chapter 6: acids/bases + mechanisms + curved arrows

Define Brønsted–Lowry acid and base.

Brønsted–Lowry Acid = proton donor. Brønsted–Lowry Base = proton acceptor.
Example: NH₃ accepts H⁺ → NH₄⁺ (base).

Define Lewis acid and Lewis base.

Lewis acid = electron pair acceptor. Lewis base = electron pair donor.
Example: BF₃ (acid) + THF (base) → BF₃·THF adduct.

What is pKₐ and what does it measure?

pKₐ = −logKₐ, measures acid strength. Lower pKₐ = stronger acid.

Example: pKₐ −3 is stronger than pKₐ 5.

How do you predict direction of acid–base equilibrium?

Equilibrium favors the side with the weaker acid (higher pKₐ).

Example: Acetic acid (pKₐ ~5) + NH₃ ⇌ acetate + NH₄⁺ (pKₐ ~10) → favors products.

Periodic trends (acidity across a row): CH₄ vs NH₃ vs H₂O vs HF

Acidity increases left → right (more electronegative atom stabilizes negative charge):

CH₄ (pKₐ ~50) < NH₃ (~40) < H₂O (~15) < HF (~5).

Example: HF is much stronger acid than methane.

Periodic trends (acidity down a column): HF vs HCl vs HBr vs HI

Acidity increases down group because H–X bond becomes weaker:
HF < HCl < HBr < HI.
Example: HCl (pKₐ ~ −7) stronger than HF (pKₐ ~5).

Halide basicity trend: F⁻, Cl⁻, Br⁻, I⁻

F⁻ is the strongest base (least stable anion), then Cl⁻, Br⁻, I⁻.

Example: F⁻ grabs H⁺ more strongly than I⁻.

Four big factors controlling acid/base strength:

(1) Charge: More positive acid / less negative conjugate base = stronger acid.

(2) Electronic effects (resonance/inductive): Resonance or electron-withdrawing groups stabilize the conjugate base.

(3) Hybridization: More s-character (sp > sp² > sp³) = stronger acid.

(4) Periodic table position: Across → more electronegative; Down → larger atom = stronger acid.

Example: A resonance-stabilized conjugate base → stronger acid.

How does resonance affect acidity?

Resonance stabilizes the conjugate base, increasing acidity.

Example: Carboxylic acids are more acidic than alcohols (carboxylate resonance).

How does inductive effect affect acidity?

Electron-withdrawing inductive effects stabilize conjugate base → stronger acid.

Example: CF₃CH₂OH is more acidic than CH₃CH₂OH (stronger −I effect).

What is a reaction mechanism (vs a reaction equation)?

Equation = what happens; mechanism = how it happens (step-by-step electron flow).

Example: Showing lone pair attack + bond breaking with arrows.

Curved arrow formalism: where do arrows start and end?

Start at electron source (lone pair or bond), end at electron sink (atom/bond).

Example: Lone pair on N → H in an acid–base step.

What does a full curved arrow (2 electrons) represent?

Movement of an electron pair.

Example: N lone pair attacking H⁺.

What’s a Lewis adduct formation example you need to recognize?

BF₃ + THF → BF₃·THF (coordination bond).

Example: THF donates a lone pair to boron.