Inductive effects and Hyperconjugation

Inductive effects are electronic effects that occur through sigma σ bonds caused by electronegativity differences of atoms.

Electron donor groups (Z) stabilize a (+) charge; Z→Y+. Electron-withdrawing groups

(W) stabilize a (–) charge; W←Y –.

Inductive effects

p208a

The following is a partial list of inductive electron-withdrawing groups:

In contrast, only a few groups are electron-donating relative to hydrogen because of their inductive effects. Two of these are electron rich because of their negatively charged oxygen atoms. In addition, alkyl groups, such as CH

3 and CH

2CH

3, behave as weak

electron-donating groups in many situations:

Carbocation Stabilities

Hyperconjugation

A second explanation for the observed trend in carbocation stability is based on orbital overlap. A 3° carbocation is more stable than a 2°, 1°, or methyl carbocation because the positive charge is delocalized over more than one atom.

Spreading out charge by the overlap of an empty p orbital with an adjacent σ bond is called hyperconjugation.

For examaple, CH3+ cannot be stabilized by hyperconjugation, but (CH3)2CH+ can:

Chapter 4 6

Carbocation Stability (Continued)

•  Stabilized by alkyl substituents in two ways:

1. Inductive effect: Donation of electron density along the sigma bonds.

2. Hyperconjugation: Overlap of sigma bonding orbitals with empty p orbital.

Chapter 4 7

Carbon Reactive Intermediates

Chapter 4 8

Carbocation Structure

•  Carbon has 6 electrons, positively charged. •  Carbon is sp2 hybridized with vacant p orbital.

Carbocation Stabilities

Chapter 4 10

Free Radicals

•  Also electron-deficient. •  Stabilized by alkyl substituents. •  Order of stability:

3° > 2° > 1° > methyl

Relative Stabilities of Alkyl Radicals

Chapter 4 12

Carbanions

•  Eight electrons on carbon: 6 bonding plus one lone pair.

•  Carbon has a negative charge.

•  Destabilized by alkyl substituents.

•  Methyl >1° > 2 ° > 3 °

Chapter 4 13

Carbenes

•  Carbon is neutral. •  Vacant p orbital, so can be electrophilic. •  Lone pair of electrons, so can be nucleophilic.