Carboxylic Acids, Esters, Amines, Amides, Thiols, Disulfides, and Phosphates: Structure, Properties, and Reactions

Carboxylic Acids

Structure and General Properties

Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH). They are more oxidized than aldehydes and ketones and play a central role in biological systems.

• General formula: R-COOH, where R is a carbon-containing group.

• Carboxyl group: Consists of a carbonyl (C=O) and a hydroxyl (O-H) group attached to the same carbon.

• Intermolecular forces: Strong hydrogen bonding due to both donor and acceptor sites.

• Physical properties: Higher boiling points than aldehydes, ketones, and esters due to hydrogen bonding.

• Occurrence: Found in fatty acids and amino acids.

Acidity of Carboxylic Acids

Carboxylic acids are weak acids, capable of donating a proton (H+) to water.

• Acid dissociation:

$\ce{RCOOH <=> RCOO^- + H^+}$

• Acid dissociation constant: $K_a = 10^{-4}$, $pK_a \approx 4$ (for acetic acid).

• Relative acidity: Carboxylic acids are stronger acids than alcohols and phenols.

Carboxylic Acids in Proteins

• Amino acids contain carboxylic acid groups, which are important for protein structure and function.

Neutralization of Carboxylic Acids

Carboxylic acids react with bases to form carboxylate salts and water.

$\ce{RCOOH + NaOH -> RCOO^- Na^+ + H_2O}$

• Carboxylate salts: More water-soluble than the parent acid.

Reactions of Carboxylic Acids

• Esterification: Reaction with alcohols to form esters (condensation reaction).

• Amide formation: Reaction with amines to form amides (substitution reaction).

Summary

• Carboxyl groups make up aldehydes, ketones, carboxylic acids, esters, and amides.

Esters

Structure and Properties

Esters are derived from carboxylic acids and alcohols. They are important in biochemistry and as fragrances/flavors.

• General formula: R-COOR'

• Intermolecular forces: Dipole-dipole interactions; no hydrogen bond donors.

• Physical properties: Lower boiling points than carboxylic acids; more volatile.

Reactions of Esters

• Hydrolysis: Esters can be hydrolyzed with water (acidic or basic conditions) to yield carboxylic acids and alcohols.

$\ce{RCOOR' + H_2O -> RCOOH + R'OH}$

• Saponification: Base-catalyzed hydrolysis of esters to form carboxylate salts and alcohols.

$\ce{RCOOR' + NaOH -> RCOO^- Na^+ + R'OH}$

Summary

• Esters do not have hydrogen bond donors and use dipole-dipole interactions.

• Alcohols can be added to carboxylic acids to form esters by esterification.

Amines

Structure and Classification

Amines are derivatives of ammonia (NH3) where one or more hydrogens are replaced by alkyl or aryl groups.

• Primary (1°) amine: One carbon group attached to nitrogen.

• Secondary (2°) amine: Two carbon groups attached to nitrogen.

• Tertiary (3°) amine: Three carbon groups attached to nitrogen.

• Quaternary ammonium: Four carbon groups attached to nitrogen (positively charged).

Properties of Amines

• 1° and 2° amines can form hydrogen bonds (both donor and acceptor).

• 3° amines only act as hydrogen bond acceptors.

• Less polar than alcohols, but still polar.

• Smelly substances: e.g., trimethylamine (C3H9N).

Basicity of Amines

• Amines are weak bases; they accept protons to form ammonium ions.

$\ce{RNH_2 + H_2O <=> RNH_3^+ + OH^-}$

Amine Salts

• Formed when amines react with acids; more water-soluble than neutral amines.

$\ce{RNH_2 + HCl -> RNH_3^+Cl^-}$

Summary

• Amines with a lone pair of electrons on N can accept a proton and act as bases.

• Amine salts are charged ions and are much more soluble in water than amines.

Amides

Formation and Structure

Amides are formed by the reaction of carboxylic acids with amines. They are important in proteins as peptide bonds.

• Condensation reaction: Carboxylic acid + amine → amide + water.

• Classification: Based on the number of carbons attached to the nitrogen (1°, 2°, 3° amides).

Properties of Amides

• 1° and 2° amides can hydrogen bond; 3° amides cannot.

• Usually water-soluble unless a large hydrophobic group is present.

• Amides are not basic due to resonance stabilization of the lone pair on nitrogen.

Hydrolysis of Amides

• Amides can be hydrolyzed under acidic or basic conditions to yield carboxylic acids or their salts and amines.

$\ce{RCONH_2 + H_2O + H^+ -> RCOOH + NH_4^+}$ $\ce{RCONH_2 + NaOH -> RCOO^- Na^+ + NH_3}$

Summary

• Amines can be added to carboxylic acids to form amides.

• Amides can be classified by the number of carbons to which they are bonded.

• Unlike amines, amides are not basic.

Thiols and Disulfides

Thiols

Thiols are sulfur analogs of alcohols, containing an -SH group. They are important in protein structure and have distinctive odors.

• General formula: R-SH

• Weaker hydrogen bonding than alcohols; more reactive.

• Characteristic 'skunky' smell.

Disulfides

• Formed by oxidation of two thiol groups.

$\ce{2 RSH <=> RSSR + 2 H^+ + 2 e^-}$

• Disulfide bonds are important in stabilizing protein structure.

Reaction with Heavy Metals

• Thiols react with heavy metals (e.g., Pb2+) to form insoluble metal-thiolates, leading to protein function loss.

Thioesters

• Thioesters are similar to esters but contain a sulfur atom in place of the oxygen atom of the ester linkage.

• Important in metabolism (e.g., acetyl-CoA).

Phosphates

Phosphate Esters and Anhydrides

Phosphate esters and anhydrides are key functional groups in biochemistry, especially in energy transfer.

• Phosphate ester: C–O–P bond, formed from reaction of phosphate with alcohol.

• Phosphoanhydride: P–O–P bond, formed from reaction of two phosphates with elimination of water.

Example: ATP (Adenosine Triphosphate)

• ATP contains phosphoanhydride bonds.

• Hydrolysis of these bonds releases energy for cellular processes.

Phosphorylation Reaction

• Transfer of a phosphate group from ATP to another molecule.

• Used to activate or deactivate proteins and enzymes.

Summary

• Phosphates are used for energy storage and transfer in cells.

• Phosphorylation is a key regulatory mechanism in metabolism and cell signaling.