Hyperconjugation constant

A mode] study has demonstrated the pathways shown in Scheme 4,17. The first cyclization step gave predominantly five-membered rings, the second a mixture of six- and seven-membered rings.155 Relative rate constants for the individual steps were measured. The first cyclization step was found to be some five-fold faster than for the parent 5-hexenyl system. Although originally put forward as evidence for hyperconjugation in 1,6-dienes, further work showed the rate acceleration to be sterie in origin.113-I3j... 

In the examples shown, the primary ion has only two hyperconjugative forms while the tertiary has six. According to rule 6 (p. 41), the greater the number of equivalent forms, the greater the resonance stability. Evidence for the hyperconjugation explanation is that the equilibrium constant (K) for this reaction... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

The rate constants for the KIEs were measured using UV spectroscopy in separate kinetic runs using the undeuterated and deuterated substrates. Although this normal secondary /3-deuterium KIE could be due to hyperconjugation, the authors, like Streitwieser and Van Sickle (1962), preferred to attribute it to an inductive effect. 

The secondary /3-deuterium KIEs observed for the reaction of the same substrate with hydroxide ion and with tris(hydroxymethyl)methylamine in aqueous solution at 25°C were small, i.e. kH/kD = 1.09 0.01 and 1.10 0.01, respectively. While Kresge argued that the EIE was primarily due to hyperconjugation, the secondary /3-deuterium KIEs were attributed partly to hyperconjugation and partly to a polar (inductive) effect. The rate constants for the evaluation of both the EIE and the KIEs were determined in separate kinetic runs by following the increase in the absorbance due to the nitronate ion by UV spectroscopy. 

Withdrawal of electrons from orbitals antisymmetric between hydrogen atoms (generally hyperconjugative effects) should lead to a negative change in the coupling constants. ... 

It is important to point out once again that explanations (rationalizations) of isotope effects which employ arguments invoking hyperconjugation and/or steric effects are completely equivalent to the standard interpretation of KIE s in terms of isotope independent force constant differences, reactant to transition state. In turn, these force constant differences describe isotope dependent vibrational frequencies and frequency differences which are not the same in reactant and transition states. The vibrational frequencies determine the partition functions and partition function ratios in the two states and thus define KIE. The entire process occurs on an isotope independent potential energy surface. This is not to claim that the... 

The electronic effects of cycloalkyl annelation can be divided into inductive and resonance effects. Few measurements of the resonance contributions of cycloalkyl substituents exist. A single determination of the resonance parameter para to substitution in o-xylene, tetralin and indan suggests that hyperconjugative release is fairly constant within a series of benzocycloalkenes, and that strain induced electronic effects must act inductively. ... 

Further support for the rehybridization model was provided by analysis of the spin densities of annelated 1,4-naphthoquinones, n hthalenes, and cyclobutaben-zene radical anions by ESR. The hyperfine coupling constants at the methylene position decrease as ring strain increases. Calculation of spin densities using the rehybridization model leads to an excellent fit of experimental spin densities, while calculation of spin densities using either the Coulson-Crawford hyperconjugation model or INDO do not accurately correlate experimental values. Such results suggest that rehybridization can account for the observed changes in spin densities. 

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