Hyperconjugation and it's Applications
Hyperconjugation (Baker-Nathan effect)
The delocalization of σ-electrons or lone pair of electrons into adjacent π-orbital or p-orbital is called hyperconjugation. It is also known as no bond resonance as there is no bond between the α-carbon atom and one of the hydrogen atoms.It is a permanent effect and its effect is stronger than the inductive effect because, in Inductive effect there is partial delocalization of charges, but in Hyperconjugation there is total transfer or delocalization of charge.
Hyperconjugation occurs due to overlapping of σ-bonding orbital or the orbital containing a lone pair with adjacent π-orbital or p-orbital.
For a molecule to show hyperconjugation, there must be an α-C-H group or a lone pair on atom adjacent to sp2 hybrid carbon or other atoms like nitrogen, oxygen etc. Greater the number of hyperconjugative structure , greater would be the stability of the molecule.
Alkenes, alkyl carbocations, alkyl free radicals, nitro compounds with α- hydrogen shows hyperconjugation.
Types of Hyperconjugation
Hyperconjugation is of three types-1. σ (C–H), π conjugation
This type of conjugation occurs in alkenes and alkyl substituted aromatic compounds.
2. σ(C–H), positive charge(vacant p-orbital) conjugation
This type of conjugation occurs in alkyl carbocations.
3. σ(C–H), odd electron (incomplete p-orbital conjugation)
This type of conjugation occurs in alkyl free radicals.
Applications of Hyperconjugation
Stability of alkenesStability of alkenes increases with increase in the number of alkyl groups containing H-atom on the double bond due to increase in the number of hyperconjugative structures.
Stability of carbocations
CH3-CH2+ is more stable than the CH3+ because, the σ-electrons of the α-C-H bond in ethyl group are delocalized into the empty p-orbital of the positive carbon center and thus by giving rise to hyperconjugative structures. Whereas hyperconjugation is not possible in methyl carbocation and hence is less stable.The stability order of carbocations can be given as-
methyl < primary < secondary < tertiary
Stability of free radicals
Due to hyperconjugation, the stability of free radicals also follow the same order as that of carbocations i.e.-methyl < primary < secondary < tertiary.
Heat of hydrogenation
Hyperconjugation decreases the heat of hydrogenation.More hyperconjugative structure, more stable the molecule and less heat of hydrogenation.
Order of Heat of hydrogenation is-
C < D < B < A
Q. Which has more heat of hydrogenation-
CH3-CH=CH2 or CH2 = CH2Dipole moment
Since hyperconjugation affects the charges development. So, also affects the dipole moment in the molecule. The dipole moment increases, when hydrogen of formaldehyde (μ = 2.3D) is replaced by methyl group, i.e., acetaldehyde (μ = 2.7D) due to hyperconjugation, which tends to charges development.Orienting influence of alkyl group in ortho-, para-positions
Ortho-para directing property of methyl group in toluene is due to hyperconjugation.Abnormal bond lengths
In hyperconjugation a single bond acquires a double bond character and vice versa, hence abnormality is observed in bond lengths in the compounds showing hyperconjugation.For example, Ethane and ethylene, C–C and C=C bonds show normal length 1.54Ao and 1.33Ao, respectively due to no hyperconjugation in the compounds but in propene, the bond lengths are 1.47Ao and 1.35Ao for C–C and C=C bonds, respectively. This change in bond lengths is due to hyperconjugation.
Anomeric effect
In α-methyl glucoside, the non bonding HOMO with a lone pair of electrons on the ring oxygen is antiperiplanar to the antibonding LUMO of C-O bond in methoxy group. This allows hyperconjugation between them and thus by stabilizing the α-form. Whereas, in β-methyl glucoside the methoxy group is at equatorial position and cannot involve in hyperconjugation as it is not antiperiplanar to the lone pair on ring oxygen. Therefore, β-methyl glucoside is less stable than the α-methyl glucoside.Points to be Remember: Number of α hydrogen ∝ Number of hyperconjugaive structure ∝ Stability ∝ Polarity ∝ Dipole Moment ∝ (1/ Bond Length) ∝ (1/ Heat of Hydrogenation)
