Acid-base chemistry is one of the main tools for predicting how organic molecules react. In organic chemistry, an acid donates H+ and a base accepts H+, but the most useful question is often which side of an equilibrium is favored. The pKa scale lets you compare acids quantitatively, so you can decide whether a proton transfer is likely to happen.
This matters because many reaction mechanisms begin with protonation or deprotonation.
Key Facts
- Lower pKa means stronger acid, and higher pKa means weaker acid.
- Acid strength increases when the conjugate base is more stable.
- For HA + B- ⇌ A- + HB, equilibrium favors the side with the weaker acid, which has the higher pKa.
- Approximate pKa values: carboxylic acids 4 to 5, phenols about 10, alcohols about 16, alkynes about 25, alkanes about 50.
- Resonance, electronegativity, induction, hybridization, and atom size can stabilize a conjugate base.
- A useful estimate is ΔpKa = pKa(product acid) - pKa(reactant acid), and favorable proton transfer usually has ΔpKa > 0.
Vocabulary
- pKa
- pKa is a number that measures acid strength, with smaller values meaning the acid gives up H+ more easily.
- Conjugate base
- A conjugate base is the species left after an acid donates a proton.
- Resonance stabilization
- Resonance stabilization occurs when charge is spread over multiple atoms by delocalized electrons.
- Inductive effect
- The inductive effect is the pulling or pushing of electron density through sigma bonds by nearby atoms or groups.
- Proton transfer
- A proton transfer is an acid-base step in which H+ moves from an acid to a base.
Common Mistakes to Avoid
- Comparing acids by the strength of the bond to hydrogen alone is wrong because acidity depends strongly on the stability of the conjugate base after H+ leaves.
- Thinking a negative charge always means a strong base is wrong because a resonance-stabilized or electronegative atom can hold negative charge with low basicity.
- Using pKa values backward is wrong because the stronger acid has the lower pKa, while equilibrium favors formation of the weaker acid with the higher pKa.
- Ignoring solvent and functional group context is wrong because acid-base behavior can change when ions are stabilized differently or when nearby groups withdraw or donate electron density.
Practice Questions
- 1 Acetic acid has pKa 4.8 and ethanol has pKa 16. Will ethoxide deprotonate acetic acid? Calculate ΔpKa using ethanol as the product acid.
- 2 Phenol has pKa 10 and acetylene has pKa 25. Which conjugate base is stronger, phenoxide or acetylide, and by about how many pKa units do their conjugate acids differ?
- 3 Explain why a carboxylic acid is much more acidic than an alcohol even though both contain an O-H bond.