subject: Hyperconjugation [print this page] Introduction to Hyperconjugation: Introduction to Hyperconjugation:
Hyperconjugation is also called as " No bond resonance" or " Baker Nathan Effect" or " resonance due to delocalisation of bonds". Hyperconjugation is called no bond resonance as the canonical forms of the resonating structures contain no bond between carbon and hydrogen. Hyperconjugation is called Baker Nathan Effect after the scientist Baker and Nathan explained the difference between conjugation and Hyperconjugation.
Required Conditions for Hyperconjugation
To exhibit Hyperconjugation
The alpha carbon should be attached to atleast one SP2 hybrid of alkene, carbocation or free radical.
The alpha carbon should possess atleast one hydrogen.
Hyperconjugation in Free Radical
Similarly we can draw hyperconjugative structures for carbonium ions.
Number of hyperconjugative structures will be equal to number of alpha hydrogens + 1.
The resulting structures suggest that there is some ionic character between carbon and hydrogen bond and C = C acquires single bond character.
More substituted (alkylated) alkenes are more stable (on the basis of Saytzeff rule) as more hyperconjugative structures are possible. The stability of alkene or carbon free radical or carbonium ion is directly proportional to number of hyperconjugative structures.
stability of alkene is inversely proportional to hydrogenation enthalpy.
Effect of hyperconjugation on chemical properties:
1) Bond length
For the shortening of sigma bond (s bonds) hyperconjugation is considered as a key factor. For example, the single CC bonds in 1,3-butadiene and methylacetylene are approximately 1.46 angstrom in length, which is much less than the length found in saturated hydrocarbons(1.54). This is mainly due to the partial double bond character given by hyperconjugation.
2) Dipole moment
In 1, 1, 1-trichloroethane there is large increase in dipole movement as compared with chloroform, this increase can be attributed to hyperconjugated structures.
3) Stability of carbocations:
(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+
The CC s bond adjacent to the cation is free to rotate,as it rotates in turn the three CH s bonds of the methyl group in undergoes the stabilization interaction.The large hyperconjugation stabilization is due to more adjacent C-H bonds.
4) Heat of formation
The heat of formation of such molecules exceeds the sum of their bond energies while and the heats of hydrogenation per double bond are less compared to the heat of hydrogenation of ethylene.
Applications of Hyper Conjugation
1. Hyper conjugation explains the stability of certain alkenes over the other.
Example:The stability of few alkenes is as follows
The hydrogenation energy is opposite to this order.Bond length due to sigma-pie resonance gets affected.Therefore the single bond length is less than expected and double bond length is more than expected.C-H bond length is longer than expected.
2.The stability of free radicals increase with increase in number of alpha hydrogens.
