The Reimer-Tiemann Reaction

What the Reimer-Tiemann reaction does

Give a phenol a hot bath in aqueous sodium hydroxide, drip in chloroform, and after a few hours you can steam-distill out an oil that smells of almonds and meadowsweet: salicylaldehyde (2-hydroxybenzaldehyde). In a single pot you have converted a plain phenol into an ortho-hydroxy aromatic aldehyde — a substituent pattern that is otherwise awkward to reach. The reaction is the classic teaching example of carbene chemistry doing useful synthesis, and for over a century it was the standard bench route to salicylaldehyde.

The overall transformation looks deceptively simple:

    phenol  +  CHCl₃  +  3 NaOH  ──(60-70 °C, aq)──→  salicylaldehyde  +  3 NaCl  +  2 H₂O

Three equivalents of base are stoichiometrically required: one to deprotonate the phenol, one to deprotonate chloroform, and one to neutralize the acid produced during hydrolysis of the benzal-chloride intermediate. The single carbon that becomes the new CHO group comes entirely from the chloroform.

The mechanism, arrow by arrow

The Reimer-Tiemann reaction is an electrophilic aromatic substitution in which the electrophile is a carbene, not a cation. Four distinct events happen in sequence:

1. Make the phenolate. Hydroxide removes the O-H proton of the phenol (pKa ≈ 10). The resulting phenoxide is far more nucleophilic than neutral phenol because the oxygen lone pair is delocalized onto the ring, building up negative charge at the ortho and para carbons.
2. Make the carbene. Hydroxide also deprotonates chloroform (CHCl₃, pKa ≈ 24 — surprisingly acidic because three chlorines stabilize the anion) to give trichloromethide, CCl₃⁻. This anion undergoes α-elimination: it expels a chloride ion from the same carbon, leaving a neutral, six-electron carbon — dichlorocarbene, :CCl₂. In its singlet ground state the carbene has a filled sp² lone pair and an empty p-orbital, so it behaves as a hungry electrophile.
3. The ring attacks the carbene. The nucleophilic ortho carbon of the phenoxide donates a pair of π electrons into the empty p-orbital of :CCl₂. This breaks aromaticity and forms a cyclohexadienyl-type intermediate bearing a -CHCl₂ (dichloromethyl) group and, temporarily, a proton on the same sp³ carbon. Loss of that proton to base rearomatizes the ring, giving an ortho-(dichloromethyl)phenolate.
4. Hydrolyze to the aldehyde. The benzal-chloride group (Ar-CHCl₂) is a masked aldehyde. Under the aqueous basic conditions, hydroxide displaces the two chlorines. The first substitution gives an unstable chlorohydrin/hemiacetal-type species; loss of the second chloride and a proton collapses the carbon to a carbonyl. The product is the salicylaldehyde phenolate, which is protonated to salicylaldehyde on acidic workup.

  1)  Ar-OH   +  OH⁻   →  Ar-O⁻          (phenolate; nucleophilic ortho/para)
  2)  CHCl₃   +  OH⁻   →  CCl₃⁻  +  H₂O
      CCl₃⁻            →  :CCl₂  +  Cl⁻   (α-elimination → dichlorocarbene)
  3)  Ar-O⁻   +  :CCl₂ →  [dienyl]  →  o-(Ar)-CHCl₂   (S_EAr at ortho carbon)
  4)  o-Ar-CHCl₂  +  2 OH⁻  →  o-Ar-CHO  +  2 Cl⁻  +  H₂O   (hydrolysis of benzal chloride)

The load-bearing insight is step 2: the reaction is really carbene chemistry wearing an electrophilic-aromatic-substitution costume. The empty p-orbital on :CCl₂ is what makes it electrophilic, and the fact that it can add to a ring carbon that already carries a substituent (see the "abnormal" pathway below) is the fingerprint that proves a carbene — not a formyl cation — is the intermediate.

