Acid-catalyzed nucleophilic acyl substitution
Fischer esterification
How the Fischer esterification 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: Carboxylic acid + excess alcohol; cat. H₂SO₄ or TsOH. Conditions: Reflux; Dean–Stark trap or excess ROH. Bonds formed: C–OR σ; H₂O; bonds broken: C–OH σ; O–H. Stereochemistry: None here (achiral). Rate and selectivity: Equilibrium (K ≈ 4 for AcOH/EtOH); driven by excess ROH or water removal.
At a glance
Step 1: Protonation of the carbonyl O
Acid catalyst (H⁺) protonates the carbonyl oxygen, making the carbonyl carbon much more electrophilic than it is uncatalyzed.
Step 2: Methanol adds
The alcohol's oxygen lone pair attacks that activated carbon, forming a tetrahedral intermediate carrying two oxygens now bonded to the same carbon.
Step 3: Proton shuffle, lose water
After proton transfers (not shown individually), water leaves and a final deprotonation gives the ester. Every step is reversible — this is why Fischer esterification is run with excess alcohol or removal of water to push the equilibrium forward.
Abbreviations and terms
- Me
- Methyl. –CH₃.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- EtOH
- Ethanol. Polar protic solvent.
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TsOH
- p-Toluenesulfonic acid. Strong organic acid catalyst, soluble in organic solvents.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
How it compares with related reactions
| Fischer esterification | Acid-catalyzed amide hydrolysis | Amide synthesis from an acid chloride and amine | |
|---|---|---|---|
| Reagents | Carboxylic acid + excess alcohol; cat. H₂SO₄ or TsOH | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | Benzoyl chloride, methylamine, aqueous NaOH |
| Conditions | Reflux; Dean–Stark trap or excess ROH | 6 M acid, reflux (hours) | CH₂Cl₂ / H₂O (biphasic), 0–25 °C |
| Stereochemistry | None here (achiral) | No stereocentre created; α-stereocentres retained | No stereocentre; planar amide |
| Rate / selectivity | Equilibrium (K ≈ 4 for AcOH/EtOH); driven by excess ROH or water removal | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis | Very fast; base neutralises HCl so the amine is not lost as its salt |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Fischer esterification?
Carboxylic acid + excess alcohol; cat. H₂SO₄ or TsOH. Conditions: Reflux; Dean–Stark trap or excess ROH.
Which bonds are formed and which are broken?
Formed: C–OR σ; H₂O. Broken: C–OH σ; O–H.
What is the stereochemical outcome?
None here (achiral)
What controls the rate or the selectivity?
Equilibrium (K ≈ 4 for AcOH/EtOH); driven by excess ROH or water removal
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
Related mechanisms
- Acid-catalyzed amide hydrolysis
- Amide synthesis from an acid chloride and amine
- Base hydrolysis (saponification) of an ester
- Base-catalyzed transesterification
- Ester synthesis from an acid chloride and alcohol
- In-line phosphoryl transfer (SN2 at phosphorus)
- Weinreb amide addition
- Acid-catalyzed hydration of an alkene