Carboxylic Acids (Physical And Chemical Properties)

Physical Properties (Carboxylic Acids)

The physical properties of carboxylic acids are influenced by the polar carboxyl group and the presence of the $O-H$ bond, which allows for hydrogen bonding.

1. Physical State:

• Lower members (formic acid, acetic acid, propanoic acid) are colorless liquids.
• Higher members (from $C_4$ onwards) are solids at room temperature.

2. Boiling Points:

• Carboxylic acids have significantly higher boiling points than corresponding hydrocarbons, haloalkanes, alcohols, aldehydes, and ketones of similar molecular weights.
• This is due to the formation of strong intermolecular dimers through two hydrogen bonds between the carboxyl groups of two molecules.
• Boiling points increase with increasing molecular weight.
• The boiling points of isomeric carboxylic acids are similar, but branching can slightly lower the boiling point.

3. Solubility in Water:

• Lower carboxylic acids (up to about 10 carbon atoms) are soluble in water.
• This solubility is due to their ability to form intermolecular hydrogen bonds with water molecules, facilitated by the polar carboxyl group.
• Solubility decreases as the length of the nonpolar hydrocarbon chain increases.
• Aromatic acids (like benzoic acid) are practically insoluble in water.

4. Acidity: Carboxylic acids are acidic due to the polar $O-H$ bond and the resonance stabilization of the carboxylate ion ($RCOO^-$). Their acidity is greater than that of phenols and alcohols.

Chemical Reactions (Carboxylic Acids)

Carboxylic acids undergo a variety of reactions involving the carboxyl group and sometimes the alkyl/aryl group attached to it.

Reactions Involving Cleavage Of O—H Bond (Acidity)

1. Reaction with Metals: React with active metals to liberate hydrogen gas.

$2RCOOH + 2Na \rightarrow 2RCOONa + H_2$

2. Reaction with Bases: React with bases to form salts and water (neutralization).

$RCOOH + NaOH \rightarrow RCOONa + H_2O$

3. Reaction with Carbonates and Hydrogencarbonates: React to liberate carbon dioxide gas.

$2RCOOH + Na_2CO_3 \rightarrow 2RCOONa + H_2O + CO_2$

$RCOOH + NaHCO_3 \rightarrow RCOONa + H_2O + CO_2$

4. Reaction with Active Metals: React with active metals to liberate hydrogen gas.

$2RCOOH + 2Na \rightarrow 2RCOONa + H_2$

Reactions Involving Cleavage Of C—O Bond (or C—C Bond)

1. Formation of Acid Derivatives: Carboxylic acids react to form acyl halides, anhydrides, esters, and amides by replacing the -OH group or by condensation reactions.

• With $PCl_5, PCl_3, SOCl_2$: Form acyl chlorides.

$RCOOH + SOCl_2 \rightarrow RCOCl + SO_2 + HCl$

• With Acetic Anhydride: Form acid anhydrides.

$2CH_3COOH \xrightarrow{P_4O_{10}} (CH_3CO)_2O + H_2O$

• Esterification: Reaction with alcohols in the presence of acid catalyst to form esters (Fischer Esterification).

$RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O$

• Amide Formation: Reaction with ammonia or amines (usually via acyl chloride or anhydride) to form amides.

$RCOCl + NH_3 \rightarrow RCONH_2 + HCl$

2. Reduction: Can be reduced to primary alcohols using strong reducing agents like $LiAlH_4$ or $B_2H_6$.

$RCOOH \xrightarrow{LiAlH_4 \ or \ B_2H_6} RCH_2OH$

3. Decarboxylation: Loss of $CO_2$. Usually requires heating, especially for $\beta$-keto acids or malonic acids.

• Beta-keto acid decarboxylation:

$RCOCH_2COOH \xrightarrow{\Delta} RCOCH_3 + CO_2$

• Malonic acid decarboxylation:

$CH_2(COOH)_2 \xrightarrow{\Delta} CH_3COOH + CO_2$

4. Hell-Volhard-Zelinsky Reaction: $\alpha$-halogenation of carboxylic acids in the presence of $P$ and $X_2$.

$RCH_2COOH + X_2 \xrightarrow{P/X_2} RCHXCOOH + HX$

Reactions Involving C—C Bond Cleavage

Description: This typically occurs under harsh conditions or specific reaction pathways that break the carbon chain.

