formic acid
Summary
Formic acid (HCOOH, methanoic acid) is the simplest carboxylic acid, with an H where other carboxylic acids have an alkyl group. It is a weak acid, but stronger than acetic acid (pKa about 3.75 versus 4.76), because no electron-donating alkyl group destabilizes its conjugate base, formate.
Detail
Meaning: Formic acid, HCOOH, is methanoic acid, the simplest carboxylic acid: a carbonyl carbon bonded to OH and to H rather than an alkyl group. It is a weak acid, yet stronger than acetic acid (pKa about 3.75 versus 4.76). An alkyl group donates electron density and slightly destabilizes the carboxylate anion, while H does not, so formate is relatively more stable and the acid dissociates more.
Equation: Ka = [H₃O⁺][HCOO⁻] / [HCOOH], where brackets are equilibrium molar concentrations (mol/L), HCOO⁻ is formate, and pKa = −log Ka.
On the MCAT: Passages ask you to rank carboxylic acid acidity. Electron-withdrawing groups (such as Cl or F near the carboxyl) raise acidity and alkyl groups lower it, so trichloroacetic acid > formic acid > acetic acid. Formic acid also appears as the product when methanol is oxidized to formaldehyde and then to the acid, and as the first member of a homologous series in boiling point comparisons.
Common confusion: Formic acid is not a strong acid. With a pKa near 3.75 it only partly dissociates in water, unlike HCl. Students also assume more carbons means more acidic, but acidity falls as alkyl groups are added.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- 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.