Pharmaceutical Chemistry and Medicinal Chemistry
Core concepts
You must be able to move between a drug's functional groups and its behavior: acidity/basicity (pKa), ionization at physiological pH, partition coefficient (logP), solubility, and stability. Know the classic structure–activity relationships: the β-lactam ring and its susceptibility to β-lactamases, the catechol/amine pattern of sympathomimetics and how methylation or ring substitution changes receptor selectivity, and the ester/amide distinction that governs hydrolytic lability. Understand prodrug design (masking a polar group to improve absorption, then relying on esterases or pH for release), stereochemistry (many pairs differ in potency or toxicity, e.g., enantiomers with different receptor affinity), and the chemistry behind degradation pathways: hydrolysis, oxidation, photolysis, and racemization.
Question traps
Questions often ask which form predominates at a given pH - remember the Henderson–Hasselbalch logic rather than a memorized percentage. A weak acid at pH below its pKa is mostly unionized (more lipid-permeable); a weak base at pH below its pKa is mostly ionized. Another trap is confusing a structural feature with a pharmacologic class: not every drug with an aromatic ring is lipophilic enough to cross the blood–brain barrier. Also watch for "which change increases oral absorption" items where the correct answer is a small, nonpolar modification, not simply "add a charge."
Worked rule of thumb
For a weak base with pKa 8, at plasma pH 7.4 the ratio ionized:unionized is roughly 10^(8−7.4) ≈ 4:1, so most is ionized and less membrane-permeable; acidifying the compartment traps more drug. Use this ratio reasoning to answer distribution and "ion trapping" questions.
Study approach
Map each drug class to its key pharmacophore and one metabolic soft spot. That single mental table answers most medicinal chemistry items.