Carboxylic Acids — Revision Notes

⚡ 30-Second Revision

Functional Group — Carboxyl group (-COOH).
General Formula — R-COOH.
Acidity Order — Carboxylic Acids > Phenols > Alcohols.
Key Acidity Factor — Resonance stabilization of carboxylate anion (R-COO$^-$).
Preparation — Oxidation of R-CH\_2OH or R-CHO (KMnO\_4, K\_2Cr\_2O\_7); Hydrolysis of R-CN or R-CONH\_2; R-MgX + CO\_2 $\xrightarrow{\text{H}^+}$ R-COOH; Oxidation of alkylbenzenes.
Reactions (O-H bond cleavage) — Acidic nature, reacts with Na, NaOH, NaHCO\_3 (effervescence).
Reactions (C-OH bond cleavage)

- Esterification: R-COOH + R'-OH $\xrightarrow{\text{H}^+}$ R-COOR' + H\_2O. - Acyl Halide: R-COOH + SOCl\_2 $\longrightarrow$ R-COCl + SO\_2 + HCl. - Anhydride: 2R-COOH $\xrightarrow{\text{P\_2O\_5, } \Delta}$ (RCO)\_2O + H\_2O. - Amide: R-COOH + NH\_3 $\xrightarrow{\Delta}$ R-CONH\_2 + H\_2O.

Reactions (Carboxyl group as a whole)

- Reduction: R-COOH $\xrightarrow{\text{1. LiAlH\_4, 2. H\_3O}^+}$ R-CH\_2OH. - Decarboxylation: R-COOH $\xrightarrow{\text{NaOH + CaO, } \Delta}$ R-H + Na\_2CO\_3 (or easy for $\beta$-keto acids).

Reactions ($\alpha$-hydrogen)

- HVZ Reaction: R-CH\_2-COOH $\xrightarrow{\text{X\_2/Red P}}$ R-CH(X)-COOH.

2-Minute Revision

Carboxylic acids, characterized by the -COOH group, are stronger acids than alcohols and phenols due to the resonance stabilization of their carboxylate anion. This stability allows them to react with bases like NaOH and even weaker bases like NaHCO\_3, producing CO\_2 gas (a key distinguishing test).

They have high boiling points due to strong intermolecular hydrogen bonding, forming dimers. Key preparation methods include the oxidation of primary alcohols and aldehydes, hydrolysis of nitriles and amides, and the reaction of Grignard reagents with carbon dioxide, which is useful for increasing the carbon chain by one carbon.

Important reactions include esterification (forming esters with alcohols), conversion to acyl halides (with SOCl\_2, PCl\_3, PCl\_5), and reduction to primary alcohols using strong reducing agents like LiAlH\_4.

Decarboxylation, the removal of CO\_2, occurs readily for $\beta$-keto acids and malonic acid derivatives, and with soda lime for simple acids. The Hell-Volhard-Zelinsky (HVZ) reaction is specific for $\alpha$-halogenation of carboxylic acids possessing an $\alpha$-hydrogen.

Understanding these reactions, their reagents, and the factors affecting acidity is crucial for NEET.

5-Minute Revision

Carboxylic acids are defined by the carboxyl group (-COOH), a combination of a carbonyl and a hydroxyl group. Their $sp^2$ hybridized carboxyl carbon leads to a planar structure. They are significantly more acidic than alcohols and phenols because the carboxylate anion (R-COO$^-$) formed after proton donation is highly stabilized by resonance, delocalizing the negative charge over two electronegative oxygen atoms.

Electron-withdrawing groups increase acidity, while electron-donating groups decrease it. For example, chloroacetic acid is more acidic than acetic acid due to the inductive effect of chlorine. Carboxylic acids exhibit high boiling points due to strong intermolecular hydrogen bonding, existing as stable dimers.

Lower members are water-soluble.

Preparation Methods:

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Oxidation — Primary alcohols (R-CH\_2OH) and aldehydes (R-CHO) are oxidized to carboxylic acids using strong agents like KMnO\_4 or K\_2Cr\_2O\_7.
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Nitriles/Amides Hydrolysis — R-CN or R-CONH\_2 can be hydrolyzed under acidic or basic conditions to R-COOH.
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Grignard Reagent — R-MgX reacts with CO\_2 (dry ice) followed by acid hydrolysis to give R-COOH, adding one carbon atom.

*Example: CH\_3MgBr + CO\_2 $\xrightarrow{\text{H}^+/H\_2O}$ CH\_3COOH*

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Alkylbenzenes Oxidation — Alkylbenzenes with at least one benzylic hydrogen are oxidized to benzoic acid by hot alkaline KMnO\_4.

