Exam Master Review Sheet - Carboxylic Acids (JEE & NEET)

CHEMCA

EXAM MASTER REVIEW SHEET

Organic Chemistry: Carboxylic Acids

Comprehensive Notes for JEE Main, Advanced & NEET

1 Structure & Physical Properties

Carboxylic acids ($R-COOH$) consist of a carbonyl group attached to a hydroxyl group. The carboxyl carbon is $sp^2$ hybridized.

Boiling Point & Dimerization

Carboxylic acids have higher boiling points than aldehydes, ketones, and even alcohols of comparable molecular masses.

This is due to extensive intermolecular hydrogen bonding.
Most carboxylic acids exist as cyclic dimers in the vapor phase or in aprotic solvents (like benzene).

B.P. Order: Acid > Alcohol > Ketone > Aldehyde

Solubility

Simple aliphatic carboxylic acids up to 4 carbon atoms are miscible in water due to the formation of hydrogen bonds with water.

Solubility rapidly decreases as the number of carbon atoms increases (the hydrophobic alkyl chain dominates).

Benzoic acid is nearly insoluble in cold water.

2 Acidity & Electronic Effects

Carboxylic acids are more acidic than phenols because the carboxylate ion ($R-COO^-$) is stabilized by two equivalent resonance structures (negative charge is delocalized over two highly electronegative oxygen atoms), whereas the phenoxide ion has non-equivalent structures with negative charge on less electronegative carbons.

Acidity: $R-COOH > Ar-OH \text{ (Phenol)} > H_2O > R-OH \text{ (Alcohol)} > HC \equiv CH$

Electron Withdrawing Groups (-I, -M)

EWGs increase acidity by stabilizing the conjugate base via delocalization of negative charge.

$CF_3COOH > CCl_3COOH > CHCl_2COOH > CH_2ClCOOH > HCOOH > CH_3COOH$

Electron Releasing Groups (+I, +M)

ERGs decrease acidity by destabilizing the conjugate base (intensifying negative charge).

$HCOOH > CH_3COOH > CH_3CH_2COOH$

The Ortho Effect (JEE Advanced): Almost all ortho-substituted benzoic acids (whether EWG or ERG) are stronger acids than benzoic acid itself. This is primarily due to steric hindrance which forces the carboxyl group out of the plane of the benzene ring, preventing resonance destabilization by the ring and increasing stabilization of the anion.

3 Key Preparation Methods

Oxidation Reactions

From $1^\circ$ Alcohols / Aldehydes using strong oxidants ($KMnO_4, K_2Cr_2O_7$, Jones reagent).

$R-CH_2OH \xrightarrow{[O]} R-COOH$

From Alkylbenzenes: Entire side chain is oxidized to $-COOH$ regardless of length, provided there is at least one benzylic hydrogen.

$Ph-CH_3 \xrightarrow{KMnO_4, KOH, \Delta} Ph-COOK \xrightarrow{H_3O^+} Ph-COOH$

Hydrolysis & Grignard

From Nitriles ($R-CN$): Acidic or basic hydrolysis yields amides, then acids.

$R-CN \xrightarrow{H_3O^+} R-CONH_2 \xrightarrow{H_3O^+, \Delta} R-COOH$

From Grignard Reagent: Excellent for ascending the series (adding 1 Carbon).

$R-MgX + O=C=O \xrightarrow{\text{Dry Ether}} R-COOMgX \xrightarrow{H_3O^+} R-COOH$

4 Chemical Reactions

Reaction Type / Cleavage	Reagents	Product & Key Notes
Reaction with Metals/Alkalies (Cleavage of O-H bond)	$Na, NaOH, NaHCO_3$	Forms carboxylate salts. $NaHCO_3$ Test: Produces brisk effervescence of $CO_2$. Used to distinguish acids from phenols (most phenols don't react).
Anhydride Formation (Cleavage of C-OH bond)	$H_2SO_4 / \Delta$ or $P_2O_5 / \Delta$	Dehydration of two acid molecules yields an Acid Anhydride ($(RCO)_2O$).
Fischer Esterification	$R'-OH + \text{Conc. } H_2SO_4$	Reversible reaction yielding Esters (Fruity smell). The $-OH$ comes from the acid and the $-H$ from the alcohol.
Formation of Acid Chlorides	$PCl_5, PCl_3$ or $SOCl_2$	Yields $R-COCl$. Thionyl chloride ($SOCl_2$) is preferred because the byproducts ($SO_2, HCl$) are gases and easily escape.
Reaction with Ammonia	$NH_3$, followed by heat ($\Delta$)	Forms ammonium salt first, which on intense heating loses water to form an Amide ($R-CONH_2$).

5 Decarboxylation, Reduction & Substitution

Reductions & Decarboxylation

Reduction to Primary Alcohols

Uses $LiAlH_4$ / ether or $B_2H_6$. Important: $NaBH_4$ does NOT reduce carboxyl groups.

$R-COOH \xrightarrow{(i) LiAlH_4, \text{ ether } (ii) H_3O^+} R-CH_2OH$

Soda-Lime Decarboxylation

Heating Na/K salts with Soda-Lime ($NaOH + CaO$, 3:1) removes $CO_2$ yielding alkanes with one less carbon.

$R-COONa \xrightarrow{NaOH+CaO, \Delta} R-H + Na_2CO_3$

Kolbe's Electrolysis

Electrolysis of aqueous solution of Na/K salts yields alkanes (at Anode) with even number of carbons.

$2R-COONa \xrightarrow{\text{Electrolysis}} R-R + 2CO_2 + ...$

Hell-Volhard-Zelinsky (HVZ)

Halogenation of aliphatic acids at the $\alpha$-position. Requires at least one $\alpha$-hydrogen.

$R-CH_2-COOH \xrightarrow{\text{(i) } X_2 / \text{Red P} \text{ (ii) } H_2O} R-CH(X)-COOH$

$X = Cl, Br$

Electrophilic Aromatic Substitution

The $-COOH$ group is strongly Deactivating and Meta-Directing.

JEE Trap: Benzoic acid does NOT undergo Friedel-Crafts alkylation or acylation because the carboxyl group strongly deactivates the ring and binds with the Lewis acid catalyst ($AlCl_3$).

Nitration: Gives m-Nitrobenzoic acid.
Bromination: Gives m-Bromobenzoic acid (using $Br_2/FeBr_3$).

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Exam Master Review Sheet - Carboxylic Acids