acetone
Summary
Acetone (propanone, CH₃COCH₃) is the simplest ketone. It is a polar aprotic solvent: it accepts hydrogen bonds at its carbonyl oxygen but has no O–H or N–H to donate. Its most testable role is favoring SN2 reactions by leaving nucleophiles poorly solvated.
Detail
Meaning: Acetone is propanone, CH₃COCH₃, the simplest ketone. Its carbonyl carbon is electrophilic, so it undergoes nucleophilic addition (a Grignard reagent gives a tertiary alcohol, and NaBH₄ reduces it to 2-propanol). The carbonyl oxygen can accept hydrogen bonds, but acetone has no O–H or N–H, so it cannot donate them. That makes it a polar aprotic solvent, miscible with water and many organic liquids. Its α hydrogens are weakly acidic, so base forms an enolate, and acetone interconverts with a minor enol tautomer (keto-enol tautomerism).
On the MCAT: Acetone usually appears as a solvent in substitution questions. Polar aprotic solvents do not cage anions through hydrogen bonding, so nucleophiles stay more reactive and SN2 is favored over SN1. It also appears in spectroscopy: a strong IR C=O stretch near 1700 cm⁻¹ and a single ¹H NMR singlet (six equivalent hydrogens) near 2 ppm. In enolate and aldol questions it is a standard ketone with α hydrogens.
Common confusion: Students call acetone protic because it contains hydrogens. Protic means hydrogen bonded to O or N that can be donated in a hydrogen bond. Acetone's hydrogens are all on carbon, so it is aprotic. For the same reason it boils lower than 2-propanol, which can hydrogen-bond to itself.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.