Organic Chemistry Functional Groups

Organic Chemistry and Drug Relevance

Organic chemistry underpins all of pharmacology. Every drug molecule contains one or more functional groups — specific arrangements of atoms that determine chemical reactivity, physical properties, and biological behaviour. Understanding these groups allows you to predict how drugs are absorbed, distributed, metabolised, and excreted.

Key Functional Groups

Hydroxyl groups (–OH) are found in alcohols. In drug molecules, they increase water solubility and enable hydrogen bonding with receptors. Ethanol and morphine both contain hydroxyl groups. Phase II metabolism often adds glucuronic acid to hydroxyl groups to aid renal excretion.

Carboxylic acids (–COOH) are weak acids (pKa ~4–5). At physiological pH 7.4 they are largely ionised (–COO⁻), limiting membrane permeability. Aspirin and ibuprofen contain carboxylic acid groups. Ionisation is critical for their renal excretion and plasma protein binding.

Amines (–NH₂, –NHR, –NR₂) are weak bases (pKa ~8–10). At physiological pH, many amines are partially protonated, affecting their ability to cross lipid membranes. Most neurotransmitters (dopamine, serotonin, adrenaline) and many drugs (amoxicillin, fluoxetine) contain amine groups. The Henderson-Hasselbalch equation predicts the ionised fraction.

Aldehydes (–CHO) are reactive electrophiles. They are uncommon in stable drugs due to reactivity but are important metabolic intermediates — acetaldehyde is the toxic product of ethanol metabolism by alcohol dehydrogenase.

Esters (–COOR) are formed from a carboxylic acid and an alcohol. They are hydrolysed by plasma esterases, making them useful as prodrugs. Aspirin is an acetyl ester; it is hydrolysed in plasma to salicylic acid. This instability limits ester drugs to topical or short-acting applications.

Amides (–CONHR) resemble esters but are far more stable to hydrolysis. Paracetamol contains an amide linkage. Amide bonds are also the peptide bonds linking amino acids in proteins.

Henderson-Hasselbalch in Pharmacology

For a weak acid: fraction ionised = 1 / (1 + 10^(pKa – pH))

At pH 7.4, aspirin (pKa 3.5) is 99.99% ionised — it stays in plasma. In the acidic stomach (pH 2), aspirin is largely unionised and absorbed. This principle guides formulation of enteric-coated tablets and understanding drug distribution across compartments.

Clinical Pearl

Functional group knowledge predicts drug interactions with metabolic enzymes. Cytochrome P450 enzymes oxidise amines, hydroxylate aromatic rings, and dealkylate ethers. Identifying these groups in a drug structure immediately flags potential CYP-mediated interactions.