Inductive and hyperconjugative effects

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

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. 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

Unfortunately, the methods for determining the electronegativity of M and the 01 constants of R3M substituents (in particular, Me3M) do not yet provide a rigorous separation of the inductive and hyperconjugation effects. 

The carbocation intermediate can be stabilised by neighbouring alkyl groups through inductive and hyperconjugation effects. However, it can also be stabilised by sharing the positive charge with the bromine atom and a second carbon atom. 

Electronic factors also help in the formation of the carbocation because the positive charge can be stabilised by the inductive and hyperconjugative effects of the three alkyl groups ... 

Both the inductive and hyperconjugation effects are greater when there are three alkyl groups connected to the carbocation centre than when there are only one or two. Therefore, tertiary alkyl halides are more likely to produce a stable coarbocation intermediate than primary or secondary alkyl halides. 

The conformation of cycloheptene oxide has been examined via the C nmr spectra of deuterated compounds on the basis of the temperature-dependence of the chemical shifts of the individual signals. In phenyl-substituted oxiranes, the C shifts have revealed the inductive and hyperconjugative effects of the oxirane ring, and thus the ring behaves as an electron-acceptor. " ... 

In general, the effects of alkyl and alkoxyl (e.g. CH3—O) groups are well known and understood. For example, ortho and para alkyls (at positions 2,4,6) stabilize the phenoxyl radical by inductive and hyperconjugative effects and, in addition, ortho groups provide steric hindrance to minimize undesirable wasting reactions such as pro-oxidation (equation 21). In addition, the conjugative effect of a heteroatom, for example at the para-position, provides stabilization through resonance (Scheme 4). 

Here SQci=c and 8Q=CiK are the changes of the induction and hyperconjugation effects, and So- is the change of the conjugation energy caused by substitution. The dependence (11.66) is not very simple. To find the effect of a one should, evidently, compare the compounds of such a structure where SQcj=c and SQ=ci k would, as far as possible, not change. 

Correlation of the first ionization energy in a series of alkyl derivatives of H2S is shown in Fig. The inductive and hyperconjugative effect of the alkyl... 

Explain how mass effects act to lower the carbonyl frequency, as well as how inductive and hyperconjugation effects act to raise the carbonyl frequency of aldehydes relative to ketones. 

An interpretation of the magnitude of p is difficult since many factors are involved. One sees, for example, the effect of inductive electron withdrawal on the p values of (31) and (32) by their comparison with the saturated isomers (33) and (38)." Hyperconjugation is also said to be a factor " Compounds stabilized hyperconjugatively [such as (36)] have low values of p, whereas compounds with little hyperconjugative stabilization [such as (23)] have high values of p. Yet it is difficult to rationalize from our present knowledge of inductive and hyperconjugative effects why the bi-cyclic and cyclic derivatives (33) and (35), which have nearly equal p values, should have a p value so unlike the structurally similar acyclic system (37)." ... 

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