Chapter 3 — Key Takeaways

Nine bullets for the master framework.

1. The Brønsted picture of acid-base chemistry is proton transfer. An acid donates $H^+$; a base accepts $H^+$. A full reaction involves two conjugate acid-base pairs.

2. $pK_a$ quantifies acidity. $pK_a = -\log_{10} K_a$. Smaller = stronger acid. Memorize the anchor values: $HCl \approx -7$, $H_3O^+ = -1.7$, $HF = 3.2$, carboxylic acids ~5, ammonium ~10, water 15.7, alcohols 16, terminal alkynes 25, ammonia 38, alkanes 50.

3. $pK_a$ is determined by conjugate-base stability. A more stable conjugate base = a stronger acid. This is the single most important idea in the chapter.

4. The ARIO factors predict $pK_a$ differences: - Atom: within a row, electronegativity dominates; within a column, size (polarizability) dominates. - Resonance: worth ~10–11 $pK_a$ units per additional delocalization. - Induction: ~2 $pK_a$ units per α-electronegative substituent, falling off rapidly with distance. - Orbital (hybridization): higher $s$ character stabilizes negative charge ($sp > sp^2 > sp^3$); ~6 $pK_a$ units per step.

5. Equilibrium direction follows the weaker acid. For $HA + B \rightleftharpoons A^- + HB^+$, if $pK_a(HB^+) > pK_a(HA)$, the forward reaction is favored. $K_{eq} = 10^{\Delta pK_a}$.

6. Choose a base whose $pK_{aH}$ is ≥ 4 units above the substrate's $pK_a$ to ensure quantitative deprotonation. Carboxylic acids (any base with $pK_{aH}$ ≥ 9). Alcohols (NaH). Ketone α-carbons (LDA). Terminal alkynes (NaNH₂ or BuLi). Alkanes (nothing standard).

7. Lewis acid-base theory generalizes Brønsted. Any electron-pair donor is a Lewis base; any electron-pair acceptor is a Lewis acid. Every mechanism arrow in organic chemistry starts at a Lewis base and ends at a Lewis acid.

8. $pK_a$ predicts nucleophilicity and leaving-group ability: - Nucleophilicity: higher $pK_{aH}$ → better nucleophile (first approximation, within a family). - Leaving-group ability: lower $pK_a$ of the conjugate acid → better leaving group. Halides (conjugate acids $pK_a < 0$) are good. Hydroxide/alkoxide (conjugate acids 16–18) are very bad.

9. $pK_a$ applies to biology. Enzyme catalysis, drug absorption, membrane crossing, protein folding — all depend on protonation states at physiological pH. The same framework.

The habit to leave with: Every time you see a new reaction, ask yourself:

• Which bond is breaking? Which is forming?
• Is there a proton transfer? If so, what are the $pK_a$ values?
• What is the leaving group? What is its $pK_a$?
• Is there an equilibrium? Which side is the weaker-acid side?

The mechanism will often answer itself.

Chapter 4 next — functional groups and nomenclature. A vocabulary chapter; short, but necessary for the rest of the book. Keep your $pK_a$ table handy.