carboxyl group
Summary
The carboxyl group is –COOH: a carbonyl carbon (C=O) also bonded to an OH. It defines carboxylic acids and is a weak acid because the carboxylate left after losing H⁺ is resonance stabilized. Most testable: pKa is typically near 4–5, so it is deprotonated at physiological pH.
Detail
Meaning: A carboxyl group is a carbonyl carbon (C=O) also bonded to a hydroxyl, written –COOH. After losing H⁺, the carboxylate (–COO⁻) shares its negative charge over two oxygens by resonance, so the group is a weak acid with a pKa typically near 4–5, far more acidic than an alcohol (about 16). Nearby electron-withdrawing atoms such as halogens stabilize the carboxylate further and increase acidity.
Equation: pH = pKa + log([RCOO⁻]/[RCOOH]), where pH and pKa are unitless and the brackets are concentrations in mol/L. If pH > pKa, the carboxylate form predominates.
On the MCAT: Expect questions that rank the acidity of substituted acids, give the charge of an amino acid or peptide C-terminus at a stated pH, or identify the group by IR (broad O–H band about 2500–3300 cm⁻¹, C=O near 1700 cm⁻¹). Reactions tested include conversion to esters and amides and reduction to primary alcohols by LiAlH₄.
Common confusion: Carboxyl is not the same as carbonyl alone, and its OH does not act as a base like hydroxide. Its acidity comes from resonance stabilization of the carboxylate. It is still a weak acid compared with HCl.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.