Hyperconjugation positive

When the /)-positions are considered it is seen that they follow the sequence of inductive effects, and not of hyperconjugation. In this respect nitration is unusual amongst electrophilic substitutions. ... 

Hammett s equation, and substituent effects, 137-43 heteromolecules, 130 Holleman s product rule, 3 hyperconjugation, in nitration of alkyl-benzenes, 165-7 in nitration of positive poles, 169... 

Only electrons in bonds that are f3 to the positively charged carbon can stabilize a car bocation by hyperconjugation Moreover it doesn t matter whether H or another carbon IS at the far end of the (3 bond stabilization by hyperconjugation will still operate The key point is that electrons m bonds that are (3 to the positively charged carbon are more stabilizing than electrons m an a C—H bond Thus successive replacement of first one... 

Methylindole has a p/sTa of -4.6 and it is therefore a weaker base than indole itself this unusual effect has been ascribed in part to the decreased hyperconjugative stabilization of the conjugate acid (38) by the one hydrogen at position 3 compared with the two hydrogens at position 3 in the 3//-indolium ion (39). 

Secondary isotope effects at the position have been especially thoroughly studied in nucleophilic substitution reactions. When carbocations are involved as intermediates, substantial /9-isotope effects are observed. This is because the hyperconjugative stabliliza-... 

FIGURE 4.16 Hyperconjugation in ethyl cation. Ethyl cation is stabilized by delocalization of the electrons in the C—H bonds of the methyl group into the vacant 2p orbital of the positively charged carbon. 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

The vinyl cation (Fig. 40 and III.13) in which the empty p orbital on the positive center is coplanar with the terminal methylene group, is clearly another candidate for hyperconjugative donation. The in-plane orbital readily overlaps with the odd p orbital, with formation of two delocalized orbitals and an energy gain... 

The calculated isotope effects for the solvolyses of t-butyl chloride-dg (previously estimated by Bartell, 1961a) and acetyl chloride-ds were much weaker than the experimentally observed ones. The authors conclude (Karabatsos et al., 1967) that in ordinary systems where hyperconjugation (from the j8-position) is possible, the effect of non-bonded interactions accounts for only a small part (less than 10%) of the observed isotope effect. 

This can happen only if the two orbitals are in the positions shown. The situation can also be represented by this type of hyperconjugation (called negative hyperconjugation) ... 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

Cieplak apphed this effect to the explanation of the shift of selectivity in the reactions of the pentamethylcyclopentadienes 21,19, and 20 having substiments of formyl, vinyl, and (hydroxyimino)methyl moiety at the 5-positions [14]. He pointed out that the observed result is consistent with the notion that in the case of formyl moiety the hyperconjugative effect is enhanced by lone pair back-donation due to ... 

This effect has been called hyperconjugation, and has been used successfully to explain a number of otherwise unconnected phenomena. It should be emphasised that it is not suggested that a proton actually becomes free in (25) or (26), for if it moved from its original position one of the conditions necessary for delocalisation to occur would be controverted (p. 20). 

The difference between the appearance potentials of the ions from methyl and ethyl radicals indicates that hyperconjugation is less effective in stabilizing an unpaired electron than a positive charge. 

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