Example: Alpha-halogenation of carboxylic acids where the $\alpha$-carbon is involved in bond cleavage/formation.

Substitution Reactions In The Hydrocarbon Part

Description: Reactions like free radical halogenation can occur on the alkyl chain of a carboxylic acid, usually at the $\alpha$-carbon, as seen in the Hell-Volhard-Zelinsky reaction.

Example:

$CH_3COOH + Br_2 \xrightarrow{P/Br_2} BrCH_2COOH + HBr$

Properties Of Ethanoic Acid (from Carbon And Its Compounds)

Ethanoic acid ($CH_3COOH$), commonly known as acetic acid, is the second simplest carboxylic acid and is widely used.

Properties Of Ethanoic Acid

Physical Properties:

• Appearance: Colorless liquid.
• Odor: Pungent, characteristic vinegar-like smell.
• Freezing Point: Pure ethanoic acid freezes below room temperature (16.6°C), hence called glacial acetic acid.
• Boiling Point: 118°C (391 K). Higher than ethanol due to stronger intermolecular hydrogen bonding (dimer formation).
• Solubility: Miscible with water, ethanol, and ether due to hydrogen bonding.
• Density: Slightly denser than water.

Chemical Properties:

1. Acidic Nature:

• Acts as a weak acid in water, ionizing to produce $H_3O^+$ and acetate ions ($CH_3COO^-$).

$CH_3COOH \rightleftharpoons H^+ + CH_3COO^-$

• Reacts with active metals like $Na$, $Mg$, $Zn$ to liberate $H_2$.

$2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$

• Reacts with bases to form salts (acetates) and water.

$CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$

• Reacts with carbonates and hydrogencarbonates to liberate $CO_2$.

$2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2$

2. Reactions of the Methyl Group ($\alpha$-Hydrogens):

• Halogenation (Hell-Volhard-Zelinsky Reaction): Reacts with $X_2$ (e.g., $Br_2$, $Cl_2$) in the presence of red phosphorus or $P$ to substitute $\alpha$-hydrogens with halogens.

$CH_3COOH + Br_2 \xrightarrow{P/Br_2} BrCH_2COOH + HBr$

3. Reactions of the Carboxyl Group:

• Esterification: Reacts with alcohols in the presence of an acid catalyst to form esters.

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$

• Formation of Acid Halides: Reacts with $PCl_5$, $PCl_3$, or $SOCl_2$ to form acetyl chloride ($CH_3COCl$).

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

• Formation of Anhydride: Heating with dehydrating agent like $P_4O_{10}$ or heating sodium acetate with concentrated sulfuric acid gives acetic anhydride.

$2CH_3COOH \xrightarrow{P_4O_{10}, \ heat} (CH_3CO)_2O + H_2O$

• Formation of Amides: Reacts with ammonia (usually via acetyl chloride or anhydride) to form acetamide.

$CH_3COCl + 2NH_3 \rightarrow CH_3CONH_2 + NH_4Cl$

• Reduction: Can be reduced to ethanol by strong reducing agents like $LiAlH_4$.

$CH_3COOH \xrightarrow{LiAlH_4} CH_3CH_2OH$

4. Reaction with strong oxidizing agents: Can be oxidized by strong oxidizing agents, but requires vigorous conditions.

Uses:

• Vinegar (5-8% solution) in food.
• Manufacture of rayon, dyes, perfumes, plastics (polyvinyl acetate), pharmaceuticals.
• Solvent.
• Glacial acetic acid is used in laboratories.