Structure

A carboxylic acid has the structural unit -COOH: a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon. That one carbon carries two different oxygen-based groups, and the electronic interaction between them is what gives carboxylic acids their distinctive chemistry.

Hybridization and Geometry

The carboxylic acid carbon is sp² hybridized. Its three sp² orbitals form:

1. One sigma bond to the C=O oxygen.
2. One sigma bond to the O-H oxygen.
3. One sigma bond to the R group (an alkyl, aryl, or H).

The remaining p orbital forms the pi bond with the C=O oxygen, completing the C=O double bond.

Because of sp² geometry, the three sigma bonds lie in a plane at 120° angles. The two oxygens (and the R group) are all coplanar with the carbon.

Bond Lengths Reveal Resonance

The C-O bonds in neutral carboxylic acids are NOT identical:

• C=O bond: about 1.21 Å (shorter, double bond).
• C-OH bond: about 1.31 Å (longer, single bond).

For comparison, a typical C-O single bond in an alcohol is 1.43 Å - longer than the carboxylic acid’s C-OH. The carboxylic acid’s C-OH is shorter because partial double-bond character from resonance of the O-H lone pair into the C=O pi system pulls them closer.

In the conjugate base (carboxylate, RCOO⁻), the two C-O bonds become EQUIVALENT - both about 1.26 Å. This averaging comes from perfect resonance: both oxygens share the negative charge and bond order equally (each C-O is 1.5 in the hybrid).

The Two Oxygens Have Different Roles

In the neutral acid:

• Carbonyl O (C=O): more electronegative partial charge concentration; accepts hydrogen bonds.
• Hydroxyl O (O-H): donates the acidic proton AND can also accept hydrogen bonds via its remaining lone pair.

In the conjugate base (after deprotonation), both oxygens are equivalent and both carry partial negative charge.

Drawing Carboxylic Acids

Common representations:

• Expanded: R-C(=O)-O-H or R-C(=O)-OH.
• Condensed: R-COOH or R-CO₂H.
• Skeletal: R with a triangle-wedge showing the carbonyl and a hydroxyl on the same carbon.

On the MCAT, recognize -COOH, -CO₂H, and the structural formula interchangeably. They all mean the same thing.

Contrast with Related Functional Groups

• Aldehyde: -CHO. One C=O, one C-H. No OH. Not acidic at the O (no OH). Alpha-H is weakly acidic.
• Ketone: R-CO-R’. One C=O, two C-R. No OH. Not acidic at O. Alpha-H is weakly acidic.
• Ester: R-CO-OR’. One C=O, one C-OR’. No OH, no acid.
• Amide: R-CO-NR’₂. One C=O, one C-NR’₂. No acidic H at the amide itself (N-H is only weakly acidic).
• Carboxylic acid: R-CO-OH. One C=O, one C-OH. The OH is acidic (pKa ~4-5).

Only the carboxylic acid has the -OH directly attached to the acyl C=O, and that is what gives it its strong acidity compared to the others.