Enolate chemistry — exhaustive halogenation
Haloform reaction of a methyl ketone
How the Haloform reaction of a methyl ketone works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: I₂ (or Br₂, Cl₂, NaOCl) and NaOH. Conditions: Aqueous base, 25 °C; excess halogen. Bonds formed: C–I (×3), C–O; CHI₃ (yellow solid); bonds broken: α C–H (×3), C–C. Stereochemistry: No stereocentre formed. Rate and selectivity: Each halogenation faster than the last (acidity increases).
At a glance
Step 1: Base removes an α-hydrogen
Base deprotonates the alpha carbon of a methyl ketone; the enolate reacts with halogen (I₂, Br₂, or Cl₂) to install one halogen at a time.
Step 2: Enolate takes I₂; repeat three times
Each halogen makes the remaining alpha-hydrogens more acidic, so this repeats three times until all three are replaced, giving a trihalomethyl ketone.
Step 3: Hydroxide cleaves C–CI₃
Hydroxide then attacks the carbonyl carbon, expelling the stabilized CX₃⁻ carbanion as haloform — the basis of the classic iodoform test for methyl ketones.
Abbreviations and terms
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
How it compares with related reactions
| Haloform reaction of a methyl ketone | Aldol condensation | Base-catalyzed aldol addition of acetaldehyde | |
|---|---|---|---|
| Reagents | I₂ (or Br₂, Cl₂, NaOCl) and NaOH | NaOH or KOH (cat.) | cat. NaOH or KOH |
| Conditions | Aqueous base, 25 °C; excess halogen | H₂O/EtOH, heat 50–100 °C | H₂O / EtOH, 0–25 °C |
| Stereochemistry | No stereocentre formed | Mostly E-enal (thermodynamic) | New stereocentre — racemic |
| Rate / selectivity | Each halogenation faster than the last (acidity increases) | E1cB; loss of HO⁻ from the enolate is rate-determining | Heating promotes dehydration to the enal (aldol condensation) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Haloform reaction of a methyl ketone?
I₂ (or Br₂, Cl₂, NaOCl) and NaOH. Conditions: Aqueous base, 25 °C; excess halogen.
Which bonds are formed and which are broken?
Formed: C–I (×3), C–O; CHI₃ (yellow solid). Broken: α C–H (×3), C–C.
What is the stereochemical outcome?
No stereocentre formed
What controls the rate or the selectivity?
Each halogenation faster than the last (acidity increases)
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go