Carboxylic Acids (Introduction & Preparation)

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group ($-COOH$).

Nomenclature

General Formula: $R-COOH$, where R can be an alkyl group, aryl group, or hydrogen.

IUPAC Nomenclature:

Parent Chain: Identify the longest continuous carbon chain containing the carboxyl group.
Numbering: Number the chain starting from the carboxyl carbon as C1.
Suffix: Replace the '-e' ending of the parent alkane name with '-oic acid'.
Substituents: Name and number other substituents as prefixes in alphabetical order.

Examples:

$HCOOH$: Methanoic acid (Common name: Formic acid)
$CH_3COOH$: Ethanoic acid (Common name: Acetic acid)
$CH_3CH_2COOH$: Propanoic acid
$CH_3CH(CH_3)COOH$: 2-Methylpropanoic acid
$C_6H_5COOH$: Benzoic acid

Dicarboxylic Acids: Named by adding '-dioic acid' to the parent alkane name (e.g., Ethanedioic acid for oxalic acid, Propanedioic acid for malonic acid).

Structure Of Carboxyl Group

Description: The carboxyl group ($-COOH$) consists of a carbonyl group ($C=O$) attached to a hydroxyl group ($-OH$).

Bonding:

The carbon atom is $sp^2$ hybridized, resulting in a trigonal planar geometry around it.
The $C=O$ bond is a double bond consisting of one $\sigma$ bond and one $\pi$ bond.
The $C-O$ bond in the $-OH$ group is a single $\sigma$ bond.

Polarity: The carboxyl group is highly polar due to the presence of two highly electronegative oxygen atoms.

The $C=O$ bond is strongly polarized towards oxygen.
The $O-H$ bond is also polar, with the hydrogen atom being partially positive ($\delta^+$) and the oxygen atom being partially negative ($\delta^-$).

Resonance: The carboxyl group exhibits resonance, where the negative charge on the carboxylate ion ($RCOO^-$) is delocalized over both oxygen atoms. This resonance stabilizes the carboxylate ion, making carboxylic acids stronger than alcohols.

$$ \underset{+}{R-C(=O)-\underset{-}{O}H} \leftrightarrow \underset{-}{R-C(-\underset{+}{O}-)-O H} \leftrightarrow \underset{+}{R-C(-\underset{-}{O}-)=O H} $$

Resonance in the carboxylate ion:

$$ \underset{-}{R-C(=O)-O^-} \leftrightarrow \underset{-}{R-C(-O^-)=O} $$

Acidity: The polarity of the $O-H$ bond and the resonance stabilization of the carboxylate anion make carboxylic acids acidic. They readily donate the proton ($H^+$).

Methods Of Preparation Of Carboxylic Acids

Carboxylic acids can be synthesized by various methods, often involving oxidation of compounds with a $CH(OH)-$ or $CH_3-$ group attached to the carbonyl or carboxyl carbon.

From Primary Alcohols And Aldehydes

1. Oxidation of Primary Alcohols: Primary alcohols are oxidized to aldehydes, which are further oxidized to carboxylic acids. This requires strong oxidizing agents.

Oxidizing Agents: Acidified potassium permanganate ($KMnO_4$), acidified potassium dichromate ($K_2Cr_2O_7$), or chromic acid ($CrO_3$).

$RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH$

2. Oxidation of Aldehydes: Aldehydes are easily oxidized to carboxylic acids.

$RCHO \xrightarrow{mild \ or \ strong \ oxidizing \ agent} RCOOH$

Tollens' Reagent: Test for aldehydes.

Fehling's Solution: Test for aliphatic aldehydes.

From Alkylbenzenes

Description: Alkyl side chains on an aromatic ring can be oxidized to a carboxyl group using strong oxidizing agents.

Reaction: Any alkyl side chain (primary or secondary) attached to the benzene ring can be oxidized to a carboxyl group.

