Hyperconjugation in carbocations

Block localized wavefunction (BLW) method s The electron delocalization to the cationic carbon and neutral boron center can be estimated by removing the vacant p-orbitals from the expansion space of molecular orbitals. Although this simple orbital deletion procedure (ODP) technique is limited to the analysis of positive hyperconjugation in carbocations and boranes, it has been generalized and extended to the block localized wavefunction (BLW) method. ... 

Bridging and hypercoordination in transition-metal complexes Hydrido-bridged carbocations 118 Carbon hypercoordinated carbocations 123 Hyperconjugation in carbocations 146 References 158... 

Draw a skeletal structure of the following carbocation. Identify it as primary, secondary, or tertiary, and identify the hydrogen atoms that have the proper orienta- tion for hyperconjugation in the conformation shown. 

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

This KIE was also attributed to hyperconjugation. The authors suggested that smaller secondary deuterium KIEs were found in radical reactions because hyperconjugation was less important in radicals than in carbocations. 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

Summary 1-Ary 1-2-trialkyIsilyl-substituted vinyl cations are characterized in solution by NMR spectroscopy. The NMR chemical shift data reveal the stabilization of the positive charge by a (5-silyl substituent. The order of hyperconjugative stabilization of a positive charge by P-substituents is H < alkyl < silyl. The P-silyl effect is dependent on the electron demand of the carbocation and decreases with better electron donating a-substituents. NMR spectroscopy is a suitable tool to investigate the competition between 7i-resonance and CT-hyperconjugation in these type of carbocations. 

The electron-donating groups attached to positively charged carbons in carbocations increase the stability of the carbocations by inductive effect and/or hyperconjugation (no bond resonance). Thus, a tertiary carbocation is more stable than a secondary carbocation, which in turn is more stable than a primary carbocation. 

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