methyl ketone
Summary
A methyl ketone is a ketone with a CH₃ group on the carbonyl carbon, R–C(=O)–CH₃; acetone is the simplest. The testable point is the iodoform test: I₂ with NaOH gives a yellow CHI₃ precipitate, which most other ketones do not give.
Detail
Meaning: The carbonyl carbon is bonded to a methyl group and to one other carbon group R. Acetone and 2-butanone are examples. The α-hydrogens on the methyl are acidic because the enolate is resonance stabilized. With base and excess I₂, all three are replaced by iodine. The resulting CI₃ group is a good leaving group, so hydroxide adds to the carbonyl and ⁻CI₃ leaves. It is then protonated to iodoform, CHI₃.
Equation: R–CO–CH₃ + 3 I₂ + 4 OH⁻ → R–COO⁻ + CHI₃ + 3 I⁻ + 3 H₂O. R is an alkyl or aryl group, and the other species have no units. Acidifying the carboxylate gives the carboxylic acid.
On the MCAT: A passage gives an unknown compound and a test result. A yellow precipitate with I₂/NaOH points to a methyl ketone. Expect to name the carboxylic acid product, which has one fewer carbon than the ketone. Spectra also point to it: IR C=O near 1715 cm⁻¹ and a 3H singlet near 2.1 ppm in ¹H NMR.
Common confusion: Students assume only methyl ketones are positive. Ethanol, acetaldehyde and secondary alcohols of the form CH₃CH(OH)R also test positive, because I₂/base first oxidizes them to a methyl carbonyl. Ketones lacking the CH₃–C=O unit, such as 3-pentanone, test negative.
Sources
- Organic Chemistry (Clayden)
- 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.