Chapter 20 Carboxylic Acids

Carboxylic Acids

Nomenclature

Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH). Their systematic naming follows IUPAC conventions:

• IUPAC Naming: Replace the final "e" of the parent hydrocarbon with "oic acid". For dicarboxylic acids, use "dioic acid".

• The carbon of the COOH group is always assigned position 1.

• Common names are often used for simple carboxylic acids (e.g., formic acid, acetic acid).

Examples:

• methanoic acid (formic acid)

• ethanoic acid (acetic acid)

• propanoic acid (propionic acid)

• butanoic acid (butyric acid)

• benzoic acid (aromatic carboxylic acid)

If the COOH group is attached to a ring, the compound is named as "carboxylic acid" with the ring carbon holding the COOH group as carbon-1.

Structures and Physical Properties

Carboxylic acids have distinct physical properties due to their ability to form hydrogen bonds:

• Hydrogen Bonding: Both the hydroxyl and carbonyl groups participate in hydrogen bonding, leading to higher boiling points than alcohols and alkanes of similar molecular weight.

• Boiling Points: For example, acetic acid (CH3COOH) boils at 118°C, compared to ethanol (97°C) and propane (0°C).

• Solubility: Carboxylic acids with four or fewer carbons are miscible with water due to hydrogen bonding (miscible = soluble in all proportions).

Acidity of Carboxylic Acids

Carboxylic acids are more acidic than alcohols due to resonance stabilization of the carboxylate ion:

• General Acid Dissociation:

$RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+$

• pKa values: Carboxylic acids: 3–5; Alcohols: 16–18; HCl: –7

• Resonance Stabilization: The negative charge on the carboxylate ion is delocalized over two oxygen atoms, making it more stable than the alkoxide ion from alcohols.

Comparative Reaction:

$R-OH + H_2O \rightleftharpoons R-O^- + H_3O^+$

Substituent Effects on Acidity

Electron-withdrawing groups increase the acidity of carboxylic acids by stabilizing the conjugate base:

• Acetic acid: pKa = 4.74

• Chloroacetic acid: pKa = 2.86

• Dichloroacetic acid: pKa = 1.26

• Trichloroacetic acid: pKa = 0.64

The closer the electron-withdrawing group is to the COOH group, the greater its effect:

Compound	pKa
2-chlorobutanoic acid	2.86
3-chlorobutanoic acid	4.05
4-chlorobutanoic acid	4.52

Salts of Carboxylic Acids

Carboxylic acids react with strong bases to form carboxylate salts:

• Deprotonation Reaction:

$RCOOH + NaOH \rightarrow RCOO^-Na^+ + H_2O$

• Carboxylate salts of alkali metals are generally soluble in water.

• Naming: Name the cation first, then the carboxylate ion by replacing "ic acid" with "ate" (e.g., sodium ethanoate, potassium benzoate).

Synthesis of Carboxylic Acids

Carboxylic acids can be synthesized by several methods:

1. Oxidation of Primary Alcohols and Aldehydes

Primary alcohols and aldehydes are oxidized to carboxylic acids using strong oxidizing agents:

$RCH_2OH \xrightarrow{Na_2Cr_2O_7, H_2SO_4} RCOOH$

$RCHO \xrightarrow{Na_2Cr_2O_7, H_2SO_4} RCOOH$

2. Oxidative Cleavage of Alkenes and Alkynes

Alkenes and alkynes undergo oxidative cleavage with potassium permanganate:

$R_2C=CR_2 \xrightarrow{KMnO_4, warm} RCOOH + R'COOH$

Alkynes can yield carboxylic acids and ketones depending on substitution.

3. Carboxylation of Grignard Reagents

Grignard reagents react with carbon dioxide to form carboxylic acids:

$R-MgBr + CO_2 \xrightarrow{H_3O^+} RCOOH$

Reactions of Carboxylic Acids

1. Formation of Esters (Fischer Esterification)

Carboxylic acids react with alcohols in the presence of acid to form esters:

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

• The equilibrium constant is not very large; the reaction can be driven to completion by using excess reactant or removing water.

2. Formation of Amides

Carboxylic acids react with amines to form amides, often requiring heat or a coupling agent like DCC (dicyclohexylcarbodiimide):

$RCOOH + R'NH_2 \xrightarrow{heat} RCONHR' + H_2O$

$RCOOH + R'NH_2 \xrightarrow{DCC} RCONHR' + N,N'-dicyclohexylurea$

3. Formation of Acid Chlorides

Carboxylic acids react with thionyl chloride to form acid chlorides:

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

4. Reactions of Acid Chlorides

Acid chlorides are reactive intermediates used to synthesize esters, amides, and alcohols:

• With Grignard reagents: $RCOCl + 2R'MgBr \xrightarrow{H_2O^+} R'RCOH$

• With alcohols: $RCOCl + R'OH \rightarrow RCOOR' + HCl$

• With amines: $RCOCl + R'NH_2 \rightarrow RCONHR' + HCl$

Mechanism for ester formation from acid chloride:

$RCOCl + R'OH \rightarrow RCOOR' + HCl$

5. Reduction of Carboxylic Acids

Carboxylic acids can be reduced to alcohols or aldehydes:

• To alcohols: $RCOOH \xrightarrow{1) LiAlH_4, 2) H_2O} RCH_2OH$

• To aldehydes: Convert to acid chloride first, then reduce with lithium tri-tert-butoxyaluminum hydride:

$RCOOH \xrightarrow{SOCl_2} RCOCl \xrightarrow{LiAlH(O-tBu)_3} RCHO$

If LiAlH4 is used, the aldehyde will be further reduced to alcohol.

Synthesis Problems

Practice problems may include providing reagents and steps for the synthesis of carboxylic acids and their derivatives from various starting materials.

Summary Table: Carboxylic Acid Reactions

Reaction Type	Reagents	Product
Oxidation of Alcohols/Aldehydes	Na2Cr2O7, H2SO4	Carboxylic Acid
Oxidative Cleavage	KMnO4 (warm)	Carboxylic Acid/Ketone
Grignard Carboxylation	CO2, H3O+	Carboxylic Acid
Fischer Esterification	Alcohol, H+	Ester
Amide Formation	Amine, heat or DCC	Amide
Acid Chloride Formation	SOCl2	Acid Chloride
Reduction	LiAlH4 or LiAlH(O-tBu)3	Alcohol/Aldehyde

Additional info: Mechanisms for some reactions (e.g., Fischer esterification, acid chloride reactions) are referenced but not fully detailed in the notes. For exam preparation, students should be familiar with the general steps and reagents for each transformation.