Key Reactions:

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Acidity — React with active metals (Na), strong bases (NaOH), and carbonates/bicarbonates (NaHCO\_3, giving CO\_2 effervescence).
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Esterification — R-COOH + R'-OH $\xrightarrow{\text{H}^+}$ R-COOR' + H\_2O (reversible).
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Formation of Acyl Halides — R-COOH + SOCl\_2 $\longrightarrow$ R-COCl + SO\_2 + HCl (preferred due to gaseous byproducts).
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Formation of Anhydrides — 2R-COOH $\xrightarrow{\text{P\_2O\_5, } \Delta}$ (RCO)\_2O + H\_2O.
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Formation of Amides — R-COOH + NH\_3 $\longrightarrow$ R-COONH\_4 $\xrightarrow{\Delta}$ R-CONH\_2 + H\_2O.
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Reduction — R-COOH $\xrightarrow{\text{1. LiAlH\_4, 2. H\_3O}^+}$ R-CH\_2OH (requires strong reducing agent).
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Decarboxylation — Removal of CO\_2. Easy for $\beta$-keto acids and malonic acid derivatives. Simple acids require soda lime (NaOH + CaO) and heat.
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Hell-Volhard-Zelinsky (HVZ) Reaction — $\alpha$-halogenation of carboxylic acids with $\alpha$-hydrogens using X\_2/Red P. *Example: CH\_3COOH $\xrightarrow{\text{Cl\_2/Red P}}$ ClCH\_2COOH*.

Remember the reactivity order of derivatives: Acyl halides > Anhydrides > Esters > Amides. Focus on distinguishing tests and the specific reagents for each transformation.

Prelims Revision Notes

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Carboxylic Acid Structure — Contains -COOH group. Carbonyl carbon is $sp^2$ hybridized, planar geometry.
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Nomenclature — IUPAC: alkane-oic acid (e.g., ethanoic acid). Common: formic, acetic, propionic, butyric. $\alpha, \beta, \gamma$ for common names. Carboxyl carbon is C1 in IUPAC.
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Acidity

* Order: Carboxylic Acids (pKa 4-5) > Phenols (pKa ~10) > Alcohols (pKa 16-18). * Reason: Resonance stabilization of carboxylate anion (R-COO$^-$) is superior (charge on two O atoms). * Factors: Electron-withdrawing groups (EWGs, e.

g., -NO\_2, -Cl, -F) increase acidity (stabilize anion). Electron-donating groups (EDGs, e.g., -CH\_3, -CH\_2CH\_3) decrease acidity (destabilize anion). Effect decreases with distance. Number of EWGs increases acidity (e.

g., CCl\_3COOH > CHCl\_2COOH > CH\_2ClCOOH).

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Physical Properties

* Boiling Point: Very high due to strong intermolecular hydrogen bonding, forming stable dimers. Higher than alcohols of comparable mass. * Solubility: Lower members (up to C4) are water-soluble due to H-bonding with water. Solubility decreases with increasing carbon chain length.

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Preparation Methods

* Oxidation: Primary alcohols (R-CH\_2OH) and aldehydes (R-CHO) with strong oxidizing agents (KMnO\_4/H$^+$/$\Delta$, K\_2Cr\_2O\_7/H$^+$/$\Delta$, CrO\_3/H\_2SO\_4). * Alkylbenzenes: Ar-R $\xrightarrow{\text{KMnO\_4/OH}^-, \Delta}$ Ar-COOH (requires benzylic H). * Nitriles/Amides: R-CN or R-CONH\_2 $\xrightarrow{\text{H}^+/H\_2O \text{ or } OH^-/H\_2O}$ R-COOH. * Grignard Reagent: R-MgX + CO\_2 (dry ice) $\xrightarrow{\text{H}^+/H\_2O}$ R-COOH (adds one carbon).

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Chemical Reactions

* Acidic Reactions: Reacts with Na, NaOH, NaHCO\_3 (brisk effervescence of CO\_2). * Esterification: R-COOH + R'-OH $\xrightarrow{\text{conc. H\_2SO\_4}}$ R-COOR' + H\_2O (reversible). * Acyl Halide Formation: R-COOH + SOCl\_2 $\longrightarrow$ R-COCl + SO\_2 + HCl (best method).

Also PCl\_3, PCl\_5. * Anhydride Formation: 2R-COOH $\xrightarrow{\text{P\_2O\_5, } \Delta}$ (RCO)\_2O + H\_2O. * Amide Formation: R-COOH + NH\_3 $\longrightarrow$ R-COONH\_4 $\xrightarrow{\Delta}$ R-CONH\_2 + H\_2O.

* Reduction: R-COOH $\xrightarrow{\text{1. LiAlH\_4, 2. H\_3O}^+}$ R-CH\_2OH (strong reducing agent). Diborane (B\_2H\_6) also works. * Decarboxylation: R-COOH $\xrightarrow{\text{NaOH + CaO, } \Delta}$ R-H + Na\_2CO\_3.

Easy for $\beta$-keto acids and malonic acid derivatives on heating. * HVZ Reaction: R-CH\_2-COOH $\xrightarrow{\text{X\_2/Red P}}$ R-CH(X)-COOH (requires $\alpha$-H).

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Reactivity of Derivatives — Acyl Halides > Anhydrides > Esters > Amides (towards nucleophilic acyl substitution).
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Distinguishing Tests — NaHCO\_3 test (effervescence) for carboxylic acids vs. phenols. FeCl\_3 test for phenols (coloration) vs. carboxylic acids (no color).

Vyyuha Quick Recall

To remember the acidity order: Carboxylic Acids Prefer Alcohols (C > P > A).

For reactions forming derivatives: Acids Eat Anhydrides, Acyl Halides, Amides, Esters (Acyl Halides, Anhydrides, Amides, Esters are derivatives).

For HVZ reaction: Halogenate Very Zealously at Alpha ($\alpha$) position (requires $\alpha$-H).