1. There are two nitrogen atoms in lidocaine, the amide nitrogen (bonded to the carbonyl [C=O] and phenyl ring), and the tertiary amine (bonded to two ethyl groups [-CH2
]). Without getting too deep into why, the tertiary amine is the only one which will demonstrate appreciable acid/base properties around physiological pH because its lone pair is more accessible. In the cationic protonated form, this nitrogen atom is bonded to three carbon atoms and one hydrogen atom. In the nonionic free base form, it is bonded only to carbons. The acid base reaction you are considering is this nitrogen atom accepting or donating a hydrogen ion (we chemists like to call them protons).
2. Compare the pKa
s of the lidocaine cation and the bicarbonate anion. The species with the lower pKa
is a better acid and will donate a proton. The species with the higher pKa
is a better base and will accept a proton. Make sure your masses and charges balance.
is the negative log of the Ka
(acid dissociation constant). An acid dissociation constant is the equilibrium constant for an acid dissolved in water. An equilibrium constant is equal to the concentration of products over the concentration of reactants when the system is in equilibrium.
Consider the cationic acid HL+
dissociating in water. The equation would be:
The Ka of this equation would be:
Ka = [H3O+][L]/[HL+]
Square brackets are short hand for molar concentration