acrylic acid

Meaning: Acrylic acid is CH₂=CH–COOH, IUPAC name propenoic acid. A vinyl group is bonded directly to the carboxyl carbon, so the C=C is conjugated with the C=O. This makes it an α,β-unsaturated carbonyl compound. By resonance the β carbon (the CH₂ end) is electron-poor, so nucleophiles can attack it in conjugate (1,4 or Michael) addition. The sp² carbon next to the carboxyl group is a little more electron-withdrawing than an sp³ carbon, so acrylic acid is slightly more acidic than propanoic acid. Its alkene can also undergo radical chain polymerization to give polyacrylic acid, and its esters (acrylates) make up many common polymers.

On the MCAT: Expect to name or draw it, rank its acidity against propanoic or acetic acid, or predict the product of adding H₂ or HBr across the C=C. Hydrogenation gives propanoic acid. A passage may ask which carbon is electrophilic, or ask you to recognize the monomer in a polymerization scheme. In spectroscopy, look for the broad O–H stretch of a carboxylic acid together with C=O and C=C absorptions.

Common confusion: Students often treat the carbonyl carbon as the only electrophilic site. In a conjugated system the β carbon is also electrophilic. Another slip is forgetting that the acidic O–H reacts first with a base. That gives the acrylate anion, which is a poorer electrophile than the neutral acid.