Hyperconjugation

Hyperconjugation (or no-bond resonance) is the delocalization of electrons with predominantly -character bonds. An unpopulated non-bonding p or antibonding or an orbital adjacent to a sigma orbital (e.g. C–H or C–C) is hyperconjugated to provide a pair of extended molecular orbitals. Negative hyperconjugation occurs when low-lying antibonding * orbitals engage with filled lone pair character (n) orbitals. Hyperconjugation promotes electron delocalization and thus system stability. The new orbital with bonding character is stabilised, resulting in overall molecule stabilisation.  Only electrons in sigma bonds can directly stabilise an atom by donating to an orbital in another atom directly linked to it. Extended hyperconjugation (like double hyperconjugation)is also significant.

It is caused by a partial overlap between the sigma bonding orbital of the carbon atom near to the electron-deficient radical centre and the half-filled 2p orbital, which is created by the carbon atom’s sigma bonding orbital. When an electron density is donated from a filled p orbital to an empty p orbital, this is referred to as hyperconjugation. Hyperconjugation is a fundamental concept in organosilicon chemistry. This type of orbital interaction leads in the stabilisation of carbanions, carbocations, and radicals by a neighbouring silicon moiety and is a result of the electronic nature of silicon, which includes its bigger size, low-lying orbitals, and decreased electronegativity, among other characteristics. 

importance of the electronic effects

 One of the most important of these occurrences, the -silicon effect, which results in the stabilisation of vicinal carbocations in vinylsilanes and related scaffolds, is perhaps the most important of these phenomena. Hyperconjugation is a stabilising interaction that occurs as a result of the interaction of electrons in a -bond (typically C-H or C-C) with an adjacent empty or partially filled p-orbital or a -orbital to produce an extended molecular orbital that increases the stability of the system. Hyperconjugation is a stabilising interaction that occurs as a result of the interaction of electrons in a -bond (typically Hyperconjugation is the term used to describe the interaction between electrons in p systems (many bonds) and electrons in nearby s bonds (single H–C bonds) of the substituent groups in organic compounds. It has a long-lasting effect. Propene, for example, exhibits hyperconjugation.

The silicon-stabilised carbocation

This carbocation is often formed after the nucleophilic addition of organosilanes to electrophiles, and it has a silicon-stabilised structure An atom’s carbocation is sustained by hyperconjugation, more precisely, by the contribution of electron density from the Si–C bond to the carbocation p-orbital of the silicon atom Due to the fact that silicon is more electropositive than carbon, the Si–C bond is polarised in the direction of carbon, further reinforcing the stabilising action. As a matter of fact, the magnitude of this stabilisation has been calculated to be approximately 38 kcal mol1, out of which 29 kcal mol1 results from hyperconjugation and the remaining amount is assigned to inductive and field effects. 5 When compared to the -stabilisation of a methyl group (13 kcal mol1), the stabilisation of a carbocation by a -silicon atom exhibits a considerable increase in the amount of energy required. Aside from that, in the solvolysis of constrained systems, the presence of a silicon atom can result in a rate enhancement of up to 1012 times greater than that of a hydrogen atom, which is one of the largest kinetic effects ever observed (only Sn and Ge provide larger effects). 6,7 In many cases, this phenomenon is referred to simply as the “silicon effect” or the “vertical effect,” and it is important in the prediction and rationalisation of reaction outcomes for the nucleophilic addition of organosilicon reagents to C–X -systems.

Carbon-Carbon Attraction

The C(2)-C(3) bond has a double bond character, but the C(1)-C(2) bond is single-bond-characteristic. This tells us that the carbon-hydrogen link is less than a single bond in strengthThe carbon-carbon “single” bond in propylene has a strength of 1.50 A, which is consistent with the partial double bond character of the chemical compound. In doubly bonded carbon, the more the number of alkyl groups present, the greater the number of contributing structures such as, the greater the delocalization of electrons, and the more stable the alkene. The type of hyperconjugation we encountered in connection with free radicals and carbocations, on the other hand, does not require the “sacrifice” of a bond and is hence referred to as isovalent hyperconjugation. Hyperconjugation is the term used to describe the interaction between electrons in p systems (many bonds) and electrons in nearby s bonds (single H–C bonds) of the substituent groups in organic compounds. It has a long-lasting effect. Propene, for example, exhibits hyperconjugation.

Reverse hyperconjugation

When it comes to Alpha-halo alkenes, the delocalization of electrons occurs towards the halogen group through a hyperconjugative mechanism, which is caused by the halogen’s electron withdrawing characteristics. Reverse hyperconjugation is the term used to describe this process. Consequently, the dipole moments of halo alkenes are significantly amplified as a result of this phenomena.Reverse hyperconjugation is the transfer of electron density from a fully filled orbital to a neighbouring orbital in the opposite direction. It has a disruptive influence on the system.

For example, alkenes, alkyl free radicals, and so on. In the case of reverse hyperconjugation, which occurs in alpha-haloalkanes, the delocalization of electrons towards the halogen group occurs as a result of the hyperconjugation active mechanism. The dipole moment of alpha-haloalkanes is increased as a result of the reverse hyperconjugation that occurs. When filled p or d orbitals contact with adjacent antibonding p* orbitals, negative hyperconjugation occurs (as opposed to “positive” hyperconjugation, as seen in the ethyl carbocation), the result is negative hyperconjugation. For instance, the trifluoromethoxy anion and the anomeric effect are both examples of this type of phenomena

Conclusion

Hyperconjugation is a fundamental concept in organosilicon chemistry. It is a stabilising interaction that occurs as a result of the interaction of electrons in a -bond (typically C-H or C-C) with an adjacent empty or partially filled p-orbital or a -orbital. When an electron density is donated from a filled p orbital to an empty p orbital, this is referred to as hyperconjugation. Propene, for example, exhibits hyperconjugation. An atom’s carbocation is sustained by hyperconjugation, more precisely, by the contribution of electron density from the Si–C bond to the carbocation p-orbital of the silicon atom.. Hyperconjugation is the term used to describe the interaction between electrons in p systems (many bonds) and electrons in nearby s bonds (single H–C bonds) of the substituent groups in organic compounds.