carbonate ion
Summary
The carbonate ion, CO₃²⁻, is a planar, resonance-stabilized anion whose −2 charge is delocalized over three oxygens. The most testable point is that all three C–O bonds are identical, with bond order 4/3 and a length between a single and a double bond.
Detail
Meaning: Carbonate is the conjugate base of bicarbonate (HCO₃⁻), which is the conjugate base of carbonic acid. Carbon is sp² hybridized, so the ion is trigonal planar with 120° O–C–O angles. Three equivalent resonance structures each place one C=O and two C–O⁻, so the real ion is a hybrid. The charge is spread over all three oxygens, which stabilizes it. Carboxylates and nitrate follow the same pattern.
Equation: C–O bond order = 4 bonds ÷ 3 C–O connections ≈ 1.33 (unitless), where 4 is the bond count in one resonance structure (one double, two single). Each oxygen carries an average charge of −2/3 (in units of e).
On the MCAT: Expect questions that ask you to draw resonance structures, predict equal C–O bond lengths, or give geometry and hybridization. In extraction passages, aqueous sodium bicarbonate or carbonate deprotonates a carboxylic acid, giving a water-soluble carboxylate salt that moves into the aqueous layer. Neutral organics stay in the organic layer. Phenols are weaker acids, so they are generally not deprotonated by bicarbonate. Carbonate salts can also act as mild bases.
Common confusion: Students freeze one structure and predict one short C=O and two long C–O bonds. Resonance says all three bonds are the same length, intermediate between single and double. Also keep carbonate (CO₃²⁻, symmetric) separate from bicarbonate (HCO₃⁻), which has an O–H and so unequal C–O bonds.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.