To determine which base will work to deprotonate a given compound (acid), the key factor is comparing the acid strength (pKa) of the compound and the conjugate acid of the base:
- The base can deprotonate the acid if the conjugate acid of the base has a higher pKa (weaker acid) than the acid being deprotonated. This means the equilibrium favors deprotonation.
- In simpler terms, deprotonation happens if the base's conjugate acid is less acidic than the compound.
For example:
- Phenol, with pKa around 10, can be deprotonated by a base whose conjugate acid has pKa greater than 10, such as ethoxide (conjugate acid ethanol pKa ~16).
- Terminal alkynes (pKa ~25) can be deprotonated by bases like sodium amide, whose conjugate acid ammonia has pKa ~38.
The general rule is:
Any base whose conjugate acid is weaker (higher pKa) than the acid to be
deprotonated can work as a base for deprotonation.
This applies broadly:
- Stronger acids require very strong bases with weak conjugate acids.
- Weaker acids require less strong bases with relatively stronger conjugate acids.
Thus, to choose an appropriate base, identify the compound's pKa and pick a base whose conjugate acid has a higher pKa value. If the conjugate acid has a lower pKa, deprotonation will not occur effectively. This principle allows determination of which bases will work to deprotonate which compounds in acid- base chemistry.
