M

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.