Purifying Purifying Acetanilide by Acetanilide by

RecrystallizationRecrystallization

DefinitionDefinition Recrystallization:Recrystallization: purification process used to remove impurities from organic purification process used to remove impurities from organic

compounds which are solid at room temperature.compounds which are solid at room temperature.

Principle:Principle: Solubility of a compound in a solvent increases with temperature.Solubility of a compound in a solvent increases with temperature. As the solution cools, crystals form and grow.As the solution cools, crystals form and grow. Molecules in a crystal have a greater affinity for molecules of the same kind than Molecules in a crystal have a greater affinity for molecules of the same kind than

for impurities.for impurities.

Criteria for selecting recrystallizing solvent

The compound being purified must be insoluble in solvent at room temp but soluble in boiling solvent.

Solvent’s boiling point must be lower than the compound’s melting point. (Why?)

An abundant quantity of crystals must be recoverable from the cool solvent.

Choosing a Choosing a Recrystallization SolventRecrystallization Solvent Options:Options:

ImpuritiesImpurities should be should be solublesoluble in the chosen solvent in the chosen solvent at all temperaturesat all temperatures so that they so that they stay in solution.stay in solution.

ORORImpuritiesImpurities should be should be insoluble at all temperaturesinsoluble at all temperatures so they can be filtered from the so they can be filtered from the hot solution.hot solution.

Example:Example: A compound X has the following characteristics in solvents A, B, and C.A compound X has the following characteristics in solvents A, B, and C.

AA BB CC

Room Room TemperatureTemperature SolubleSoluble InsolubleInsoluble InsolubleInsoluble

Solvent’s Solvent’s Boiling PointBoiling Point SolubleSoluble SolubleSoluble SolubleSoluble

Note:Note:

1- Be careful that your 1- Be careful that your compound does not melt before compound does not melt before the solvent boils.the solvent boils.

2- Make sure that you record 2- Make sure that you record your observations in a chart your observations in a chart like the one shown here.like the one shown here.

Step 1 – Choosing the Right Step 1 – Choosing the Right SolventSolvent

WaterWater EthanolEthanol AcetoneAcetone Petroleum Petroleum EtherEther

Dissolves at Room Temperature?

YES NO

STOPBad Solvent

Dissolves?

YES NO

STOPBad Solvent

CRYSTALS?

YESGood Solvent

NOBad Solvent

Heat

Ice-water bath

Sample added to solvent

Step 2 – Dissolving the Step 2 – Dissolving the CompoundCompound Dissolving the CompoundDissolving the Compound

Dissolve the compound in a Dissolve the compound in a MINIMUMMINIMUM volume of boiling solvent. volume of boiling solvent. After all of the compound dissolves, add 5% excess to prevent premature After all of the compound dissolves, add 5% excess to prevent premature

crystallization. (For example: if 10 mL of boiling solvent is required to just dissolve a crystallization. (For example: if 10 mL of boiling solvent is required to just dissolve a compound, 0.5 mL will be added)compound, 0.5 mL will be added)

11- Heat some solvent to - Heat some solvent to boiling. Place the solid boiling. Place the solid to be recrystallized in an to be recrystallized in an Erlenmeyer flask.Erlenmeyer flask.

22- Add small amount (1 - Add small amount (1 mL) of the hot solvent mL) of the hot solvent into the flask.into the flask.

(Use a Pasteur pipet)(Use a Pasteur pipet)

33- Swirl the flask- Swirl the flask

44- Place the flask on the - Place the flask on the hot plate to keep the hot plate to keep the solution warmsolution warm

55- Repeat steps 2 to 4 until you have - Repeat steps 2 to 4 until you have dissolved the compound, and then add the 5% dissolved the compound, and then add the 5% excess.excess.

Step 3 - DecolorizingStep 3 - Decolorizing

Decolorizing the SolutionDecolorizing the Solution Add a spatula tip of activated carbon (or decolorizing carbon) Add a spatula tip of activated carbon (or decolorizing carbon)

after the solution is removed from the heat and has stopped after the solution is removed from the heat and has stopped boiling !!! boiling !!! Stir in the carbon and then allow it to sit briefly.

Activated carbon has a surface area that adsorbs dissolved Activated carbon has a surface area that adsorbs dissolved organic substances: avoid adding too much because carbon can organic substances: avoid adding too much because carbon can also adsorb your compound!!also adsorb your compound!!

Hot Filtration – Right After DecolorizingHot Filtration – Right After Decolorizing The hot solution is filtered (in this step) by gravity filtration through The hot solution is filtered (in this step) by gravity filtration through

a funnel a funnel (stemless)(stemless) containing a fluted filter paper. Keep the containing a fluted filter paper. Keep the paper and funnel warm to avoid crystals forming during filtration.paper and funnel warm to avoid crystals forming during filtration.

Decolorizing the SolutionDecolorizing the Solution

Assemble the gravity filtration Assemble the gravity filtration apparatus using a beaker.apparatus using a beaker.

Preheat: Boil 20 mL of solvent Preheat: Boil 20 mL of solvent (boiling chip) and pour the boiling (boiling chip) and pour the boiling solvent over filter paper. Leave the solvent over filter paper. Leave the apparatus on hot plate.apparatus on hot plate.

Remove the solvent used to preheat Remove the solvent used to preheat the apparatus from the beaker fast the apparatus from the beaker fast and then filter the solution containing and then filter the solution containing the carbon.the carbon.

