Reductive amination — formic acid variant
Eschweiler-Clark reaction
How the Eschweiler-Clark 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: Amine, excess CH₂O (formalin), excess HCO₂H. Conditions: Aqueous, 80–100 °C. Bonds formed: N–CH₃ (×2); CO₂; H₂O; bonds broken: N–H; C=O (CH₂O); H–CO₂H. Stereochemistry: Chiral centres α to N are not racemised. Rate and selectivity: Hydride transfer to the iminium is rate-limiting; CO₂ loss makes it irreversible.
At a glance
Step 1: Amine + CH₂O → iminium ion
An amine condenses with formaldehyde to form an iminium ion — the same Schiff-base chemistry covered throughout this library.
Step 2: Formate delivers hydride, loses CO₂
Formic acid, rather than a metal hydride, serves as the hydride donor: it transfers a hydride directly to the iminium carbon while itself losing CO₂ (a decarboxylative hydride transfer, related in spirit to the Cannizzaro's hydride shift).
Step 3: Repeat until tertiary
The result is exhaustive N-methylation — because this cycle repeats with each remaining N–H, a primary amine is driven all the way to a tertiary N,N-dimethylamine using nothing but formaldehyde and formic acid as reagents.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Eschweiler-Clark reaction | Leuckart-Wallach reaction | Borch reductive amination | |
|---|---|---|---|
| Reagents | Amine, excess CH₂O (formalin), excess HCO₂H | Ketone, HCO₂NH₄ (or HCONH₂ / R₂NH + HCO₂H) | Amine, ketone/aldehyde, NaBH₃CN |
| Conditions | Aqueous, 80–100 °C | Neat, 160–185 °C | MeOH, pH 6–7 (AcOH), 25 °C |
| Stereochemistry | Chiral centres α to N are not racemised | Racemic product at a new stereocentre | New stereocentre usually racemic (unless chiral amine/ketone) |
| Rate / selectivity | Hydride transfer to the iminium is rate-limiting; CO₂ loss makes it irreversible | High temperature needed; iminium formation reversible, hydride transfer irreversible | Iminium formation is rate-limiting; iminium reduced much faster than ketone at this pH |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Eschweiler-Clark reaction?
Amine, excess CH₂O (formalin), excess HCO₂H. Conditions: Aqueous, 80–100 °C.
Which bonds are formed and which are broken?
Formed: N–CH₃ (×2); CO₂; H₂O. Broken: N–H; C=O (CH₂O); H–CO₂H.
What is the stereochemical outcome?
Chiral centres α to N are not racemised
What controls the rate or the selectivity?
Hydride transfer to the iminium is rate-limiting; CO₂ loss makes it irreversible
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