Medicinal Chemistry: Acids, Bases, and Drug Absorption (Cheat Sheet)

This set of vocabulary flashcards reviews the definitions of acids and bases, the distinction between protonated and unprotonated forms, and the relationship between pKa, pH, and lipid solubility for drug absorption.

Acid

A compound that can donate a proton ($H^+$) to water.

Base

A compound that can accept a proton ($H^+$) from water.

Protonated (P)

The chemical species with the extra hydrogen atom.

Unprotonated (U)

The chemical species with one less hydrogen atom.

$$HA$$

The protonated ($P$) and unionized form of an acid.

$$A^-$$

The unprotonated ($U$) and ionized form of an acid.

$$BH^+$$

The protonated ($P$) and ionized form of a base.

B

The unprotonated ($U$) and unionized form of a base.

Stronger Acid

Characterized by a larger $Ka$ and a smaller $pKa$ value.

$$pKa + pKb = 14$$

The mathematical relationship between the $pKa$ and $pKb$ of conjugate pairs.

Stronger Base (in terms of $pKa$)

A base that has a LARGE $pKa$ value.

The Golden Equation

The medicinal chemistry version of the Henderson-Hasselbalch equation: $P/U = 10^{(pKa - pH)}$.

Effect of lowering pH on Acids

Favors the formation of more $HA$, which is the unionized form.

Effect of lowering pH on Bases

Favors the formation of more $BH^+$, which is the ionized form.

Acid in Acid / Base in Base

Environments where the compound becomes neutral.

Acid in Base / Base in Acid

Environments where the compound becomes ionized.

Unionized Form

The lipid soluble form of a drug that diffuses into the blood stream for absorption.

Ionized Form

The water soluble form of a drug that is NOT absorbed well.

Stomach (Acidic Environment)

The location where acidic drugs absorb best.

Intestine (Basic Environment)

The location where basic drugs absorb best.