Inductive effects and Hyperconjugation Inductive effects

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 –. 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 and CH CH , behave as weak electron-donating groups in many situations: 3 2 3 Carbocation Stabilities Hyperconjugation

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

For examaple, CH + cannot be stabilized by hyperconjugation, but (CH ) CH+ can: 3 3 2 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 6 Carbon Reactive Intermediates

Chapter 4 7 Carbocation Structure

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

Chapter 4 8 Carbocation Stabilities Free Radicals

• Also electron-deficient. • Stabilized by alkyl substituents. • Order of stability: 3° > 2° > 1° > methyl

Chapter 4 10 Relative Stabilities of Alkyl Radicals 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 12 Carbenes

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

Chapter 4 13