Topic 20 · page 11 of 15

Reflux, washing and solvent extraction

Reflux lets a slow reaction run hot without losing anything. Washing then removes acid and water-soluble impurities from the crude product, and solvent extraction pulls a product out of water into a liquid that does not mix with it.

15 min read20.5(i)20.5(ii)20.5(iii)

Heating under reflux 20.5(i)

Many organic reactions are slow at room temperature and need hours of heating. The reactants and the solvent are often volatile, and many are flammable. In an open flask they would boil away. Reflux solves this.

A reflux apparatus. A round-bottomed flask holds the reaction mixture and anti-bumping granules and sits in a heating mantle. A vertical Liebig condenser stands on the flask and is open at the top. Cooling water enters at the bottom of the condenser jacket and leaves at the top. Arrows show vapour rising and liquid running back. — Reflux. Water in at the bottom, out at the top. The top of the condenser is open.

PracticalHeating under reflux

Put the reactants and a few anti-bumping granules in a round-bottomed flask, no more than half full.
Clamp the flask over the heater. Fit a vertical condenser to it, held upright with a clamp.
Connect the water so that it enters at the bottom of the condenser and leaves at the top.
Leave the top of the condenser open.
Heat until the mixture boils gently, so that the vapour condenses in the lower part of the condenser. Continue for the time stated.
Turn off the heat and let the apparatus cool before taking it apart.

Feature	Reason
vertical condenser	vapour condenses and returns to the flask, so nothing is lost
water in at the bottom, out at the top	the jacket stays full of water, so cooling is effective
top open	a sealed apparatus builds up pressure and could explode
anti-bumping granules	the liquid boils smoothly, without sudden large bubbles
electric heater or water bath for flammable liquids	no naked flame near flammable vapour

Reflux keeps everything in the flask. Distillation (page 12) is the opposite: it takes a substance out of the flask as it boils.

Purification by washing 20.5(ii)

After a preparation the crude liquid product is rarely pure. It contains water, unreacted acids, a catalyst such as sulfuric acid, and by-products. Washing removes soluble impurities by shaking the crude liquid with another liquid, usually an aqueous solution, in a separating funnel. The product must not mix with the washing liquid, so that the layers can be separated.

Wash with	Removes
water	water-soluble impurities: salts, small alcohols and acids that dissolve
sodium carbonate solution	acid impurities, such as unreacted carboxylic acid or sulfuric acid catalyst

Sodium carbonate removes acid by turning it into a salt that dissolves in the water layer. Carbon dioxide forms.

\ce{2CH3COOH(aq) + Na2CO3(aq) -> 2CH3COONa(aq) + H2O(l) + CO2(g)}

\ce{2H+(aq) + CO3^2-(aq) -> H2O(l) + CO2(g)}

PracticalWashing with sodium carbonate solution

Pour the crude liquid into a separating funnel and add sodium carbonate solution. Put the stopper in.
Shake gently, then invert the funnel and open the tap to release the carbon dioxide. Close the tap and repeat until the pressure no longer builds up.
Let the layers settle. Remove the stopper and run off the lower layer.
Wash again with water to remove any remaining sodium carbonate or salts.

The gas stops forming when all the acid has been removed.

Worked examplePurifying a crude ester

The crude ester made from ethanoic acid, ethanol and concentrated sulfuric acid contains ethanol, ethanoic acid and sulfuric acid, as well as water.

Shake with sodium carbonate solution: it removes the ethanoic acid and the sulfuric acid, which dissolve as salts, giving CO₂. Open the tap regularly to release the gas.
Shake with water: it removes the remaining ethanol, which is soluble in water, and any salt.
Run off the lower aqueous layer and keep the layer that contains the ester.
Dry the ester (page 12) and distil it.

Solvent extraction 20.5(iii)

Washing removes impurities from the organic layer. Solvent extraction does the reverse: it takes the product out of an aqueous mixture into an organic solvent.

A good extraction solvent is:

immiscible with water, so two layers form,
one in which the product is more soluble than in water,
unreactive towards the product, and
volatile, so it can be removed easily afterwards, for example by distillation.

A separating funnel with a stopper at the top and a tap at the bottom. It holds two layers. The upper layer is labelled organic layer, less dense. The lower layer is labelled aqueous layer, more dense. A beaker below the tap catches the lower layer. — A separating funnel. The denser liquid is the lower layer and leaves first through the tap.

MethodSolvent extraction

Put the aqueous mixture in the funnel and add a small portion of solvent.
Stopper, shake and vent. Let the layers separate.
Run off the lower layer. Keep both layers until the product is found.
Return the aqueous layer to the funnel and repeat with a fresh portion of solvent.
Combine the solvent portions. Dry them and remove the solvent.

The denser liquid is the lower layer. Water is about 1.0 g cm⁻³. Most organic solvents such as ether and hexane are less dense and form the upper layer, but dichloromethane is denser than water and forms the lower layer.

Using several small portions of solvent is better than one large portion. Each fresh portion takes out a fraction of the product that is left, so more is removed in total.

Check yourself

Describe what happens in a reflux apparatus and why the condenser is vertical.
A student fits a bung to the top of the condenser before heating a flammable liquid under reflux. Explain the danger.
Why is a crude ester washed with sodium carbonate solution? Include an equation and say why the tap is opened.
A student connects the cooling water so that it enters at the top of the condenser. Give two problems this causes.
Explain why three extractions with 30 cm³ of ether remove more product than one extraction with 90 cm³.
Dichloromethane (density 1.33 g cm⁻³) is shaken with an aqueous solution of a product in a separating funnel. Which layer holds the product, and how do you collect it?

Answers cover these until you have tried the questions

The mixture boils and the vapour rises into the condenser, where it is cooled, condenses and runs back into the flask. The condenser is vertical so the liquid drains back by gravity. Nothing is lost, so the mixture can be heated for a long time.
The apparatus would be sealed. As the liquid heats, the vapour pressure rises with nothing to release it, and the flask or condenser could burst or explode, spreading flammable liquid and vapour.
The crude ester contains acid impurities (unreacted carboxylic acid and the sulfuric acid catalyst). Sodium carbonate removes them as water-soluble salts: $\ce{2H+(aq) + CO3^2-(aq) -> H2O(l) + CO2(g)}$ . The tap is opened to release the CO₂ that builds up pressure in the funnel.
The jacket does not fill with water, so the condenser cools badly and vapour escapes. The water also leaves unevenly, with air bubbles. (Water should enter at the bottom and leave at the top.)
Each fresh portion of solvent starts with none of the product, so it takes out a fraction of the product still in the water. Three portions remove a fraction three times, which takes out more in total than one extraction with the same total volume of solvent.
Dichloromethane is denser than water, so it is the lower layer, and the product (more soluble in it) is there. Open the tap (stopper out) and run the lower layer into a flask, then keep the water layer until you are sure of the product.