Nucleophilic acyl substitution — simple ester synthesis
Ester synthesis from an acid chloride and alcohol
How the Ester synthesis from an acid chloride and alcohol 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: Acid chloride + alcohol; pyridine or Et₃N (optional). Conditions: CH₂Cl₂ or Et₂O, 0–25 °C. Bonds formed: Ester C–O; HCl; bonds broken: C–Cl, O–H, C=O π (temporarily). Stereochemistry: Alcohol stereocentre retained (C–O bond not broken). Rate and selectivity: Very fast and irreversible; HCl scavenged by base.
At a glance
Step 1: Alcohol attacks the carbonyl carbon
An alcohol's oxygen lone pair attacks the acid chloride's highly electrophilic carbonyl carbon — acid chlorides are among the most reactive carbonyl electrophiles in this library, since chloride is such a good leaving group.
Step 2: Collapse expels chloride
This forms a tetrahedral intermediate that collapses almost immediately, expelling chloride as the C=O reforms — the same addition-elimination pattern as every nucleophilic acyl substitution covered throughout this library, just proceeding unusually fast.
Step 3: Ester and HCl
The result is an ester, formed cleanly and irreversibly (unlike the Fischer esterification's acid-catalyzed equilibrium) since HCl is evolved as a gas and chloride is a poor enough nucleophile not to reverse the reaction — often run with a mild base like pyridine present just to mop up the HCl byproduct.
Abbreviations and terms
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Et3N
- Triethylamine. Common tertiary-amine base that scavenges HCl.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
How it compares with related reactions
| Ester synthesis from an acid chloride and alcohol | Acid-catalyzed amide hydrolysis | Amide synthesis from an acid chloride and amine | |
|---|---|---|---|
| Reagents | Acid chloride + alcohol; pyridine or Et₃N (optional) | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | Benzoyl chloride, methylamine, aqueous NaOH |
| Conditions | CH₂Cl₂ or Et₂O, 0–25 °C | 6 M acid, reflux (hours) | CH₂Cl₂ / H₂O (biphasic), 0–25 °C |
| Stereochemistry | Alcohol stereocentre retained (C–O bond not broken) | No stereocentre created; α-stereocentres retained | No stereocentre; planar amide |
| Rate / selectivity | Very fast and irreversible; HCl scavenged by base | 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 Ester synthesis from an acid chloride and alcohol?
Acid chloride + alcohol; pyridine or Et₃N (optional). Conditions: CH₂Cl₂ or Et₂O, 0–25 °C.
Which bonds are formed and which are broken?
Formed: Ester C–O; HCl. Broken: C–Cl, O–H, C=O π (temporarily).
What is the stereochemical outcome?
Alcohol stereocentre retained (C–O bond not broken)
What controls the rate or the selectivity?
Very fast and irreversible; HCl scavenged by base
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