Overview
This lecture explains curved arrow notation (electron pushing arrows) in organic chemistry, focusing on how to represent the flow of electrons in acid-base, radical, and resonance reactions.
Curved Arrow Notation Basics
- A full arrow shows the movement of two electrons (an electron pair).
- A half-arrow indicates the movement of one electron, often used in radical reactions.
- Arrows are drawn from electron-rich (nucleophile) to electron-poor (electrophile) sites.
Acid-Base Reactions Example
- In acid-base reactions, bases (like hydroxide) abstract a proton from acids (like acetic acid).
- Arrow is drawn from hydroxide's lone pair to the acidic hydrogen.
- Bond electrons from the breaking acid's O-H bond go to the oxygen atom.
- Products: water and acetate ion.
Bond Cleavage: Homolytic vs Heterolytic
- Homolytic cleavage: bond breaks evenly, each atom gets one electron (use half-arrows); produces radicals (e.g., Br₂ + UV → 2Br•).
- Heterolytic cleavage: bond breaks unevenly, both electrons go to the more electronegative atom (use full arrows).
- Example: C-Br bond breaks, electrons go to Br (Br⁻ formed); C becomes a carbocation.
Nucleophilic Substitution Example
- Hydroxide acts as a nucleophile, attacking methyl bromide's carbon.
- Arrow from hydroxide to carbon; C-Br bond electrons go to Br.
- Products: methanol and bromide ion.
Acidic Alpha Hydrogens and Resonance
- Hydrogens next to carbonyl groups (alpha hydrogens) are acidic; pKa drops if next to two carbonyls.
- Hydroxide abstracts the acidic hydrogen; C-H bond electrons go to carbon (carbanion formed).
- Negative charge on carbanion stabilizes via resonance, delocalizing to oxygen atoms.
Drawing Resonance Structures Using Arrows
- Identify high electron density (lone pairs/negative charges) and low electron density (carbocation/empty p orbital).
- Move electrons with arrows to generate resonance forms.
- Example: Allylic carbocation—double bond electrons move to empty p orbital, forming new pi bonds and shifting positive charge.
- Example: Lone pair moves to form new double bond, shifting negative charge to another atom.
Key Terms & Definitions
- Curved Arrow Notation — Symbols used to show electron flow in reaction mechanisms.
- Nucleophile — Electron-rich species donating electron pairs.
- Electrophile — Electron-poor species accepting electron pairs.
- Homolytic Cleavage — Bond breaks evenly; each atom gets one electron, forming radicals.
- Heterolytic Cleavage — Bond breaks unevenly; both electrons go to one atom.
- Resonance Structure — Different Lewis structures showing electron delocalization in a molecule.
Action Items / Next Steps
- Practice drawing curved arrow mechanisms for acid-base, nucleophilic substitution, and resonance examples.
- Complete assigned practice problems or worksheets if available.
- Review relevant organic chemistry videos on resonance and exam preparation as suggested.