Electrophilic aromatic substitution — ortho-formylation
Duff reaction: ortho-selective phenol formylation
How the Duff reaction 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: Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺. Conditions: Heat, 100–150 °C; aqueous hydrolysis. Bonds formed: Ar–C (ortho CH=N then CHO); bonds broken: C–N (HMTA cage); C=N (hydrolysis). Stereochemistry: Ortho-selective (OH directs; H-bonded iminium). Rate and selectivity: Moderate; electron-rich phenols only.
At a glance
Step 1: HMTA gives an iminium; phenol attacks
Hexamethylenetetramine and a phenol combine under acid; the phenol's very electron-rich ring attacks an iminium-like electrophile generated from the hexamine cage — mechanistically related to the Delépine reaction's use of the same hexamine reagent, just as an electrophile source here instead of a nucleophile.
Step 2: Ortho aminomethylation, then oxidation
This installs an aminomethyl group specifically ortho to the phenol's OH, directed there by the strong activating and ortho/para-directing effect of the hydroxyl — the same directing-group logic that governs every EAS reaction in this library.
Step 3: Hydrolysis to the o-hydroxy aldehyde
Acidic hydrolysis of the resulting imine reveals the aldehyde directly at that ortho position — this gives phenolic ortho-aldehydes (like salicylaldehyde derivatives) more reliably than the Reimer-Tiemann reaction's dichlorocarbene chemistry covered earlier.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
- TFA
- Trifluoroacetic acid. A strong organic acid (pKa ≈ 0.5), used to remove Boc groups and protonate carbonyls.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
How it compares with related reactions
| Duff reaction | Bromination of benzene (FeBr₃-catalyzed) | Friedel-Crafts acylation (contrast with alkylation) | |
|---|---|---|---|
| Reagents | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ | Br₂ with FeBr₃ (Lewis-acid catalyst) | RCOCl, AlCl₃ (≥1 equiv) |
| Conditions | Heat, 100–150 °C; aqueous hydrolysis | Anhydrous, 25 °C, dark | CH₂Cl₂ or CS₂, 0–25 °C; aqueous work-up |
| Stereochemistry | Ortho-selective (OH directs; H-bonded iminium) | None (planar, achiral) | No rearrangement; ketone deactivates ring (no poly-acylation) |
| Rate / selectivity | Moderate; electron-rich phenols only | Slow step = arenium formation; deactivated rings react poorly | Acylium formation fast; arenium formation is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Duff reaction?
Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺. Conditions: Heat, 100–150 °C; aqueous hydrolysis.
Which bonds are formed and which are broken?
Formed: Ar–C (ortho CH=N then CHO). Broken: C–N (HMTA cage); C=N (hydrolysis).
What is the stereochemical outcome?
Ortho-selective (OH directs; H-bonded iminium)
What controls the rate or the selectivity?
Moderate; electron-rich phenols only
Study guides that help with this reaction
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam