1. Acidity and Resonance Carboxylic acids are the most acidic common organic functional group (pKa 4–5). The carboxylate ion (RCOO–) is stabilized by resonance: charge equally delocalized over two equivalent C–O bonds (bond order 1.5). This far exceeds the stability of RO– (no resonance) or ArO– (less effective ring delocalization).

2. pKa Ladder RCOOH (4–5) > ArOH (~10) > H2O (15.7) > ROH (16–18) > RC≡CH (~25). Know all five values. Lower pKa = stronger acid = more stable conjugate base.

3. Halogen Substitution Increases Acidity Cl > no group > alkyl. Cl3CCOOH (0.65) > Cl2CHCOOH (1.26) > ClCH2COOH (2.87) > HCOOH (3.75) > CH3COOH (4.76) > (CH3)3CCOOH (5.05). Each Cl provides additional –I stabilization of the carboxylate.

4. Distance Effect The –I effect of a halogen on acidity decreases with increasing distance from –COOH: 2-chloro > 3-chloro > 4-chloro.

5. Formic Acid is Strongest Simple Carboxylic Acid H in HCOOH has no +I effect. Any alkyl group (+I) attached to the carboxyl carbon weakens the acid.

6. LiAlH4 Reduces RCOOH; NaBH4 Does Not RCOOH + LiAlH4 → RCH2OH (primary alcohol). RCOOH + NaBH4 → No reaction (NEET trap).

7. HVZ Requires Alpha-H HVZ: X2/Red P → alpha-halo acid. HCOOH (no alpha-C) and ArCOOH (sp2 alpha-C) cannot undergo HVZ.

8. Fischer Esterification is Reversible RCOOH + R'OH ⇌ RCOOR' + H2O (H2SO4, heat). K ≈ 1. Maximize yield: excess R'OH or remove H2O.

9. Saponification is Irreversible RCOOR' + NaOH → RCOONa + R'OH. Gives carboxylate salt (not free acid). Irreversible because NaOH deprotonates the acid as it forms.

10. SOCl2 for Acyl Chlorides RCOOH + SOCl2 → RCOCl + SO2↑ + HCl↑. Preferred over PCl5 (which gives liquid POCl3).