Upper secondary · 20 min

Reflux synthesis of an ester

Heat a carboxylic acid with an alcohol under a vertical condenser: vapour condenses and returns to the flask, so volatile reagents can boil for a long time without loss.

Goal

Explain why esterification is run at reflux, name each part of the apparatus, and connect the technique to the equilibrium character of the reaction.

Apparatus and reagents

Acetic acid and ethanol, a few drops of concentrated sulfuric acid, round-bottom flask, reflux (Liebig) condenser with water hoses, heating bath and boiling chips.

Procedure

Identify each part of the rig: the round flask holding the reaction mixture, the vertical water condenser, the water inlet/outlet and the heat source.
Follow the path of the vapour: it leaves the boiling liquid, enters the condenser jacket, liquefies on the cold wall and drips back — the flask contents never escape.
Compare with simple distillation: in reflux the condensate returns to the same flask (mass is conserved), while in distillation it is collected in a receiver (components are separated).
Relate the duration of reflux to the slow, reversible esterification: keeping the mixture at its boiling point for a long time lets the equilibrium approach its maximum conversion.

What to observe

Vapour rises only part-way up the condenser and liquefies there; the liquid level in the flask stays nearly constant over time.
Cooling water enters at the bottom of the jacket and leaves at the top, so the condenser is always filled and efficiently cooled.

Explanation

Reflux keeps a reaction at the boiling point without losing material: the condenser returns every drop of vapour to the flask. For the equilibrium CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O this lets a slow reaction run long enough to reach equilibrium; the apparatus drawn in the simulation is the teaching version with a vertical Liebig condenser.

History of the experiment

The water-cooled condenser became standard laboratory glassware after Justus von Liebig popularised his design around 1830, finally allowing chemists to boil volatile mixtures for hours. Emil Fischer exploited this in the 1890s when he worked out the acid-catalysed esterification that still bears his name, turning reflux into the workhorse of organic synthesis.

Chemists behind it

Justus von Liebig Emil Fischer