Let the filtrate cool at room Let the filtrate cool at room temperature, and then in an ice-temperature, and then in an ice-water bath.water bath.

Folding a Fluted Filter Folding a Fluted Filter PaperPaper

1- Fold the paper in half. 1- Fold the paper in half. Crease sharply.Crease sharply.

2- Fold in half again.2- Fold in half again.

3- Fold each quarter in 3- Fold each quarter in half again, with all half again, with all creases facing up.creases facing up.

4- Fold each eighth in half again but this 4- Fold each eighth in half again but this time make the folds in the opposite time make the folds in the opposite direction to the one you have been doing.direction to the one you have been doing.

Step 4 – Forming CrystalsStep 4 – Forming Crystals Recrystallizing Pure CompoundRecrystallizing Pure Compound

The solution is now allowed to The solution is now allowed to slowly coolslowly cool at room temperature. (If at room temperature. (If crystal formation occurs too rapidly, impurities may become trapped in crystal formation occurs too rapidly, impurities may become trapped in the crystals).the crystals).

Then the solution is further cooled in an ice-water bath to maximize Then the solution is further cooled in an ice-water bath to maximize crystal formation.crystal formation.

TIP:TIP: If crystals do not form, induce crystallization yourself:If crystals do not form, induce crystallization yourself:

- scratch the inside of the flask with a glass stirring rod- scratch the inside of the flask with a glass stirring rod

- or add a seed crystal of pure compound- or add a seed crystal of pure compound

Collecting, Washing, and Drying the CrystalsCollecting, Washing, and Drying the Crystals Vacuum filtration is used to separate the crystals from the mother Vacuum filtration is used to separate the crystals from the mother

liquor. liquor. Pre-weigh your filter paper and record mass.Pre-weigh your filter paper and record mass.

Clamp the filter flask to a stand. A Büchner funnel is fitted to the Clamp the filter flask to a stand. A Büchner funnel is fitted to the flask with a rubber adaptor. Place filter paper in the funnel, and flask with a rubber adaptor. Place filter paper in the funnel, and finally connect the flask by tubing to a vacuum source.finally connect the flask by tubing to a vacuum source.

Step 5 – Isolating CrystalsStep 5 – Isolating Crystals

Isolating CrystalsIsolating Crystals

Disconnect the vacuum. Carefully remove the filter paper and Disconnect the vacuum. Carefully remove the filter paper and solid from the funnel. Set the filter cake on a watch glass to solid from the funnel. Set the filter cake on a watch glass to dry.dry.

Wet filter paper with solvent. Turn on vacuum. The solution containing the Wet filter paper with solvent. Turn on vacuum. The solution containing the crystals is poured onto the filter. After the mother liquor has been pulled crystals is poured onto the filter. After the mother liquor has been pulled through the filter, wash crystals with small portions of through the filter, wash crystals with small portions of ice cold solventice cold solvent..

Isolating CrystalsIsolating Crystals

Sandwich your crystals between two Sandwich your crystals between two filter papers. Let your crystals dry for filter papers. Let your crystals dry for 1 week in your lab drawer.1 week in your lab drawer.

Next WeekNext Week Measure and record the mass and Measure and record the mass and

melting point.melting point.

Submit your crystals to the TA in a Submit your crystals to the TA in a labeled vial.labeled vial.

Note:Note: Label your vial with your Label your vial with your name, TA’s last name, mass of name, TA’s last name, mass of compound, and MPcompound, and MP

Step 6 – Recovery AnalysisStep 6 – Recovery Analysis (Next Week)(Next Week)

Percent RecoveryPercent Recovery is calculated by dividing the mass of the is calculated by dividing the mass of the recrystallized compound by the mass of the crude compound before recrystallized compound by the mass of the crude compound before recrystallization.recrystallization.

mass of recrystallized compound, gmass of recrystallized compound, g

% Recovery = % Recovery = ————————————————— x 100————————————————— x 100

mass of crude compound, gmass of crude compound, g

Assessing Purity:Assessing Purity: Observe colorObserve color Measure melting pointMeasure melting point

Procedure CautionsProcedure Cautions Wear your goggles at all time.Wear your goggles at all time.

Chemicals:Chemicals: Acetanilide is toxic and irritating. Don’t touch it with your bare hands use Acetanilide is toxic and irritating. Don’t touch it with your bare hands use

appropriate tools to handle it. Avoid inhaling dust.appropriate tools to handle it. Avoid inhaling dust. All of the organic solvents are flammable, keep them away from heat All of the organic solvents are flammable, keep them away from heat

sources. sources.

Be careful to avoid burns from the hot solvents, hot glassware, or hot plates!Be careful to avoid burns from the hot solvents, hot glassware, or hot plates!

Good to know if you want to have a good grade:Good to know if you want to have a good grade: Moving a flask from the hot plate to a Moving a flask from the hot plate to a cold benchcold bench may cause breaking. may cause breaking.

Place your flask on a paper towel instead of directly on the bench.Place your flask on a paper towel instead of directly on the bench. Adding Adding activated carbonactivated carbon to a boiling solution, may cause boiling over. Let to a boiling solution, may cause boiling over. Let

it cool a little bit before adding the carbon.it cool a little bit before adding the carbon. Be quick at the hot filtration step to avoid Be quick at the hot filtration step to avoid premature crystallizationpremature crystallization in the in the

filter.filter. You will be graded on your percent recovery and purity.You will be graded on your percent recovery and purity.

Have Fun!Have Fun!