Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Boiling Points: — High due to cyclic dimerization via two H-bonds. $R-COOH cdots O=C(R)-OH cdots O=C(R)-OH$
Solubility: — Lower members soluble (H-bonding with $H_2O$), higher members insoluble.
Acidity: — $R-COOH \rightleftharpoons R-COO^- + H^+$. Carboxylate anion resonance stabilized.
Acidity Order: — Mineral acids > Carboxylic acids > Carbonic acid > Phenols > Alcohols.
Effect of Substituents: — EWG increase acidity (stabilize $R-COO^-$), EDG decrease acidity (destabilize $R-COO^-$).
$NaHCO_3$ Test: — $R-COOH + NaHCO_3 \rightarrow R-COONa + H_2O + CO_2 \uparrow$ (effervescence).
Esterification: — $R-COOH + R'-OH \xrightarrow{H^+} R-COO-R' + H_2O$
Acyl Chloride: — $R-COOH + SOCl_2 \rightarrow R-COCl + SO_2 + HCl$
Reduction: — $R-COOH \xrightarrow{1. LiAlH_4, 2. H_3O^+} R-CH_2OH$ (not $NaBH_4$)
Decarboxylation (Soda-lime): — $R-COONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3$ (loss of 1 carbon)
HVZ Reaction: — $R-CH_2-COOH \xrightarrow{X_2/Red P} R-CH(X)-COOH$ ($alpha$-halogenation, requires $alpha$-H)
Aromatic Substitution: — COOH is meta-directing, deactivating.

2-Minute Revision

Carboxylic acids possess a unique carboxyl group ($-COOH$) that dictates their properties. Physically, they exhibit unusually high boiling points due to strong intermolecular hydrogen bonding, forming stable cyclic dimers.

Lower members are water-soluble because the carboxyl group can hydrogen bond with water, but solubility decreases as the non-polar alkyl chain lengthens. Chemically, their most important property is their acidity.

They are stronger acids than alcohols and phenols because the resulting carboxylate anion ($R-COO^-$) is highly stabilized by resonance, delocalizing the negative charge over two oxygen atoms. This acidity allows them to react with active metals, bases, and crucially, with sodium bicarbonate to produce $CO_2$ gas, a key distinguishing test.

They undergo nucleophilic acyl substitution reactions, forming derivatives like esters (with alcohols), acyl chlorides (with $SOCl_2$), and amides (with $NH_3$). The entire carboxyl group can be reduced to a primary alcohol using $LiAlH_4$ or removed via decarboxylation (e.

g., soda-lime, losing one carbon). Aliphatic carboxylic acids with an $alpha$-hydrogen undergo the Hell-Volhard-Zelinsky (HVZ) reaction to form $alpha$-halo carboxylic acids. Aromatic carboxylic acids undergo meta-directed electrophilic substitution.

5-Minute Revision

Carboxylic acids are defined by the $-COOH$ functional group, a hybrid of carbonyl and hydroxyl. Their physical properties are dominated by strong intermolecular hydrogen bonding, leading to the formation of stable cyclic dimers.

This dimerization explains their exceptionally high boiling points compared to alcohols or aldehydes of similar molecular mass. For example, ethanoic acid ($118^circ\text{C}$) boils higher than propan-1-ol ($97^circ\text{C}$).

Lower carboxylic acids (up to $C_4-C_5$) are water-soluble due to hydrogen bonding with water molecules, but solubility rapidly decreases as the hydrophobic alkyl chain length increases. Odour varies from pungent for lower members to odourless waxy solids for higher ones.

Chemically, carboxylic acids are weak acids, significantly stronger than phenols and alcohols. Their acidity stems from the resonance stabilization of the carboxylate anion ($R-COO^-$) formed upon deprotonation, where the negative charge is delocalized over two equivalent oxygen atoms.

Electron-withdrawing groups (e.g., halogens, nitro) increase acidity by stabilizing the anion, while electron-donating groups (e.g., alkyl) decrease it. They react with active metals (e.g., Na, Mg) to liberate $H_2$, with strong bases (e.

g., NaOH) to form salts, and famously with sodium bicarbonate ($NaHCO_3$) to produce $CO_2$ effervescence, a diagnostic test.

