Carbene addition to aromatics
Reimer-Tiemann reaction
How the Reimer-Tiemann 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: CHCl₃, NaOH (aq). Conditions: Biphasic H₂O/CHCl₃, 60–70 °C; acid work-up. Bonds formed: Ar–CHO (ortho C–C); bonds broken: C–H (CHCl₃); 2 C–Cl. Stereochemistry: Ortho-selective (Na⁺ ion-pair guides carbene). Rate and selectivity: Carbene formation is rate-limiting; modest yields, some para product.
At a glance
Step 1: Base makes dichlorocarbene
Base deprotonates chloroform; alpha-elimination of chloride generates a highly reactive dichlorocarbene — a neutral carbon with only six electrons and an empty orbital.
Step 2: Phenoxide ring attacks the carbene
The phenoxide's electron-rich ring (activated by the negatively charged oxygen) attacks the electrophilic carbene carbon directly, forming a ring-carbon bond and a new C–Cl-substituted intermediate.
Step 3: Hydrolysis → o-hydroxybenzaldehyde
Hydrolysis of the resulting dichloromethyl group under the basic conditions converts it to an aldehyde — a way to formylate phenols specifically at the position ortho to the OH.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
How it compares with related reactions
| Reimer-Tiemann reaction | Achmatowicz rearrangement | Biginelli reaction | |
|---|---|---|---|
| Reagents | CHCl₃, NaOH (aq) | NBS, THF/H₂O, NaHCO₃ (or mCPBA; or VO(acac)₂/t-BuOOH; or Oxone/KBr) | Aldehyde, urea, ethyl acetoacetate, HCl or Lewis acid |
| Conditions | Biphasic H₂O/CHCl₃, 60–70 °C; acid work-up | 0 °C, then 25 °C | EtOH, reflux (or neat, 90 °C) |
| Stereochemistry | Ortho-selective (Na⁺ ion-pair guides carbene) | New anomeric center (α/β mix); substituent at C2 retained | C4 stereocentre formed (racemic unless chiral acid catalyst) |
| Rate / selectivity | Carbene formation is rate-limiting; modest yields, some para product | Oxidative furan opening is fast; hemiketalization is rapid and reversible | Iminium formation or its Mannich-type capture is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Reimer-Tiemann reaction?
CHCl₃, NaOH (aq). Conditions: Biphasic H₂O/CHCl₃, 60–70 °C; acid work-up.
Which bonds are formed and which are broken?
Formed: Ar–CHO (ortho C–C). Broken: C–H (CHCl₃); 2 C–Cl.
What is the stereochemical outcome?
Ortho-selective (Na⁺ ion-pair guides carbene)
What controls the rate or the selectivity?
Carbene formation is rate-limiting; modest yields, some para product
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