Organic Chemistry Lab Final Experiment 3

What is the purpose of Experiment 3?

To purify the unknown component from Experiment 2 and determine its identity and final purity.

What percentage of Panacetin is the unknown component?

50% by mass.

Which lab categories apply to Experiment 3?

Purification and Analysis.

Is an organic reaction essential in Experiment 3?

No.

What purification method is used in Experiment 3?

Recrystallization.

What is recrystallization?

A technique used to purify solid compounds by dissolving an impure solid in a hot solvent and allowing pure crystals to form upon slow cooling.

When is recrystallization most effective?

When the solid is contaminated with relatively small amounts of solid impurities.

When does recrystallization become difficult?

When the solubility of the impurity and the product are very similar.

What is required for successful recrystallization?

A solvent where the compound is very soluble when hot and poorly soluble when cold.

What are common downsides of recrystallization?

Low product recovery and trial-and-error solvent selection.

What solvent is used in Experiment 3?

Water

Why is water a good recrystallization solvent in Experiment 3?

It does not react with the compound, dissolves the unknown at high temperature but not low temperature, boils below the compound’s melting point, and is volatile enough to dry crystals.

What happens if too much solvent is used in recrystallization?

The compound remains too soluble and recrystallization may not occur.

What happens if too little solvent is used in recrystallization?

The solid will not completely dissolve.

Why should solubility data be used to calculate solvent volume?

To ensure enough solvent is used to dissolve the compound without preventing recrystallization.

What is the first step in recrystallization?

Add near-boiling solvent to the solid and heat until dissolved.

What should be done if insoluble solid remains in hot solvent?

Perform hot gravity filtration to remove insoluble impurities.

Why must the solution cool slowly to room temperature?

Slow cooling allows pure crystals to form and excludes impurities.

Why is the solution placed in an ice bath after cooling?

To ensure complete recrystallization of the compound.

What filtration technique is used to collect recrystallized solid?

Vacuum filtration.

Why is minimal cold solvent used when rinsing crystals?

To prevent loss of product due to solubility.

How is percent recovery calculated in Experiment 3?

Mass of recrystallized product divided by initial mass of crude sample × 100%.

What types of impurities can exist during recrystallization?

Soluble, insoluble, and slightly soluble impurities.

How are soluble impurities removed during recrystallization?

They remain dissolved in the solvent and are removed with the filtrate.

How are insoluble impurities removed during recrystallization?

They are removed by hot gravity filtration.

Why are slightly soluble impurities difficult to remove?

They may recrystallize along with the desired compound.

Why is recrystallization still effective despite slightly soluble impurities?

Slow crystal growth favors formation of purer crystals.

What are the two possible identities of the unknown in Experiment 3?

Acetanilide or phenacetin.

What technique is used to determine identity and purity of the unknown?

Melting point analysis.

Why is melting point useful in Experiment 3?

It quickly distinguishes between acetanilide and phenacetin and indicates purity.

What are the melting points of the possible unknowns?

Acetanilide: ~114 °C
Phenacetin: ~135 °C

How do impurities affect melting point?

They lower the melting point and broaden the melting range.

Why must the sample be completely dry before melting point analysis?

Residual solvent acts as an impurity and lowers/broadens the melting point.

How is melting point reported?

As a temperature range from first sign of melting to complete liquefaction.

Why are mixed melting point tests performed?

To confirm identity by comparing melting behavior with known compounds.

What does it mean if a 1:1 mixture with acetanilide has no melting point depression?

The unknown is acetanilide.

What does it mean if a mixed sample shows melting point depression?

The compounds are different.

Why does melting point analysis work in this experiment but not others?

Because there are only two known solid possibilities with distinct melting points.

What would residual aspirin contamination do to melting point results?

Lower and broaden the melting point range.

What would happen if too much water was used for phenacetin recrystallization?

Phenacetin may remain dissolved, reducing recovery.

Which purification methods can be used for solids?

Recrystallization, chromatography, sublimation

Which purification methods can be used for liquids?

Distillation and chromatography.

Why is recrystallization preferred in Experiment 3?

It requires no specialized instrumentation and effectively purifies solids.

Why must purification be done before analysis in Experiment 3?

Purification must be completed before analysis because impurities would interfere with melting point and lead to incorrect identification.

How does purification improve the accuracy of analysis in Experiment 3?

Purification removes contaminants so physical properties reflect only the unknown compound.

Why are literature melting points necessary for identifying the unknown in Experiment 3?

Melting point values are compared to previously reported literature values to identify unknowns.

How do impurities affect melting point measurements?

Impurities lower and broaden the melting point range.

What does a narrow melting point range indicate about purity?

A pure compound melts sharply over a narrow temperature range.

What happens if the unknown is not completely dry before melting point analysis?

Residual solvent acts as an impurity and depresses the melting point.

How would water remaining in the sample affect the melting point result?

The melting point would be lower and broader than the literature value.

What effect would residual aspirin have on the unknown’s melting point?

Aspirin contamination would depress and broaden the melting point range.

Why does adding excess water decrease percent recovery in recrystallization?

Too much solvent can dissolve the product and reduce crystal formation.

What specific risk occurs if excess water is used when recrystallizing phenacetin?

Phenacetin may stay dissolved if too much solvent is used.

What can happen if recrystallization occurs too quickly?

The compound may oil out or fail to crystallize properly.

Why is slow cooling preferred over rapid cooling in recrystallization?

Slow cooling allows orderly crystal formation that excludes impurities.

Why is hot filtration performed if solid remains in boiling solvent?

Hot gravity filtration removes insoluble impurities before crystallization.

Why can rapid cooling reduce purity during recrystallization?

Cooling the solution too quickly can trap impurities in crystals.

How is percent recovery calculated in Experiment 3?

Percent recovery compares recrystallized mass to the initial impure mass.

Why must purification be done before analysis in Experiment 3?

Purification removes contaminants so physical properties reflect only the unknown compound.

What effect would residual aspirin have on the unknown’s melting point?

Aspirin contamination would depress and broaden the melting point range.

What happens if there is residual solvent in the unknown during melting point analysis?

Residual solvent acts as an impurity and depresses the melting point.

What is the purpose of using hot gravity filtration in recrystallization?

Hot gravity filtration removes insoluble impurities before crystallization occurs.

What happens if too much solvent is used during recrystallization?

Too much solvent can dissolve the product and reduce crystal formation, lowering recovery.

Why does slow cooling allow for pure crystal formation?

Slow cooling enables the crystal lattice to form orderly, excluding impurities.

What is the ideal solvent for recrystallization?

An ideal solvent should dissolve the compound at high temperatures and allow it to crystallize upon cooling.