Reactions involving the C-OH bond cleavage lead to derivatives:

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Esterification: — With alcohols ($R'-OH$) in presence of acid catalyst ($H_2SO_4$) to form esters ($R-COO-R'$).

$$CH_3COOH + CH_3OH \xrightarrow{H^+} CH_3COOCH_3 + H_2O$$

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Acyl Chloride formation: — With $SOCl_2$, $PCl_5$, or $PCl_3$ to form acyl chlorides ($R-COCl$). $SOCl_2$ is preferred due to gaseous byproducts.

$$CH_3COOH + SOCl_2 \rightarrow CH_3COCl + SO_2 + HCl$$

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Anhydride formation: — With dehydrating agents like $P_2O_5$ or strong heating.

$$2CH_3COOH \xrightarrow{P_2O_5, \Delta} (CH_3CO)_2O + H_2O$$

Reactions involving the entire -COOH group:

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Reduction: — To primary alcohols ($R-CH_2OH$) using strong reducing agents like $LiAlH_4$ (not $NaBH_4$).

$$CH_3COOH \xrightarrow{1. LiAlH_4, 2. H_3O^+} CH_3CH_2OH$$

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Decarboxylation: — Removal of $CO_2$. Heating sodium salts with soda-lime ($NaOH + CaO$) yields an alkane with one less carbon.

$$CH_3COONa + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3$$

Reactions involving the alkyl/aryl part:

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Hell-Volhard-Zelinsky (HVZ) Reaction: — $alpha$-halogenation of aliphatic carboxylic acids with $alpha$-hydrogens using $X_2$/Red P.

$$CH_3CH_2COOH \xrightarrow{Br_2/Red P} CH_3CH(Br)COOH$$

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Aromatic Substitution: — The -COOH group is meta-directing and deactivating for electrophilic substitution.

Prelims Revision Notes

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Physical Properties:

* Boiling Points: Carboxylic acids have higher boiling points than alcohols, aldehydes, and ketones of comparable molecular mass. Reason: Strong intermolecular hydrogen bonding forming stable cyclic dimers (two H-bonds per dimer).

* Melting Points: Generally increase with molecular mass. Alternation effect: Even-numbered carbon chains pack better, leading to higher melting points than odd-numbered ones. * Solubility: Lower members ($C_1-C_5$) are water-soluble due to H-bonding with water.

Solubility decreases rapidly with increasing alkyl chain length (hydrophobic part dominates). * Odour: Lower members (e.g., formic, acetic) are pungent. Butyric acid has rancid butter smell. Higher members are odourless, waxy solids.

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Chemical Properties (Reactivity):

* Acidic Nature: * Proton donation from -OH group: $R-COOH \rightleftharpoons R-COO^- + H^+$. * Reason for acidity: Resonance stabilization of carboxylate anion ($R-COO^-$). * Acidity Order: Mineral acids > Carboxylic acids > Carbonic acid ($H_2CO_3$) > Phenols > Alcohols.

* Effect of Substituents: Electron-Withdrawing Groups (EWGs) like $-NO_2, -CN, -X$ (halogens) increase acidity. Electron-Donating Groups (EDGs) like alkyl groups decrease acidity. Effect decreases with distance.

* Reactions: With active metals ($H_2$ gas), bases (salts), carbonates/bicarbonates ($CO_2$ effervescence – diagnostic test). * Reactions involving C-OH bond cleavage (Nucleophilic Acyl Substitution): * Esterification: $R-COOH + R'-OH \xrightarrow{H^+} R-COO-R' + H_2O$.

* Acyl Chloride formation: $R-COOH + SOCl_2 \rightarrow R-COCl + SO_2 + HCl$ (preferred). Also $PCl_5, PCl_3$. * Anhydride formation: $2R-COOH \xrightarrow{P_2O_5, \Delta} (R-CO)_2O + H_2O$.

* Amide formation: $R-COOH + NH_3 \rightarrow R-COONH_4 \xrightarrow{\Delta} R-CONH_2 + H_2O$. * Reactions involving -COOH group as a whole: * Reduction: $R-COOH \xrightarrow{1. LiAlH_4, 2.

H_3O^+} R-CH_2OH$(primary alcohol). Note:$NaBH_4$is ineffective. * **Decarboxylation:** Removal of$CO_2$. * **Soda-lime:**$R-COONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3$ (alkane with one less carbon).

* Kolbe's Electrolysis: $2R-COONa \xrightarrow{Electrolysis} R-R + 2CO_2 + 2NaOH + H_2$. * Reactions involving alkyl/aryl part: * Hell-Volhard-Zelinsky (HVZ) Reaction: $alpha$-halogenation of aliphatic carboxylic acids with $alpha$-hydrogens.

$R-CH_2-COOH \xrightarrow{X_2/Red P} R-CH(X)-COOH$. * Ring Substitution (Aromatic): -COOH is a meta-directing, deactivating group for electrophilic substitution.

Vyyuha Quick Recall

Carboxylic Acids Properties: Hydrogen Bonds Double Acidity Reactions Everywhere!

Hydrogen Bonds: High Boiling Points (due to Dimerization), Solubility (lower members).
Acidity: Stronger than phenols/alcohols (due to Resonance stabilization of carboxylate anion).
Reactions Everywhere:

* Esterification (alcohol + acid $\rightarrow$ ester) * Very Zealous (HVZ) reaction ($alpha$-halogenation) * Elimination of $CO_2$ (Decarboxylation, soda-lime) * Reduction ($LiAlH_4$ to alcohol) * With Carbonates ($NaHCO_3$ test, $CO_2$ effervescence)