Reagents, conditions, and practical recipe

• The base. Aqueous NaOH or KOH, typically 3-6 M and used in large excess (5-10 equivalents). The base has three jobs: form the phenolate, generate the carbene, and hydrolyze the benzal chloride. Too little base and the reaction stalls; the carbene needs a continuous supply.
• The one-carbon source. Chloroform (CHCl₃) for aldehydes; carbon tetrachloride (CCl₄) if you want the carboxylic acid variant. Chloroform is used in modest excess (1.5-3 equivalents) because much of it is wasted to direct hydrolysis and to carbene decomposition.
• Temperature. 60-70 °C is standard. The mixture is biphasic (aqueous base over a chloroform layer), so vigorous stirring or reflux is needed to keep the phases in contact; the carbene is born at the interface.
• Solvent / phase-transfer. Classic conditions are simply water/chloroform. Modern improvements add a phase-transfer catalyst (a quaternary ammonium salt) or run in ethanol-water to raise the effective carbene concentration in the organic phase, pushing yields up.
• Workup. Acidify with dilute H₂SO₄ or HCl to protonate the phenolate, then steam-distill: salicylaldehyde is volatile and co-distills, while the less-volatile para-hydroxybenzaldehyde stays behind. This distillation step is itself a clean way to separate the ortho from the para product.

Selectivity: why ortho wins

The phenoxide oxygen carries a full negative charge that resonance delivers to the ortho and para ring positions. Both are activated toward the electrophilic carbene, yet the Reimer-Tiemann reaction is decidedly ortho-selective — typically the ortho:para ratio runs from about 2:1 to 4:1 depending on conditions. Two effects favor ortho attack:

• Ion pairing / chelation. The sodium counterion sits on the phenolate oxygen. The incoming dichlorocarbene, and later the developing benzal-chloride/dichloromethide, can coordinate to that same sodium, holding the electrophile next to the oxygen — geometrically on the ortho carbon. This templating effect is absent at the para position.
• Short, tight transition state. The ortho approach lets the reacting atoms cluster close to the oxygen anion where the electron density is highest at the moment of attack.

When both ortho positions are blocked — as in 2,6-dimethylphenol — the reaction is forced to the para carbon and gives the para aldehyde. And when the para position is blocked (a para-substituted phenol), the carbene may attack an already-substituted carbon and be trapped as an abnormal cyclohexadienone (below). No stereocenters are created, so there is no stereochemistry to control: the product is a flat aromatic aldehyde.

Reimer-Tiemann vs other ring formylations

	Reimer-Tiemann	Duff reaction	Vilsmeier-Haack	Gattermann-Koch
Electrophile	Dichlorocarbene :CCl₂	Iminium from HMTA	Chloroiminium (from DMF + POCl₃)	Formyl cation surrogate (CO/HCl)
Reagents	CHCl₃ + NaOH (aq)	Hexamethylenetetramine + acid	DMF + POCl₃	CO + HCl + AlCl₃/CuCl
Substrate scope	Phenols, some electron-rich heterocycles	Phenols and anilines	Electron-rich arenes (phenols, amines, heterocycles)	Simple arenes (benzene, toluene)
Regioselectivity	ortho > para	ortho	usually para	para
Typical yield	Low (20-40%)	Moderate (30-60%)	High (70-95%)	Moderate-high
Conditions	Aqueous, biphasic, 60-70 °C	Reflux in acetic acid / glyceroborate	0 °C → reflux, anhydrous	High-pressure CO, Lewis acid
Signature product	Salicylaldehyde	Salicylaldehyde, o-vanillin	p-Dimethylaminobenzaldehyde	p-Tolualdehyde

Worked example: phenol → salicylaldehyde

Salicylaldehyde is the historical target and the reaction that made Reimer and Tiemann famous.

    C₆H₅OH  +  CHCl₃  ──NaOH (aq, ~6 eq), 65 °C, 3 h──→  2-HO-C₆H₄-CHO   (salicylaldehyde)
                                                          +  minor 4-HO-C₆H₄-CHO (para)

• Charge. Phenol (1.0 equiv) dissolved in aqueous NaOH (roughly 5-6 equiv, ~4 M). Chloroform (about 1.5-2 equiv) added dropwise with vigorous stirring, keeping the pot near 65 °C.
• Reaction. The carbene is generated in situ at the aqueous/organic interface and captured by the phenolate. Excess chloroform and heat sustain a steady carbene flux over 2-4 h.
• Workup. Acidify to pH < 3 with dilute H₂SO₄; steam-distill. The ortho aldehyde carries over with the steam (it is intramolecularly hydrogen-bonded, hence volatile and non-polar); the para isomer, which hydrogen-bonds intermolecularly, remains in the pot.
• Yield. Typically 20-40% of salicylaldehyde on a classic prep — modest, but the starting materials are cheap and the separation is clean.

