Overview
This lecture covers curved arrow notation (electron pushing arrows) to show electron movement in organic reaction mechanisms, including acid-base reactions, bond cleavage (homolytic and heterolytic), nucleophilic substitution, and resonance structures.
Curved Arrow Notation Basics
- A full-headed arrow represents the movement of two electrons (an electron pair).
- A half-headed arrow ("fishhook") denotes the movement of one electron, typically in radical reactions.
- Arrows are drawn from the nucleophile (electron-rich) to the electrophile (electron-poor).
- Electron flow in arrows shows bond formation or bond breaking.
Acid-Base Reactions and Arrow Pushing
- In an acid-base reaction (e.g., acetic acid + hydroxide), hydroxide abstracts a proton, forming water and acetate ion.
- Arrows illustrate lone pair from hydroxide forming a bond with hydrogen; bond electrons between O-H return to oxygen.
Bond Cleavage: Homolytic vs. Heterolytic
- Homolytic cleavage: Bond breaks evenly, each atom gets one electron (forms radicals, shown with half-arrows).
- Example: Br2 under UV gives two Br· radicals.
- Heterolytic cleavage: One atom gets both electrons (shown with full arrow).
- Example: C–Br bond breaks, electrons go to Br (more electronegative), forming Br⁻ and a carbocation.
Electronegativity and Arrow Direction
- Electrons flow toward more electronegative atoms during bond breakage.
- C–H bond: When broken, electrons go to carbon (more electronegative than H), forming a carbanion.
Nucleophilic Substitution Example
- Hydroxide attacks methyl bromide; arrow from hydroxide to carbon forms O–C bond, C–Br bond breaks with electrons going to Br⁻.
Acidity and Resonance in Diketones
- Diketones have acidic alpha hydrogens due to resonance stabilization.
- Hydroxide deprotonates alpha hydrogen; electrons from C–H bond go to carbon, forming a resonance-stabilized carbanion.
- Resonance arrows show negative charge delocalizing onto adjacent carbonyl oxygen.
Resonance Structures and Arrow Notation
- Curved arrows indicate electron pair movement in resonance.
- Allylic carbocation: Pi electrons move to empty p orbital, shifting positive charge.
- Lone pair with negative charge can form double bond while moving pi electrons to another atom.
- Resonance hybrids depicted with dashed bonds and partial charges.
Key Terms & Definitions
- Curved Arrow Notation — Symbolic arrows showing the movement of electrons in reaction mechanisms.
- Nucleophile — Electron-rich species donating electrons.
- Electrophile — Electron-poor species accepting electrons.
- Homolytic Cleavage — Bond split giving each atom one electron (radicals formed).
- Heterolytic Cleavage — Bond split with both electrons to one atom (ions formed).
- Resonance Structure — Alternative Lewis structures showing electron delocalization.
- Radical — Atom/molecule with an unpaired electron.
- Resonance Hybrid — Combined structure representing all resonance forms.
Action Items / Next Steps
- Practice drawing curved arrow mechanisms for example reactions.
- Use arrow notation to draw resonance structures for provided ions.
- Review resonance and acid-base concepts for upcoming assessments.