Lecture Notes: Reactivity of Aldehydes and Ketones

Overview

• Focus on the reactivity of aldehydes and ketones.
• Review of the bonding in a carbonyl group.

Carbonyl Bonding

Hybridization of Carbon

• Carbonyl: carbon double bonded to oxygen.
• Example: Formaldehyde molecule.
• Hybridization State
  • Count sigma bonds and lone pairs.
  • Sigma bonds:
    • 1 sigma bond to each hydrogen.
    • 1 sigma bond in the double bond.
  • Total: 3 sigma bonds, 0 lone pairs.
  • Steric Number = 3 → 3 hybrid orbitals → sp² hybridized.
  • Draw sp² hybrid orbitals and un-hybridized p orbital.

Role of Hydrogens

• Bonded to carbonyl carbon.
• Electron in s orbital (spherically shaped).
• Overlap forms sigma bond.

Hybridization of Oxygen

• Count sigma bonds and lone pairs.
  • 1 sigma bond to carbon.
  • 2 lone pairs.
  • Steric Number = 3 → sp² hybridized.
  • Oxygen has 3 sp² hybrid orbitals.
  • Lone pairs occupy sp² orbitals.
  • Overlap with carbon forms sigma bond.
  • Un-hybridized p orbital contributes to pi bond.

Bonding Summary

• Carbon is sp² hybridized → same plane, bond angles ~120°.

Polarization of Carbonyl

• Oxygen is more electronegative than carbon → electrons pulled towards oxygen.
• Oxygen gets partial negative charge.
• Carbon gets partial positive charge.

Effect of Alkyl Groups

• Alkyl groups are electron donating.
• Stabilize partial positive charge on carbonyl carbon.
• More alkyl groups increase stabilization (like with carbocations).

Comparison: Aldehydes vs. Ketones

• Aldehydes:
  • More polarized than ketones.
  • Carbonyl carbon is more positive.
  • More reactive due to easier nucleophile attack.
• Ketones:
  • More R groups stabilize partial positive charge.
  • Less reactive than aldehydes.

Nucleophilic Addition Reaction to Carbonyl

• Geometry: Trigonal planar at carbonyl carbon.
• Reactivity
  • Nucleophile attacks partially positive carbonyl carbon.
  • Forms bond, pi electrons move to oxygen.

Intermediate Formation

• Tetrahedral Intermediate
  • Change in geometry: sp³ hybridized carbon.
  • Bond angle ~109° (from 120°).
  • Tetrahedral geometry impacts reactivity.

Factors Affecting Reactivity

Polarization

• Aldehydes more polarized, thus more reactive.

Steric Hindrance

• Bulky R groups in ketones hinder nucleophile attack.
• Affects formation of tetrahedral intermediate.
• Aldehydes face less steric hindrance, thus more reactive.