What happens when a ketone is mixed with NaOH?

1 Answer
Dec 28, 2015

It undergoes an aldol condensation with itself.

Explanation:

Let's use acetone as an example. The reaction involves several steps.

Step 1. Proton abstraction to form a resonance-stabilized enolate ion.

#"HO"^(-) + underbrace("CH"_3"COCCH"_3)_color(red)("acetone") ⇌ underbrace([stackrelcolor (blue)("-")("C")"H"_2"COCH"_3 ⟷ "CH"_2"=C(CH"_3")-"stackrelcolor (blue)("-")("O")])_color(red)("enolate ion") + "H"_2"O"#

Step2. The enolate anion attacks the carbonyl carbon in another acetone molecule.

#"CH"_3"CO" stackrelcolor (blue)("-")("C")"H"_2 + "CH"_3"COCH"_3 → "CH"_3"COCH"_2"C(CH"_3")"_2"-"stackrelcolor (blue)("-")("O")#

Step 3. Protonation of the enolate ion to form an α-hydroxyketone.

#"CH"_3"COCH"_2"C(CH"_3")"_2"-"stackrelcolor (blue)("-")("O") + "H"_2"O" → underbrace("CH"_3"COCH"_2"-C"("CH"_3)_2"-OH")_color(red)("4-hydroxy-4-methylpentan-2-one") + "OH"^-#

Step 4. Dehydration to form an α,β-unsaturated ketone.

#"CH"_3"COCH"_2"-C"("CH"_3)_2"-OH" → underbrace("CH"_3"COCH=C(CH"_3")"_2)_color(red)("4-methylpent-3-en-2-one") + "H"_2"O"#