ingold qualitative solvent theory because these reactions involve dispersion of charge in forming the transition state. 

Hughes, Ingold, and their co-workers also attempted to generalize about the mechanism by which elimination of substituents, rather than substitution for substituents, occurs in the aliphatic molecule, resulting in the formation of an olefin in the course of what initially was predicted to be a SN1 mechanism. 

The direction and extent of the effect of solvent polarity on reaction rates of nucleophilic substitution reactions are summarized by the Hughes-Ingold rules, shown in Table 1.9 [26], These rules do not account for the entropic effects or any specific solvent-solute interactions such as H-bonding, which may lead to extra stabilization of reactants or transition states [27],... 

Table 1.9 Hughes-Ingold rales for solvent effects in nucleophilic substitution reactions [27, 28]...

HUGHES-INGOLD THEORY FOR SOLVENT EFFECTS ON REACTIVITY... 

HUGHES-INGOLD THEORY EOR SOLVENT EEEECTS ON REACTIVITY HUMMEL-DREYER TECHNIQUE HYALURONATE LYASE HYALURONIDASES HYBRIDIZATION HYDRATION ATMOSPHERE HYDRATION NUMBER Hydrazone reduction,... 

Cowdrey Hughes Ingold Masterman Scott J. Chem. Soc. 1937, 1252. The idea that the addition of one group and removal of the other are simultaneous was first suggested by Lewis in Valence and the Structure of Atoms and Molecules Chemical Catalog Company New York, 1923, p. 113. The idea that a one-step substitution leads to inversion was proposed by Olsen J. Chem. Phys. 1933, /, 418. 

M7Cowdrey Hughes Ingold Masterman Scott 3. Chem. Soc. 1937, 1264 Long Purchase 3. Am. Chem. Soc 1950, 73, 3267. 

For an exhaustive study of this reaction, see Hughes Ingold and co-workers J. Chem. Soc. 1950, 2400-2684. 

Baciocchi Clcmcnii Scbastiani Ruzziconi J. Org. Chem. 1979, 44, 32. MCoopcr Hughes Ingold MacNulty J. Chem. Sex-. 1948, 2038. 

For summaries of this position, sec Ingold Proc. Chem. Soc. 1962, 265-274 Banthorpc Hughes Ingold J. Chem. Soc. 1966, 4054. 

In 1933 the two still widely accepted mechanisms for nucleophilic displacement reactions were proposed by Hughes, Ingold, and Patel.4 They found that decomposition of quartenary ammonium salts, R4N+Y, to give R3N and RY exhibited two different kinds of kinetic behavior depending on the ammonium salt used. For example, when methyl alcohol was formed from trimethyl-n-decylammonium hydroxide (Equation 4.3), the rate of formation of methyl alcohol was found to be second-order, first-order each in trimethyl-n-decylam-monium cation and in hydroxide ion as in Equation 4.4. On the other hand, the rate of formation of diphenylmethanol from benzhydryltrimethylammonium... 

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