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please explain hyperconjugation in detail

9 years ago

Answers : (1)

Vikas TU
12283 Points
							

In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbitalor filled π orbital, to give an extended molecular orbital that increases the stability of the system. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by hyperconjugation.

Hyperconjugation has an effect on several properties.

  1. Bond length: Hyperconjugation is suggested as a key factor in shortening of sigma bonds (σ bonds). For example, the single C–C bonds in 1,3-butadiene and methylacetylene are approximately 1.46 angstrom in length, much less than the value of around 1.54 Å found in saturated hydrocarbons. This is due mainly to hyperconjugation that gives partial double-bond character of the bond.
  2. Dipole moments: The large increase in dipole moment of 1,1,1-trichloroethane as compared with chloroform can be attributed to hyperconjugated structures.
  3. The heat of formation of molecules with hyperconjugation are greater than sum of their bond energies and the heats of hydrogenation per double bond are less than the heat of hydrogenation of ethylene.
  4. Stability of carbocations:

(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+
The C–C σ bond adjacent to the cation is free to rotate, and as it does so, the three C–H σ bonds of the methyl group in turn undergoes the stabilization interaction. The more adjacent C-H bonds are, the larger hyperconjugation stabilization is.

 

9 years ago
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