chapter 5: resonance, inductive effects, molecular dipoles

What is resonance?

A way to represent electron delocalization using multiple valid Lewis structures that differ only in π bonds / lone pairs, not atom positions.

Example: Carboxylate: two resonance forms where the negative charge is shared by both oxygens.

What rule tells you whether two drawings are resonance forms vs different molecules?

Same atom connectivity (same skeleton). Only electrons move (π bonds, lone pairs, formal charges).

Example: Moving a double bond in benzene = resonance; moving an atom = different molecule.

What does a double-headed arrow (↔) mean between structures?

Resonance contributors, not an equilibrium mixture of different compounds.

Example: Two benzene Kekulé forms are not “two benzenes.”

Which electrons “participate” in resonance?

π bonds and lone pairs (in properly aligned p orbitals).

Example: Lone pair next to a π bond can delocalize (allylic lone pair situation).

What makes a resonance contributor “better” (more important)?

More complete octets, fewer formal charges, minimal charge separation, negative charge on more electronegative atoms.

Example: A contributor with full octets beats one with a carbocation missing an octet.

What is the inductive effect?

Electron donation/withdrawal through σ bonds due to electronegativity differences.
Example: CF₃–CH₂–OH has a more polarized O–H bond than CH₃–CH₂–OH.

How does inductive effect compare to resonance (strength + pathway)?

Inductive: through σ framework, generally weaker and falls off with distance. Resonance: through π system, can be stronger when conjugated.
Example: A halogen 2 carbons away affects acidity less than one directly adjacent.

What does an “electron-withdrawing group” (EWG) do inductively?

Pulls electron density, stabilizes negative charge, increases acidity.

Example: Trifluoroacetic acid is much more acidic than acetic acid.

How does distance affect inductive acidity trends?

The farther the EWG is from the acidic proton, the smaller the effect.

Example: α-halocarboxylic acid > β-halocarboxylic acid > γ-halocarboxylic acid (acidity).

What is a bond dipole?

A dipole from unequal sharing of electrons in a bond (δ⁺ to δ⁻).

Example: In C–F, carbon is δ⁺ and fluorine is δ⁻.

What is a molecular dipole?

The vector sum of all bond dipoles + lone pair contributions → overall polarity.

Example: CO₂ has polar bonds but no net dipole (linear, cancels).

How can symmetry cancel molecular dipoles?

If bond dipoles are equal and opposite, net dipole can be 0.

Example: CCl₄ is nonpolar overall.