📜 Carboxylic Acids & Acyl Derivatives

Carboxylic acids and their acyl derivatives ($R-COX$) undergo Nucleophilic Acyl Substitution ($S_N\text{Ac}$) through tetrahedral intermediates. Essential competitive exam topics include acyl derivative reactivity hierarchies, Hell-Volhard-Zelinsky (HVZ) $\alpha$-halogenation, ester hydrolysis mechanisms ($A_{AC}2,B_{AC}2$), and acidic strength rankings.

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1. ⚡ Relative Reactivity of Acyl Derivatives ($S_N\text{Ac}$)

$$\text{Mechanism: }R-CO-L+N{u}^{-}\rightleftharpoons [R-C(O^{-})(Nu)(L){]}_{\text{Tetrahedral intermediate}}\stackrel{-{\text{L}}^{-}}{\to }R-CO-Nu$$$$\text{Reactivity Order: }\text{Acyl Chloride }\mathbf{(}\mathbf{-}\mathbf{Cl}\mathbf{)}\mathbf{>}\text{Acid Anhydride }\mathbf{(}\mathbf{-}\mathbf{OCOR}\mathbf{)}\mathbf{>}\text{Ester }\mathbf{(}\mathbf{-}{\mathbf{OR}}^{\prime }\mathbf{)}\mathbf{>}\text{Amide }\mathbf{(}\mathbf{-}{\mathbf{NH}}_{\mathbf{2}}\mathbf{)}$$

Physical Rationale:

1. Leaving Group Ability: $C{l}^{-}>RCO{O}^{-}>R^{\prime }{O}^{-}>N{H}_2^{-}$ (weaker conjugate base is a better leaving group).
2. Resonance Stabilization in Ground State: Amide lone pair is strongly delocalized into carbonyl ($>N^{+}=C-O^{-}$), providing huge resonance stability and making the carbonyl carbon less electrophilic.

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2. 🧪 Hell-Volhard-Zelinsky (HVZ) Reaction

Selective halogenation at the $\alpha$-position of carboxylic acids containing $\alpha$-hydrogens:

$$R-C{H}_2-COOH+X_2\stackrel{\text{Red }P/\mathrm{\Delta }}{\to }R-CH(X)-COOH+HX(X=Cl,Br)$$

• Role of Red Phosphorus: Converts small amount of acid into acyl halide ($R-C{H}_2-COCl$), which enolizes rapidly into reactive enol form.
• $\alpha$-Bromo acids on treatment with aqueous $KOH$ yield $\alpha$-hydroxy acids; with excess $N{H}_3$ yield $\alpha$-amino acids.

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3. 🔥 Decarboxylation Reactions

1. Soda Lime Decarboxylation ($NaOH+CaO,3:1$ ratio, $\mathrm{\Delta }$):$$R-COONa+NaOH(CaO)\stackrel{\mathrm{\Delta }}{\to }\mathbf{R}\mathbf{-}\mathbf{H}\mathbf{+}{\mathbf{Na}}_{\mathbf{2}}{\mathbf{CO}}_{\mathbf{3}}$$
   • Reaction proceeds via Carbanion intermediate ($R^{-}$). Rate $\propto$ stability of carbanion.
2. $\beta$-Keto Acid Decarboxylation:
   • Carboxylic acids with a carbonyl group at the $\beta$-position ($R-CO-C{H}_2-COOH$) undergo extremely rapid thermal decarboxylation via a 6-membered cyclic transition state upon mild warming ($50-{100}^{\circ }\text{C}$):$$R-CO-C{H}_2-COOH\stackrel{\mathrm{\Delta }}{\to }R-CO-C{H}_3+C{O}_2\uparrow$$