Organic Chemistry: Nature of Bonding and Stereochemistry: Hyperconjugation Part 2

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Learning Outcomes

After studying this lesson, you shall be able to:

  • Consequences and Applications of Hyperconjugation
  • Reverse hyperconjugation

Consequences and Applications of Hyperconjugation

  • In general, As the number of alkyl groups (containing hydrogen) attached to the positively charged carbon increases, the stability of carbonium ions increases because of increase in the number of contributing structures to hyperconjugation.
  • Note: This kind of hyperconjugation can also be known as isovalent hyperconjugation.
  • As there is no decrease in the number bonds in the no bond resonance forms.
  • Thus, the increasing order of stability of carbocations can be given as: methyl < primary < secondary < tertiary as depicted below:
Stability of Free Radicals
  • Stability of free radicals: The stability of free radicals is influenced by hyperconjugation just as in case of carbonium ions. One of the two σ-electrons of the α-C-H bond can be delocalized into the p-orbital of carbon containing an odd electron.
  • Due to hyperconjugation, the stability of free radicals also follows the same order as that of carbonium ions i.e.. , methyl < primary < secondary < tertiary.
  • Dipole moment & bond length: The dipole moment of the molecules is affected due to hyperconjugation since the contributing structures show considerable polarity.
  • The bond lengths are also altered due to change in the bond order during hyperconjugation. The single bond might develop partial double bond character and vice versa.
  • E. g. The experimental dipole moment of nitro methane is higher than the calculated value because of hyperconjugation.
  • The experimental C – N bond length is also less than the expected value because of same reason.
Hyperconjugation in Nitromethane
  • The same explanation can be given to decreasing of C-C bond adjacent to -C ≡ N in acetonitrile and the C-C bond adjacent to the -C ≡ C in propyne.
  • Here also, the observed dipole moments are different from their expected values due to hyperconjugation.
Acetonitrile and Propyne
  • Reactivity & orientation of electrophilic substitution on benzene ring: In case of Toluene, the reactivity of the ring towards electrophilic substitution increases and the substitution is directed at ortho and para positions to the methyl group.
  • This can be explained by hyper conjugative effect.
  • The methyl group releases electrons towards the benzene ring partly due to inductive effect and due to hyperconjugation.
  • The no bond resonance forms of toluene due to hyperconjugation are shown below.
  • From hyper conjugative structures, it can be seen clearly that the electron density on benzene ring is increased at ortho and para positions. These positions thus attract the electrophiles more than meta position. The inductive effect is comparatively weaker effect and hyperconjugation overpowers it.
Hyperconjugation in Toluene

As a proof, consider the substitution (e. g. nitration) on the following disubstituted benzene occurs ortho to the methyl group and not ortho to the tert-butyl group. This is because, although tert-butyl group has greater inductive effect than methyl, the tert-butyl group cannot exhibit hyperconjugation.

Anomeric Effect
  • Also note that the tert-butyl group is bulky and hinders the approach of electrophile.
  • Anomeric effect: The overall tendency of anomeric substituents to favour an axial position is called Anomeric effect.
  • For example, the α-methyl glucoside is more stable than the ß-methyl glucoside due to hyperconjugation.
Hyperconjugation in D-Methylglucoside
Anti Periplanar Arrangement
  • The non-bonding HOMO with a pair of electrons on the ring of α-methyl glucoside oxygen is antiperiplanar to the antibonding LUMO of C-O bond in methoxy group.
  • This arrangement of orbitals allows hyperconjugation between them and thus stabilizing the αform.
  • Whereas, in β-methyl glucoside the methoxy group is at equatorial position and cannot be involved in hyperconjugation since it is not antiperiplanar to the lone pair on ring oxygen.
  • Therefore, β-methyl glucoside is less stable than the α-methyl glucoside.

Reverse Hyperconjugation

  • For α-halo alkenes, the delocalization of electrons takes place towards the halogen group through hyper conjugative mechanism because of electron withdrawing nature of halogen.
  • It is known as reverse hyperconjugation. The dipole moments of α -halo alkenes are remarkably increased because of this phenomenon.
Reverse Hyperconjugation

MCQs

1. On increasing the number of α- hydrogens, the number of hyperconjugation structures will ________

a) Increases

b) Decreases

c) Remains same

d) None of the above

Ans: a)

2. Hyperconjugation involves delocalization of ________.

a) Sigma bond orbital

b) Pi bond orbital

c) Both sigma & pi bond orbital

d) None of the above

Ans: a)

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