$C_6H_5CH_3 \xrightarrow{KMnO_4, OH^-, \ heat \ then \ H^+} C_6H_5COOH$

$(CH_3)_2C_6H_4 \xrightarrow{KMnO_4, OH^-, \ heat \ then \ H^+} C_6H_4(COOH)_2$ (for para-xylene)

From Nitriles And Amides

1. Hydrolysis of Nitriles: Nitriles ($R-C \equiv N$) are hydrolyzed by acids or bases to yield carboxylic acids.

$R-C \equiv N + 2H_2O \xrightarrow{H^+ \ or \ OH^-} RCOOH + NH_3 \text{ or } NH_4^+$

2. Hydrolysis of Amides: Amides ($R-CONH_2$) are hydrolyzed by acids or bases to yield carboxylic acids.

$R-CONH_2 + H_2O \xrightarrow{H^+ \ or \ OH^-} RCOOH + NH_3 \text{ or } NH_4^+$

From Grignard Reagents

Description: Grignard reagents ($RMgX$) react with carbon dioxide ($CO_2$, dry ice) followed by acid hydrolysis to yield carboxylic acids.

$RMgX + CO_2 \xrightarrow{ether} RCOOMgX \xrightarrow{H_3O^+} RCOOH + Mg^{2+} + X^- + H_2O$

Note: This method yields carboxylic acids with one more carbon atom than the alkyl halide used to prepare the Grignard reagent.

From Acyl Halides And Anhydrides

1. Hydrolysis of Acyl Halides: Acyl halides ($RCOCl$) readily hydrolyze in water to form carboxylic acids and $HCl$.

$RCOCl + H_2O \rightarrow RCOOH + HCl$

2. Hydrolysis of Acid Anhydrides: Acid anhydrides ($RCO-O-COR$) hydrolyze to form two molecules of carboxylic acid.

$(RCO)_2O + H_2O \rightarrow 2RCOOH$

From Esters

Hydrolysis of Esters: Esters ($RCOOR'$) can be hydrolyzed under acidic or basic conditions to yield carboxylic acids.

Acid Hydrolysis:

$RCOOR' + H_2O \xrightarrow{H^+} RCOOH + R'OH$

Base Hydrolysis (Saponification):

$RCOOR' + NaOH \rightarrow RCOONa + R'OH \xrightarrow{H^+} RCOOH$

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 has many practical uses.

Properties Of Ethanoic Acid

Physical Properties:

Appearance: Pure ethanoic acid is a colorless liquid.
Odor: It has a pungent, vinegar-like smell.
Freezing Point: Glacial acetic acid (pure anhydrous $CH_3COOH$) freezes at 289.8 K (16.6°C) to form ice-like crystals.
Boiling Point: 391 K (118°C).
Solubility: Miscible with water, ethanol, and ether.
Density: Slightly denser than water.
Acidity: It is a weak acid.

Chemical Properties:

1. Acidic Nature:

Ionizes in water to produce $H_3O^+$ and $CH_3COO^-$ ions.

$CH_3COOH + H_2O \rightleftharpoons H_3O^+ + CH_3COO^-$

Reacts with bases to form acetate salts and water.

$CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$

Reacts with active metals to liberate hydrogen gas.

$2CH_3COOH + 2Na \rightarrow 2CH_3COONa + H_2$

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

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

$CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + H_2O + CO_2$

2. Reactions of the -CH3 group:

Halogenation: Reacts with halogens in the presence of red phosphorus or $PCl_3$ (Hell-Volhard-Zelinsky reaction) to substitute $\alpha$-hydrogens.

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

3. Esterification: Reacts with alcohols in the presence of an acid catalyst (like conc. $H_2SO_4$) to form esters.

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$ (Fischer Esterification)

4. Reaction with Reducing Agents: Can be reduced to ethanol ($C_2H_5OH$) using strong reducing agents like $LiAlH_4$.

$CH_3COOH \xrightarrow{LiAlH_4} CH_3CH_2OH$

5. Formation of Acid Derivatives: Reacts to form acid halides, anhydrides, amides, etc.

With $PCl_5$: $CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl$
With Acetic Anhydride: $2CH_3COOH \xrightarrow{P_4O_{10}} (CH_3CO)_2O + H_2O$
With Ammonia: $CH_3COOH + NH_3 \rightarrow CH_3COONH_4 \xrightarrow{\Delta} CH_3CONH_2 + H_2O$

Uses:

Vinegar (5-8% acetic acid solution) is used as a food preservative and flavoring agent.
Used in the manufacture of vinegar, rayon, dyes, perfumes, plastics (polyvinyl acetate), and pharmaceuticals.
Used as a solvent.
Glacial acetic acid is used in laboratories.