CH 16

Carboxylic Acids and Esters

Chapter Overview

• Lecture #7

Assignments

• Reading: Chapter 16, Pages 560-579

• Focus Areas:

  • Structure of carboxylic acids

  • Acidic behavior of carboxylic acids

  • Structures of carboxylic acid derivatives

Carboxylic Acids

• Carboxylic acids are a class of compounds containing the C=O group.

• Common examples include:

  • Butanal: CH3CH2C(=O)H

  • Propanoic Acid: CH3CH2C(=O)OH

  • 2-Butanone: CH3CH2C(=O)CH3

Structure of Carboxylic Acids

• Important functional group in biological systems, especially proteins.

• General formula representation: R-COOH (R represents any carbon and hydrogen containing group).

Simple Carboxylic Acid Nomenclature

• Named from the longest carbon chain:

  • Replace the terminal -e of parent alkane with -oic acid.

  • Number carbons starting at -COOH as carbon #1.

• Example Names:

  • Hexanoic acid: CH3(CH2)4COOH

  • 5-Hydroxyhexanoic acid: CH3CH(OH)(CH2)3COOH

• Note: Substituent priorities are crucial (not named as alcohol).

Acyclic Dicarboxylic Acids

• Contain two acid groups:

  • Ethanedioic acid (Oxalic Acid): HOOC-COOH

  • Hexanedioic acid (Adipic Acid): HOOC-(CH2)4-COOH

Aromatic Carboxylic Acid Nomenclature

• Named as derivatives of benzoic acid.

• Example: 4-Aminobenzoic Acid, COOH at position 1, NH2 at position 4.

Common Names of Carboxylic Acids

• Typically found in nature and given common names:

  • Methanoic acid (Formic Acid): HCOOH

  • Butanoic acid (Butyric Acid): CH3(CH2)2COOH

  • Dodecanoic acid (Lauric Acid): CH3(CH2)10COOH

  • Hexandecanoic acid (Palmitic Acid): CH3(CH2)14COOH

  • Octadecanoic acid (Stearic Acid): CH3(CH2)16COOH

Example Problems

• Draw the structures for:

  • 2-bromo-3-methylbutanoic acid

  • 2,3-dinitrobenzoic acid

Physical Properties of Carboxylic Acids

• Polarity:

  • Higher boiling points than other organic compounds due to strong hydrogen bonds.

  • Solubility:

    • More soluble in water than alcohols, ketones, aldehydes, or ethers.

    • Solubility decreases with increasing carbon chain size.

• Smaller aliphatic carboxylic acids have sharp odors; butanoic acid contributes to the smell of stale sweat.

Properties of Carboxylic Acids

• Form strong hydrogen bonds.

  • In gas phase, exist as dimers. In liquid, aggregate due to hydrogen bonds.

  • In aqueous solutions, form hydrogen bonds with water.

• Intermolecular hydrogen bonding significantly raises boiling points compared to alcohols of similar molecular weight:

  • CH3COOH (Acetic Acid) Bp: 117.9° C

  • CH3CH2OH (Ethanol) Bp: 78° C

H-Bonding of Acetic Acid

• Illustrates the stabilization provided by hydrogen bonding in a multicompound environment.

Salts of Carboxylic Acids

• React with strong bases for neutralization:

  • Forms carboxylate ions and water.

  • Small carboxylates are water-soluble.

Acid-Base Reactions

• Carboxylic acids react with strong bases to form carboxylate salts, a process known as neutralization.

• Removal of acid protons by OH- results in the more water-soluble carboxylate ions.

• Equilibrium shifts right with the removal of water.

Reactions of Carboxylic Acids

• Besides base reactions, can undergo esterification, known as Fischer Esterification:

  • Reaction with low molecular weight alcohols in the presence of mineral acid.

  • General structure: RCOOH + R'OH → RCOOR' + H2O

Esterification Process

• Carboxylic acids react with alcohols to form esters and water through dehydration.

Structure of Esters

• Derived from carboxylic acids:

  • General structure: RCOOR'.

Ester Nomenclature

• Names derived from parent carboxylic acid:

  • Alkyl group attached to oxygen named first.

  • Replace -ic acid with -ate.

Examples of Esters

• Ethyl Ethanoate (Ethyl Acetate): CH3COOCH2CH3

• Octyl Ethanoate: CH3COO(CH2)7CH3 (used for aroma).

Example Problem - Naming

• Provide IUPAC name for:

  • CH3(CH2)8COCH3: Methyl decanoate.

Physical Properties of Esters

• Aroma examples:

  • Ethyl butanoate: Pineapple

  • Methyl salicylate: Wintergreen

  • Octyl ethanoate: Orange

• Solubility: More soluble than alkanes because of their ability to form hydrogen bonds with water.

• Lower boiling points than corresponding alcohols due to the absence of hydroxyl groups.

Preparation of Esters from Carboxylic Acids

• Fischer Esterification requires removal of water to prevent reversing the reaction:

  • RCOOH + ROH → RCOOR' + H2O.

Hydrolysis of Esters

• Main reaction involves hydrolysis with water:

  • Reverse of esterification and requires heat for acceleration.

  • Acid acts as a catalyst.

Base Hydrolysis of Esters

• Known as saponification (soap-making):

  • Involves strong bases leading to the formation of soaps (acid salt and alcohol).

Reactions of Esters - Saponification

• Cleavage of an ester by a strong base during soap manufacture:

  • Products: Acid salt, alcohol, and no free acid present in basic conditions.

Boiling Points of Esters

• Have higher boiling points than alkanes and ethers but lower than alcohols and carboxylic acids of similar mass due to absence of hydroxyl groups.

Solubility of Esters in Water

• Esters with 2-5 carbon atoms are soluble in water.

• Solubility decreases as the number of carbon atoms increases.