Classic dye synthesis
Baeyer-Drewson indigo synthesis
How the Baeyer-Drewson indigo synthesis 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: o-Nitrobenzaldehyde, acetone, aq. NaOH. Conditions: H₂O/acetone, 20–25 °C, then air. Bonds formed: C–C (aldol), C–N, C=C (indigo central bond); bonds broken: α C–H, N–O, C–C(O)CH₃ (as acetate). Stereochemistry: Product is the E-indigo, planar and strongly H-bonded. Rate and selectivity: Aldol step fast; indigo precipitates as deep blue solid.
At a glance
Step 1: Base forms the acetone enolate
Base deprotonates acetone at its methyl group, forming an enolate — the nucleophile in this reaction.
Step 2: Aldol addition, then cyclization
That enolate attacks the carbonyl carbon of ortho-nitrobenzaldehyde in an ordinary aldol-type addition, and intramolecular cyclization involving the nitro group (reduced in the process) closes a five-membered indoxyl ring — the nitro group essentially plays the same internal-electrophile role that a tethered ester or amide plays in other named cyclizations throughout this library.
Step 3: Air oxidation: indoxyl dimerizes to indigo
Two molecules of the resulting indoxyl then dimerize through air oxidation, forming the deep blue indigo dye directly — this simple, base-mediated one-pot sequence, using nothing more exotic than aldol and cyclization chemistry already covered, is the basis of indigo's industrial manufacture for dyeing blue jeans.
Abbreviations and terms
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
How it compares with related reactions
| Baeyer-Drewson indigo synthesis | Julia-Lythgoe olefination | Aza-Wittig reaction | |
|---|---|---|---|
| Reagents | o-Nitrobenzaldehyde, acetone, aq. NaOH | 1) PhSO₂CH₂R, n-BuLi; R′CHO 2) Ac₂O 3) Na(Hg), MeOH | R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone |
| Conditions | H₂O/acetone, 20–25 °C, then air | THF, –78 °C; then Na/Hg, –20 °C | THF or toluene, 25–110 °C, anhydrous |
| Stereochemistry | Product is the E-indigo, planar and strongly H-bonded | E-alkene regardless of sulfone diastereomer (radical/anion equilibration) | Imine E/Z set at ring collapse; no stereocentre needed |
| Rate / selectivity | Aldol step fast; indigo precipitates as deep blue solid | Single-electron transfer to the sulfone is key | Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Baeyer-Drewson indigo synthesis?
o-Nitrobenzaldehyde, acetone, aq. NaOH. Conditions: H₂O/acetone, 20–25 °C, then air.
Which bonds are formed and which are broken?
Formed: C–C (aldol), C–N, C=C (indigo central bond). Broken: α C–H, N–O, C–C(O)CH₃ (as acetate).
What is the stereochemical outcome?
Product is the E-indigo, planar and strongly H-bonded
What controls the rate or the selectivity?
Aldol step fast; indigo precipitates as deep blue solid
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go