Ch. 16: Carboxylic Acids and Esters: Structure, Properties, and Reactions

Carboxylic Acids

Functional Group and Structure

Carboxylic acids are organic compounds containing the carboxyl group (-COOH), which can be represented in several ways:

• Structural formulas: O=C–OH, –COOH, or –CO2H

• The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (O–H) group attached to the same carbon.

Carboxylic acids are weak acids and partially dissociate in water:

• General dissociation equation: $\mathrm{CH_3COOH} + \mathrm{H_2O} \rightarrow \mathrm{CH_3COO^-} + \mathrm{H_3O^+}$

• The acid dissociation constant ($K_a$) and $pK_a$ values for carboxylic acids typically range from 4 to 5 for unsubstituted hydrocarbons.

Acidity of Carboxylic Acids

Acid Dissociation and Influencing Factors

Carboxylic acids can release protons (H+) in water, making them weak acids. The acidity is quantified by the acid dissociation constant ($K_a$):

• $K_a = \frac{[\mathrm{CH_3COO^-}][\mathrm{H_3O^+}]}{[\mathrm{CH_3COOH}]} = 1.74 \times 10^{-5}$

• $pK_a = -\log K_a = 4.75$

The acidity of carboxylic acids is affected by substituents near the carboxyl group:

• Electron-withdrawing groups (e.g., –Cl, –OH, –NH2) increase acidity by stabilizing the conjugate base.

• Electron-donating groups decrease acidity.

Comparison Table: Effect of Substituents on Acidity

Compound	Substituent	pKa
CH3COOH	None	4.76
ClCH2COOH	Cl	2.86
NH2CH2COOH	NH2	2.34
OHCH2COOH	OH	2.34

Additional info: Differences in acidity are important for the properties of amino acids, especially aspartate and glutamate.

Fatty Acids

Structure and Biological Importance

Fatty acids are long-chain carboxylic acids derived from animal fats, vegetable oils, or phospholipids of biological membranes.

• Most natural fatty acids have an even number of carbons (typically 12–20) in an unbranched chain.

• Unsaturated fatty acids contain cis double bonds, which affect their physical properties and biological functions.

Example: Oleic acid (cis-9-octadecenoic acid) is a common unsaturated fatty acid found in olive oil.

Reaction with Bases

Formation of Carboxylate Salts

Carboxylic acids react with strong bases (e.g., NaOH, KOH) to form water-soluble salts called carboxylates:

• $\mathrm{COOH} + \mathrm{NaOH} \rightarrow \mathrm{COO^-Na^+} + \mathrm{H_2O}$

• They also react with ammonia and amines to form ammonium or amine salts:

• $\mathrm{COOH} + \mathrm{NH_3} \rightarrow \mathrm{COO^-NH_4^+}$

Example: Sodium benzoate is a common food preservative formed from benzoic acid and sodium hydroxide.

Reduction of Carboxylic Acids

Conversion to Alcohols

Carboxylic acids can be reduced to primary alcohols using strong reducing agents such as lithium aluminum hydride (LiAlH4):

• $\mathrm{CH_3COOH} \xrightarrow{LiAlH_4} \mathrm{CH_3CH_2OH}$

• LiAlH4 is a stronger reductant than NaBH4, which cannot reduce carboxylic acids.

Additional info: Other reagents capable of reducing aldehydes and ketones include NaBH4 and catalytic hydrogenation.

Decarboxylation

Loss of Carbon Dioxide

Decarboxylation is the loss of CO2 from a carboxyl group. Most carboxylic acids undergo thermal decarboxylation at very high temperatures:

• $\mathrm{RCOOH} \xrightarrow{\text{high temperature}} \mathrm{RH} + \mathrm{CO_2}$

• Thermal decarboxylation is not common in biological systems.

However, β-ketoacids decarboxylate under much milder conditions due to enol tautomerization:

• $\text{3-Oxobutanoic acid} \rightarrow \text{Acetone} + \mathrm{CO_2}$

Biochemical relevance: Decarboxylation of β-ketoacids occurs during the oxidation of foods in the tricarboxylic acid (TCA) cycle and is important in metabolism.

Physical Properties of Carboxylic Acids

Boiling Points and Hydrogen Bonding

Carboxylic acids have three polar covalent bonds: C=O, C–O, and O–H. The polarity of these bonds determines their major physical properties:

• Carboxylic acids have significantly higher boiling points than other organic compounds of similar molecular weight.

• Hydrogen bonding between two carboxyl groups creates a dimer, behaving like a larger molecule and further increasing boiling point.

Comparison Table: Boiling Points

Compound Type	Boiling Point (Relative)
Alkane	Lowest
Aldehyde	Low
Alcohol	High
Carboxylic Acid	Highest

Additional info: The ability to form hydrogen-bonded dimers is unique to carboxylic acids among small organic molecules.

Carboxyl Derivatives

Anhydrides, Esters, and Amides

Three important classes of compounds can be derived from carboxylic acids: anhydrides, esters, and amides. Each is formed by combining a carboxylic acid with another organic functional group via a dehydration reaction (loss of H2O):

Derivative	General Structure	Formation
Anhydride	R–CO–O–CO–R'	2 carboxylic acids
Ester	R–CO–O–R'	Carboxylic acid + alcohol
Amide	R–CO–NH–R'	Carboxylic acid + amine

Anhydrides are formed by joining two carboxylic acids and may be symmetrical or asymmetrical.

Fischer Esterification

Preparation of Esters

Fischer esterification is a common method for preparing esters. In this reaction, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst (usually concentrated sulfuric acid):

• $\mathrm{CH_3COOH} + \mathrm{CH_3CH_2OH} \xrightarrow{H_2SO_4} \mathrm{CH_3COOCH_2CH_3} + \mathrm{H_2O}$

• The reaction is reversible; removal of water drives the equilibrium toward ester formation.

Role of acid: The acid catalyst protonates the carbonyl oxygen, increasing its electrophilicity and facilitating nucleophilic attack by the alcohol.

Preparation of Esters

Alternative Methods

Esters can also be synthesized by the reaction of anhydrides with alcohols (acid catalysis not required):

• $\mathrm{CH_3COOCOCH_3} + \mathrm{CH_3CH_2OH} \rightarrow \mathrm{CH_3COOCH_2CH_3} + \mathrm{CH_3COOH}$

Example: Ethyl acetate is prepared from acetic anhydride and ethanol.

Summary Table: Carboxylic Acids and Derivatives

Compound	Functional Group	Preparation	Key Properties
Carboxylic Acid	–COOH	Oxidation of aldehydes/primary alcohols	Weak acid, high boiling point
Carboxylate Salt	–COO–	Acid + base	Water-soluble
Ester	–COOR	Acid + alcohol (Fischer esterification)	Fragrant, less polar
Anhydride	–CO–O–CO–	2 acids, dehydration	Reactive, not common in biochemistry
Amide	–CONH2	Acid + amine	Stable, peptide bonds