Overview
This lecture covers the concepts of electronegativity, polarity, dipoles, resonance structures, and arrow pushing in organic chemistry, highlighting how electron distribution influences molecular properties and reactivity.
Electronegativity and Polarity
- Electronegativity is an atom’s tendency to attract electrons in a bond compared to other atoms.
- Linus Pauling created a scale ranking elements by electronegativity, with fluorine being the most electronegative.
- A large electronegativity difference creates ionic bonds; a small difference creates nonpolar covalent bonds; intermediate differences create polar covalent bonds.
- In water, oxygen’s higher electronegativity creates a polar covalent bond with hydrogen, resulting in partial charges (δ+ and δ–).
- Water’s bent shape makes it a polar molecule with a molecular dipole (separation of charges).
- Carbon dioxide has polar bonds, but its linear, symmetrical shape means the molecule itself is nonpolar.
Dipoles and Molecular Structure
- A dipole is a molecule with two regions of opposite charge due to unequal electron sharing.
- The direction of a dipole is shown with an arrow: head toward negative, cross toward positive.
- Molecules like 1-chloropropane have both polar and nonpolar regions, making them slightly polar.
Resonance Structures and Hybrids
- Resonance structures are different valid ways to draw the arrangement of electrons (not atoms) in a molecule.
- Resonance is indicated with a double-headed arrow between structures.
- The real molecule is a resonance hybrid, a weighted average of all resonance forms, more stable than any individual structure.
- Bond lengths in resonance hybrids are intermediate between single and double bonds.
- Electrons involved in resonance are shared across three adjacent atoms, often involving pi bonds and lone pairs.
Arrow Pushing (Electron Movement)
- Arrow pushing uses curved arrows to represent the movement of electron pairs during formation of resonance structures.
- Arrows start from electron pairs and point to where the electrons move.
- Only electrons in pi bonds or lone pairs move; single bond electrons do not.
- All atoms must obey the octet rule and valence limitations during electron pushing.
Resonance Form Preferences
- Neutral resonance forms are preferred over those with charges.
- Oxygen and nitrogen should maintain full octets, while carbon can have fewer electrons if needed.
- Negative charges are more stable on more electronegative atoms and positive charges on less electronegative atoms.
Key Terms & Definitions
- Electronegativity — Tendency of an atom to attract electrons in a bond.
- Dipole — Separation of charge across a molecule due to uneven electron sharing.
- Resonance Structure — Different possible electron arrangements in the same molecule.
- Resonance Hybrid — Actual molecule, which is a blend of all resonance structures.
- Arrow Pushing — Technique for showing electron movement using curved arrows.
Action Items / Next Steps
- Practice drawing and identifying resonance structures in molecules.
- Prepare for the next lecture on acid-base chemistry and resonance’s role in acid strength.