Esterification & Hydrolysis — Bond Breaking with ¹⁸O Isotopic Tracing
Watch the acid-loses-OH, alcohol-loses-H mechanism unfold in 3D. Track oxygen-18 through the reaction to see who really donates the water molecule.
About this simulation
- What
- An interactive Chemistry simulation of Esterification & Hydrolysis — Bond Breaking with ¹⁸O Isotopic Tracing.
- Who
- Designed for AP, IB, and A‑Level Chemistry students.
- How
- Runs in any modern browser — drag, adjust, and explore in real time.
Updated 2026-05-18
Key Concepts
Acyl-Oxygen Cleavage
During Fischer esterification the carboxylic acid loses its entire -OH group (acyl-oxygen bond breaks), while the alcohol only loses its -H. The released -OH and -H combine to form water.
Isotopic Labelling with ¹⁸O
By replacing the oxygen in ethanol with the heavier ¹⁸O isotope, chemists tracked its fate after reaction. The ¹⁸O ended up in the ester, not in the water, proving that the acid contributes the oxygen in H₂O.
Reversible Equilibrium
Esterification is an equilibrium reaction. Under acidic conditions, water attacks the same C-O ester bond (acyl-oxygen cleavage in reverse), regenerating the original acid and alcohol.
Exam specification coverage
Why Does It Matter Which Bond Breaks?
In Fischer esterification, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to produce an ester and water. The overall equation — CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O — hides a subtle question: both reactants contain a hydroxyl group (-OH), so which one actually donates its oxygen to the water molecule?
The answer was settled by isotopic labelling experiments. Chemists synthesised ethanol with oxygen-18 (¹⁸O) and ran the esterification. Mass spectrometry of the products revealed that ¹⁸O appeared exclusively in the ester, while the water contained only ordinary ¹⁶O. This proved definitively that the carboxylic acid loses its entire hydroxyl group, and the alcohol only loses a hydrogen atom.
Understanding the exact site of bond cleavage — known as acyl-oxygen fission — is critical for predicting the outcomes of related reactions such as saponification (alkaline hydrolysis), transesterification, and polyester synthesis.