Ethyl cation hyperconjugation

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. 

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or... 

Stabilization of a carbocation by hyperconjugation The electrons of an adjacent C—H bond in the ethyl cation spread into the empty p orbital. Hyperconjugation cannot occur in a methyl cation. 

Hyperconjugation occurs only if the cr bond orbital and the empty p orbital have the proper orientation. The proper orientation is easily achieved because there is free rotation about a carbon-carbon a bond (Section 2.10). In the case of the tert-butyl cation, nine C—Ho- bond orbitals can potentially overlap with the empty p orbital of the positively charged carbon. The isopropyl cation has six such orbitals, and the ethyl cation has three. Therefore, there is greater stabilization through hyperconjugation in the tertiary tert-butyl cation than in the secondary isopropyl cation and greater stabilization in the secondary isopropyl cation than in the primary ethyl cation. 

The ethyl cation is the prototype system for demonstrating the effect of hyperconjugation. Consider classical CH3CH2 as a combination of a methyl group and a -CH2 centre. The group orbitals of the methyl group (equivalent to the MOs of NH3) include the 7CcH3-orbital shown schemat-... 

Fig. 3.19 Interaction diagram illustrating the hyperconjugation interaction in the classical ethyl cation...

The combination of experimental and calculated determination of NMR chemical shifts and spin-spin coupling constants allows the characterization of the -l-cyclopropyl-2-(triisopropylsilyl)-ethyl cation (1) and an unequivocal assignment of its stereochemistry. Due to the stabilizing p-hyperconjugative interaction of both the strained cyclopropyl C-C bonds and the P-C-Si bond with the vacant orbital at Ca the rotation around the Ca-Cp and Co-Cp bonds is frozen under the experimental conditions between -150°C and -80°C. 

The effect of alkyl substiments on the stabilities of carbenium ions provides the electronic basis of the textbook Markovnikov s rule. The stabilizing effect of positive hyperconjugation increases for stronger o-donors. For example, the stabihzing effect of a silyl substituent in p-silylethyl cation is calculated to be ca. 38kcal/mol stronger than a C-H donor of the ethyl cation in the gas phase (see Section 6.3). The effects of Ge, Sn, and Hg are also substantial. For example, hyperconjugative activation by a Sn-C bond can accelerate a reaction by a factor of >10 . ... 

The increased importance of hyperconjugation in cations affects rotational barriers, albeit not in a straightforward way. Computations at HF levels with symmetry restraints suggested that the barrier for ethyl cation itself is, in fact, quite small. The rotational barrier in the ethyl radical is also very small. Although initially unexpected, this observation can be explained by different symmetry of orbital interactions involved in negative and positive hyperconjugation. With p-orbitals, the notions of syn- and antiperiplanarity disappear (Figure 6.43) and the equivalence of all periplanar conformations results in a 6-fold symmetry of the rotational profile. 

In the hyperconjugative resonance form for the ethyl cation, the double part of the double bond is not made up of 2p/2p overlap, but of 2p/sp overlap... 

FIGURE 9.26 A detailed look at hyperconjugative stabilization of the ethyl cation. 

Perhaps the most classic example of hyperconjugation, certainly invoked in all introductory organic textbooks, is the trend in stabilities observed for substituted carbocations (see Carbocation Stabilities Comparison of Theory and Experiment). Thus, as illustrated in Figure 1, the ethyl cation (a primary carbocation) is more stable than the methyl cation because a electrons associated with C-H bonds in the attached methyl group may delocalize into the empty p orbital on the cationic center. In the limit of complete delocalization, the C-H bond is broken and the pair of electrons that formerly gave rise to it is instead employed in the formation of a rr bond between the formerly cationic carbon and the former methyl carbon. Such a resonance structure (mesomer) is called a bond/no-bond structure in recognition of the detached status of the proton whose bonding electrons have been redistributed within the carbon framework. 

Another hallmark of hyperconjugation that may be identified both computationally and experimentally is the shift in IR stretching frequencies consistent with the bond weaken-ing/strengthening effects of a delocalization. So, for example, in the ethyl cation the C-H stretching frequencies are reduced and the C-C stretching frequency is increased because of the redistribution of bonding density. 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

---