Organic Chemistry 2 Lab Midterm Review

What is refluxing?

a technique to heat a reaction mixture without losing volatile components

• uses a condenser to cool and return vapors to the reaction flask

Why is refluxing important?

allows prolonged heating for reactions while retaining solvent

When would you use reflux in a lab experiment?

extraction of caffeine from tea leaves

Extraction basics

separates compounds based on solubility in two immiscible layers (organic vs aqueous)

• often involves acid-base chemistry to manipulate solubility

Separatory funnel

used to separate layers based on physical property

How do you determine which layer is organic or aqueous?

water addition confirms the aqueous layer

What happens when you add a base like sodium bicarbonate to a mixture?

to deprotonate the benzoic acid

• sodium bicarbonate reacts with benzoic acid to form a water-soluble salt, aiding in separation

Layers separate in a separatory funnel based on

density differences (organic vs aqueous)

What is Thin Layer Chromatography (TLC)?

a method to analyze mixtures and monitor reactions

• measures how far compounds travel in a stationary phase (Rf values)

What does high Rf value suggest about a compound?

higher Rf indicates less affinity for the polar stationary phase, leading to faster elution

Chlorophyll vs. Carotenoid

Carotenoid elutes first from an alumina column

• Carotenoids are less polar than chlorophyll, interacting less with the polar alumina, thus eluting faster

Why measure melting points?

indicates purity of a solid compound

Pure compounds have

sharp, specific melting points

What happens with impurties?

impurties affect the melting point range and behavior

How would contamination change your melting point results?

it will broaden in range

• impurities lower and broaden the melting point range due to disrupted crystal lattice

Recrystallization

purifies solids by dissolving in a hot solvent and allow crystals to form as it cools

Steps of Recrystallization

1. add compound to solvent

2. heat to dissolve

3. cool slowly

4. wash crystals

5. vacuum filtration

What makes a good recrystallization solvent?

the solvent dissolves compound, solvent doesn’t dissolve impurities

Caffeine Extraction

tea leaves are boiled in a calcium carbonate solution to neutralize tannins, which can form complexes with caffeine, reducing the yield

Calcium carbonate precipitates…

tannins as insoluble salts freeing caffeine for extraction

Neutralizing tannins improves….

caffeine solubility in the organic solvent

Sublimation

transition of a solid directly to a gas, used to purify compounds like caffeine

Why perform sublimation under vacuum?

vacuum lowers the pressure, reducing the sublimation temperature and preventing decomposition

Azeotrope

a mixture with a constant boiling temperature that cannot be separated by a simple distillation

Why might vacuum conditions help in sublimation?

lower pressure facilitates sublimation at a lower temperature, preserving the compound

Sublimation Apparatus

includes a

• sublimation chamber

• cold finger (condensor)

• vacuum pump connection

• heating source

  • the sample is heated at the bottom and purified crystals collect on the cold finger under vacuum

Theoretical Yield

maximum product possible based on limiting reactant

Percent Yield

(Actual yield / theoretical yield) x 100

Common lab challenges

• difficultly separating layers in a separatory funnel

• setting up a column chromatography apparatus correctly

Practical lab tips

• always check equipment (stopcock) before use

• ensure proper layering of materials in chromatography

What might cause issues when draining a separatory funnel?

the student likely forgot to remove the stopper from the top of the separatory funnel, preventing proper drainage due to pressure buildup

• removing the stopper allows air to enter, facilitating layer separation

Key uses of TLC

• identifying compounds in a mixture

• predicting elution order in column chromatography

If 12g of salicylic acid and an excess of acetic anhydride were used, what would the theoretical yield be?

12 g SA x (1 mol SA / 132.12 g SA) x (1 mol AA / 1 mol SA) x (180.16 g AA / 1 mol AA) = 15.65 g Acetic Anhydride