Organic Chemistry Lab Techniques and Concepts

Recrystallization

A purification technique based on solubility differences of a solid in hot vs cold solvent.

Ideal solubility behavior in recrystallization

High solubility in hot solvent, poor solubility in cold.

Impurities in recrystallization

So they're either filtered out or remain dissolved without contaminating crystals.

Pure compounds crystallization

Pure compounds form well-ordered lattices, excluding impurities.

Slow crystallization

To allow orderly lattice formation and better exclusion of impurities.

Purpose of a solvent test

To identify the best solvent based on solubility trends.

Four properties of a good recrystallization solvent

(1) Dissolves compound when hot; (2) Poor solubility when cold; (3) Doesn't react with compound; (4) Dissolves impurities well or not at all.

Role of activated charcoal

Removes colored impurities by adsorption.

Purpose of hot gravity filtration

To remove insoluble impurities before crystallization.

Purity check

By measuring melting point (narrow range = pure).

Too much solvent added

Use minimum solvent and cool slowly.

No crystals after cooling

Scratch flask or add a seed crystal.

Crystals formed during hot filtration

Solution cooled too much—reheat and re-filter.

Broad melting point causes

Impurities or solvent residue (wet crystals).

Dark-colored crystals

Colored impurities weren't removed—use charcoal.

Ring of solid at top of flask

Crystallization on cooler glass—maintain even heat.

Solution turned cloudy during cooling

Rapid crystallization or precipitation of impurities.

Skipping hot filtration

Insoluble impurities remain and contaminate crystals.

Drying crystals before weighing

Solvent residue skews mass and melting point.

Effect of excess charcoal

Compound may be lost by adsorption, lowering yield.

Steps of recrystallization

Dissolve → Charcoal → Hot filter → Cool slowly → Ice bath → Vacuum filter → Dry → Record melting point.

Poor solvent choice effects

Too soluble = low recovery; too insoluble = poor purification.

Principle of chromatography

Separation based on differing affinities for stationary vs mobile phases.

Stationary phase in TLC

Silica gel or alumina (polar).

Mobile phase

Solvent or solvent mixture.

Compounds separation in TLC

Different polarities = different interaction with the stationary phase.

Solvent level in TLC

So samples don't dissolve into solvent.

Filter paper in the chamber

To saturate vapor and aid even development.

Rf calculation

Rf = (distance traveled by compound) ÷ (distance traveled by solvent front)

High Rf compounds

Nonpolar.

Effect of polarity on Rf (silica TLC)

Polar compounds have lower Rf.

Two ways to visualize TLC spots

UV light, iodine staining.

Two spots have same Rf. Why?

Same compound or poor solvent system.

Tailing spots. Cause?

Overloaded sample or solvent too polar.

No spots under UV. Next step?

Try iodine stain or other visualization.

All spots near baseline. Fix?

Increase polarity of the solvent system.

Solvent runs off top. Result?

Inaccurate Rf values.

Spot A: Rf = 0.2; Spot B: Rf = 0.8. Which is more polar?

Spot A is more polar.

All Rf ≈ 1. Problem?

Solvent system too polar.

Spot moved 3 cm; solvent front = 6 cm. Rf?

0

Smearing observed. Cause?

Overloading or impure sample.

Unsealed TLC chamber. Problem?

Uneven solvent evaporation = poor results.

Spotting below solvent level. Result?

Sample dissolves, not carried up.

Poor separation. Fix?

Adjust solvent polarity or use new solvent system.

Touched plate face. What happens?

Oils disrupt separation.

Define melting point

Temperature range where solid turns to liquid under atmospheric pressure.

Sharp melting point

High purity.

Effect of impurities

Lower and broaden melting point range.

Why is it diagnostic?

Confirms identity and assesses purity.

Acceptable MP range for pure compound

1-2 °C

Broad range (>5 °C) indicates

Impurity or poor technique.

Why dry sample before MP test?

Moisture acts like an impurity.

How much sample in tube?

1-2 mm height.

Common apparatus

Mel-Temp.

Pure MP = 132-134 °C; sample = 124-129 °C. Interpretation?

Impure.

MP = 112-117 °C. Likely cause?

Too much sample or heated too fast.

How to tell if two compounds with same MP are identical?

Mixed melting point test.

Why heat slowly near melting point?

To avoid overshooting and inaccurate readings.

MP = 150-151 °C. Conclusion?

Pure sample.

Bubbling before melting. Cause?

Moisture or solvent residue.

Instant melting at low temp. Likely cause?

Heated too fast.

Too much sample in capillary. Effect?

Broad or inaccurate MP range.

Sample not powdered. Effect?

Uneven heating → inaccurate MP.

Heated too fast. Result?

Overshooting true MP.

Reused capillary. Why not?

Cross-contamination risk.

Steps for Mel-Temp MP determination

Load dry powdered sample into capillary → Insert into Mel-Temp → Heat quickly to 10-15 °C below expected MP → Then heat slowly (1-2 °C/min) → Record start and end of melting.

How does a mixed melting point test work?

Mix unknown with known. Sharp MP = same compound; lowered/broader = different.

Three factors for accurate MP

Dry sample, small sample, slow heating near MP.