The same setup on o-cresol gives 3-methylsalicylaldehyde; on guaiacol (2-methoxyphenol) it gives o-vanillin, a flavor compound. Swap chloroform for carbon tetrachloride and the product becomes salicylic acid — the direct precursor to aspirin — via a trichloromethyl intermediate that hydrolyzes to -COOH.

Limitations and the "abnormal" side reaction

The Reimer-Tiemann reaction is prized for teaching and criticized for yield. The main leaks:

• Carbene wastage. Free dichlorocarbene is hydrolyzed by hydroxide to carbon monoxide and formate faster than it is captured by the ring, so most of the :CCl₂ that forms never reaches the substrate. This is the single biggest reason yields sit at 20-40%.
• Direct chloroform hydrolysis. Some CHCl₃ is simply hydrolyzed by the aqueous base (to formate and chloride) without ever forming productive carbene.
• The abnormal reaction. With a para-substituted phenol, the carbene can add to the para carbon that already bears a group (an ipso attack). That carbon becomes sp³ and cannot rearomatize by losing a proton — there is no proton to lose. Instead the intermediate is trapped as a stable 4-substituted-4-(dichloromethyl)cyclohexa-2,5-dienone. p-Cresol, for instance, yields this abnormal dienone alongside the normal ortho aldehyde. Its existence is the classic mechanistic proof that a carbene, capable of leaving a gem-dichloride behind, is the true intermediate.
• Tar and over-reaction. The strongly basic, hot conditions degrade sensitive substrates and the aldehyde product itself (Cannizzaro-type disproportionation of the aldehyde under base is possible), lowering isolated yield further.

    Normal (ortho, rearomatizes):        Abnormal (ipso at blocked para, cannot rearomatize):

        O⁻                                    O
        |                                     ‖
      [ring]-CHCl₂  ──→  [ring]-CHO         [ring]=  with sp³ C bearing R and CHCl₂
      (loses H⁺, aromatic)                  (a 4-R-4-CHCl₂-cyclohexa-2,5-dienone; trapped)

History: Reimer, Tiemann, and 1876

The reaction was published in 1876 by Karl Reimer and Ferdinand Tiemann, working in August Wilhelm von Hofmann's circle in Berlin. Reimer first reported that chloroform and base converted phenol into salicylaldehyde; Tiemann, a specialist in flavor and fragrance chemistry, extended and characterized the reaction (he is also co-credited with the first commercial synthesis of vanillin from coniferin). The Reimer-Tiemann reaction arrived in the same fertile decade as many named aromatic reactions, and it was the first practical laboratory route to a hydroxy-aromatic aldehyde. Its enduring value is pedagogical: it is one of the cleanest demonstrations that a divalent-carbon carbene can be generated in water and put to synthetic use.

Applications and modern practice

• Salicylaldehyde and salen ligands. Salicylaldehyde is a building block for salen ligands (from salicylaldehyde + a diamine), which chelate metals for Jacobsen-type asymmetric epoxidation catalysts and many coordination complexes.
• Fragrance and flavor. o-Vanillin and related ortho-hydroxy aldehydes made this way carry warm, vanilla-almond notes; Tiemann's own career linked the reaction to the fragrance industry.
• Coumarins and chromones. Salicylaldehyde condenses (Perkin, Knoevenagel) to coumarin and related benzopyranones — dyes, fluorescent probes, and anticoagulant scaffolds.
• Heterocycle formylation. The reaction also formylates electron-rich heterocycles such as pyrrole (giving 2-formylpyrrole) and indole, extending it beyond simple phenols.
• When yield matters, chemists switch. Because 20-40% is unappealing at scale, industrial and research syntheses of salicylaldehyde today usually favor higher-yielding formylations (Duff with hexamethylenetetramine, or a Vilsmeier route) or, for the carboxylic acid, the industrial Kolbe-Schmitt carboxylation of sodium phenolate with CO₂ under pressure that makes salicylic acid on the megatonne scale. The Reimer-Tiemann reaction survives as a teaching classic and a convenient one-pot bench method rather than a workhorse process.