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.
Understanding Chemistry: Acid-Base Chemistry of Organics
A proton transfer changes the charge pattern of a molecule. That change can decide whether a carbon atom becomes reactive, whether a leaving group can depart, or whether a nucleophile is switched off. For example, a carbonyl compound often has hydrogens next to the carbonyl group.
A base can remove one of these hydrogens to form an enolate. The negative charge in an enolate is shared between carbon and oxygen, so resonance makes this species much more stable than a simple carbon anion. This is why hydrogens next to carbonyl groups are far more acidic than most hydrogens on an alkane.
When comparing acids, draw the conjugate bases and inspect where the negative charge can go. Resonance is especially important when several valid structures spread the charge across different atoms. A charge placed on oxygen is usually more stable than one placed on carbon because oxygen attracts electrons more strongly.
Nearby electron withdrawing groups can help too. Chlorine atoms, fluorine atoms, and positively charged groups pull electron density through single bonds.
Their effect becomes weaker as they are placed farther from the acidic hydrogen. This distance effect explains why substitution patterns can noticeably change acidity.
The orbital holding a negative charge matters for carbon acids. Carbon with more s character holds electrons closer to the nucleus. An sp carbon in a terminal alkyne therefore stabilizes negative charge better than an sp2 carbon in an alkene or an sp3 carbon in an alkane.
This trend helps explain why a very strong base can remove the hydrogen from a terminal alkyne, producing an acetylide ion. That acetylide can then form a new carbon to carbon bond with a suitable electrophile. In this way, acidity data leads directly to a common bond building method.
Solvents can change the practical result of an acid base step. Water and alcohols surround charged particles well, which can stabilize ions. Some organic solvents do not do this as effectively, so bases may behave differently from expectations based on aqueous values.
Students should identify the actual acid, base, conjugate acid, and conjugate base before using any numbers. Then compare the acid on each side, not the basic species by themselves.
A small difference in acidity may give a mixture at equilibrium, while a large difference usually makes the preferred direction clear. In multistep mechanisms, track every proton transfer because it often prepares the molecule for the next reaction step.
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.