Nature of Bonding and Stereochemistry: Hyper-Conjugation Part 3

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Learning Outcomes

After studying this lesson, you shall be able to:

  • Know about the structural requirements in a molecule to show hyperconjugation.
  • Consequences and Applications of Hyperconjugation

Requirements for Hyperconjugation

Requirements for Hyperconjugation

Notice that in the above hyper conjugative structures: (II) , (III) & (IV) of propene, the bond between an α-carbon and one of the hydrogen atom is absent. Thus, the hyperconjugation is also called as “no bond resonance” .

  • These equivalent contributing structures i.e.. , (II) , (III) & (IV) are also polar in nature and hence are responsible for the dipole moment of propene (0.36 D) . The bond length of C-C in propene is equal to 1.48. The range is 1.54 (of C-C) -1.34 (of C = C) .
  • Thus, due to hyperconjugation, the approximate bond order of C-C is 1.5.
  • This kind of hyperconjugation is also known as sacrificial hyperconjugation as one bond is missing.
  • Hyperconjugation is also exhibited by carbocations attached to α-carbon having hydrogen atom and also by free radicals attached to α-carbon having hydrogen atom. Carbanions, however, do not exhibit hyperconjugation.

Consequences and Applications of Hyperconjugation

Stability of Alkenes

  • As a general rule, greater the number of alkyl groups (containing hydrogens) attached on the double bond greater is the stability of alkene.
  • This is because of increase in number of contributing no bond resonance structures.
  • Therefore, 2-butene is more stable than 1-butene.
  • For example, 2-butene is more stable than 1-butene. This is because in 2-butene, there are six hydrogens involved in hyperconjugation whereas there are only two hydrogens involved in case of 1-butene.
  • Hence the contributing structures in 2-butene are more and is more stable than 1-butene.
Stability of Alkenes

With the increase in the number of methyl groups on the double bonds, the stability of alkenes increases as shown below.

Stability of Alkenes

The above order has been justified on the basis of heat of hydrogenation data of the given alkenes. With rise in the stability of alkene, the value of heat of hydrogenation deceases. For more substituted alkenes, heat of formation is higher than expected.

It must be kept in mind that the important condition for hyperconjugation to take place in alkenes is that the alkyl group attached to the double bond must have minimum one hydrogen atom.

Stability of Alkenes

For example, in case of the following alkene containing a tert-butyl group on doubly bonded carbon, the hyperconjugation is not possible.

  • The effect of hyperconjugation is stronger than the inductive effect. It is because, in inductive effect there is partial delocalization of charges, but in hyperconjugation there is total transfer/delocalization of charge.
  • For example, considering inductive effect one may conclude that 1-butene is more stable than propene as the positive inductive effect of ethyl group is stronger than that of methyl group. But this is not correct; propene is more stable than 1-butene.
  • This is due to the fact that there are three hydrogens on a-methyl group involved in hyperconjugation. However, in 1-butene only two hydrogen atoms are there on -CH2 group that can take part in hyperconjugation
Stability of Alkenes

Stability of Carbocations (Carbonium Ions)

  • The ethyl carbocation, CH3-CH2 + is more stable than the methyl carbocation, CH3 + .
  • This is 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 ‘no bond resonance structures’ as shown below.
  • But, hyperconjugation is not possible in methyl carbocation and hence is less stable.
Stability of Carbocations (Carbonium Ions)

MCQs

1. Which of the following is known as Baker- Nathan effect?

a) Mesmeric effect

b) Inductive effect

c) Hyperconjugation effect

d) Electrometric effect

Ans: c)

2. Larger the number of hyperconjugation structures, the stability of free radicals will ________

a) Increases

b) Decreases

c) Remains same

d) None of the above

Ans: a)

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