Carboxylic Acids

Structure and Properties

Carboxylic acids are organic compounds containing a carbonyl group (C=O) bonded to a hydroxyl group (–OH), forming the carboxyl functional group (–COOH). They are more oxidized than aldehydes and ketones and are key intermediates in biological and chemical processes.

• Hydrogen Bonding: Carboxylic acids can both donate and accept hydrogen bonds, leading to strong intermolecular forces.

• Solubility: They are usually water-soluble unless the hydrophobic portion of the molecule is large (e.g., fatty acids).

• Acidity: Carboxylic acids are weak acids, typically with pKa values around 3–4. They can donate a proton to water, forming carboxylate ions.

Example: Acetic acid (CH3COOH) is a common carboxylic acid with a pKa of about 4.8.

Acid Strength Comparison: HCl > Carboxylic acid > Phenol (in terms of acid strength).

Carboxylate Salts

When carboxylic acids react with bases (such as NaOH), they form carboxylate salts and water. These salts are ionic and generally more water-soluble than the parent acid.

• Reaction:

• Application: Soaps are sodium or potassium salts of fatty acids (carboxylate salts).

Biological Relevance

Carboxylic acids are found in amino acids (e.g., glutamate) and play a role in protein structure and function. The ionization state of carboxylic acids in proteins depends on the pH relative to their pKa.

Esters

Structure and Properties

Esters are derived from carboxylic acids and alcohols. They contain a carbonyl group bonded to an oxygen atom, which is further bonded to another carbon (–COOR).

• Intermolecular Forces: Esters have dipole-dipole interactions but lack hydrogen bond donors, making them more volatile and lower boiling than carboxylic acids.

• Solubility: Esters can hydrogen bond with water (as acceptors), but are less soluble than carboxylate salts.

Example: Triacylglycerols (triglycerides) are esters formed from glycerol and fatty acids.

Formation and Hydrolysis

• Esterification: (acid-catalyzed)

• Hydrolysis: Esters can be hydrolyzed back to acids and alcohols by water or base (saponification).

• Saponification:

Amines

Structure and Classification

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

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

• Secondary (2°): Two carbons attached

• Tertiary (3°): Three carbons attached

• Quaternary (4°) ammonium: Four carbons, positively charged

Physical Properties

• 1° and 2° amines can hydrogen bond (donor and acceptor), while 3° amines only accept hydrogen bonds.

• Amines are usually water-soluble unless they have large hydrophobic groups.

• Many amines have strong odors (e.g., putrescine, cadaverine).

Chemical Properties

• Basicity: Amines are weak bases and can accept protons to form ammonium ions.

• Amine Salts: Reaction with acids forms water-soluble amine salts (e.g., drug formulations).

Example: Fluoxetine-HCl (Prozac) is an amine salt, increasing its water solubility for oral administration.

Biological Relevance

• Amines are present in amino acids (e.g., lysine) and nucleotides (heterocyclic amines in DNA/RNA).

• Many neurotransmitters and hormones are amines (e.g., serotonin, epinephrine).

Amides

Structure and Properties

Amides are formed from the reaction of carboxylic acids and amines. They contain a carbonyl group bonded to a nitrogen atom (–CONH2, –CONHR, or –CONR2).

• Classification: Primary (1°), secondary (2°), and tertiary (3°) amides, depending on the number of carbons attached to nitrogen.

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

• Basicity: Amides are not basic due to resonance delocalization of the nitrogen lone pair.

Biological Relevance

Amide bonds (peptide bonds) link amino acids in proteins. Hydrolysis of amides (acidic or basic) yields carboxylic acids and amines or their salts.

Thiols and Thioesters

Thiols

Thiols are sulfur analogs of alcohols (–SH group). They have a strong odor and are more reactive than alcohols.

• Intermolecular Forces: Dipole-dipole interactions; no hydrogen bonding due to the size of sulfur.

• Oxidation: Thiols can be oxidized to form disulfides (–S–S–), a reversible reaction important in protein structure (e.g., cysteine to cystine).

Thioesters

Thioesters are similar to esters but contain a sulfur atom in place of the single-bonded oxygen. They are formed from carboxylic acids and thiols and are important in metabolism (e.g., acetyl-CoA).

Phosphate Compounds

Phosphoesters and Phosphoanhydrides

Phosphoesters contain a C–O–P bond, formed from the reaction of phosphate with alcohols. Phosphoanhydrides contain a P–O–P bond, formed from the condensation of two phosphates.

• Phosphoesters: Found in nucleotides and nucleic acids (DNA/RNA backbone).

• Phosphoanhydrides: High-energy bonds in molecules like ATP; breaking these bonds releases energy for cellular processes.

Phosphorylation

Phosphorylation is the transfer of a phosphate group from one molecule (often ATP) to another, a key regulatory and metabolic process in cells.

• Example: The first step of glycolysis is the phosphorylation of glucose to glucose-6-phosphate by ATP.

Summary Table: Key Functional Groups and Properties