Organic Chemistry Key Concepts: Melting Point, Recrystallization, Chromatography, Distillation, and More

What does a narrow melting point range indicate about a compound?

It indicates high purity; pure compounds melt over a narrow range (≤2°C).

How can impurities affect the melting point of a compound?

Impurities lower and broaden the melting point range due to disruption of the crystal lattice.

What is the purpose of a mixed melting point test?b

To identify an unknown compound by mixing it with a known compound; unchanged melting point indicates they are the same.

What is the purpose of recrystallization?

To purify a solid by dissolving it in hot solvent and letting pure crystals form upon cooling

What are the three steps in recrystallization?

1 Dissolve the impure solid in hot solvent to create a saturated solution.

2 Cool, pure crystals form, impurities stay in solution

3 Collect by filtration

What are the criteria for choosing a solvent in recrystallization?

The solvent should dissolve the compound when hot, not when cold, and dissolve impurities either very well or not at all.

What are the three solubility behaviors? Note them as good/bad or best

Soluble hot and cold (bad)

Insoluble hot and cold (bad)

Soluble hot, insoluble cold (best)

Why do impurities stay dissolved in recrystallization?

Impurities typically remain in the mother liquid (solution) because their solubility properties differ from the main compound

Why does slow cooling lead to better crystal formation?

It allows molecules to arrange into a purer, well-ordered crystal lattice, trapping fewer impurities.

What is the stationary phase in Thin-Layer Chromatography (TLC)?

Silica or alumina, which is polar.

What is the mobile phase in Thin-Layer Chromatography (TLC)?

When the solvent rises by capillary action

How is the Rf value calculated in TLC?

Rf = distance traveled by the compound / distance traveled by the solvent.

In Thin-Layer Chromatography (TLC) what do components separate based on? (3 things)

Polarity, solubility in mobile phase, strength of absorption to stationary phase

Why is the Rf factor useful in Thin-Layer Chromatography (TLC)?

It provides a consistent measure of how far a compound travels under certain conditions, useful for identification (compare to unknowns to the known)

What does a lower boiling point indicate in distillation?

The liquid is more volatile, has weaker intermolecular forces, and will distill first during distillation.

When does liquid boil?

when vapor pressure equals atmospheric pressure

What is the difference between simple and fractional distillation?

Simple distillation is for liquids with boiling points differing by >70°C; fractional distillation is for liquids with close boiling points.

Does fractional or simple distillation give better separation

Fractional

How do impurities affect boiling point?

They raise/broaden the bp because they interfere with vapor pressure

In our distillation lab, which distilled first, and why: cyclohexane or toluene

Cyclohexane because it has a lower bp

What is the purpose of steam distillation?

To isolate volatile, heat-sensitive, high-boiling organic compounds at lower temperatures.

Why do essential oils separate into two layers during steam distillation?

Most essential oils are immiscible with water, leading to the formation of two liquid layers (water and organic compound)

Why is boiling point lower in steam distillation?

Because the compound and water boil together, so each one only needs part of the total vapor pressure.

Why is steam distillation useful for heat sensitive compounds?

They can be distilled below their normal boiling point, preventing decomposition

What is the goal of column chromatography?

To separate compounds based on polarity and adsorption to a stationary phase.

What happens to more polar compounds in column chromatography?

They move slower because they stick more strongly to the polar stationary phase.

How does solvent choice affect separation in column chromatography ?

A more polar solvent reduces interactions with the stationary phase; compounds elute faster but with less separation

In our column chromatography lab, which eluted first and why: ferrocene or acetylferrocene?

Ferrocene because it was less polar

What are the three steps in the chlorination (free radical) mechanism?

1. Initiation (radicals formed), 2. Propagation (radicals react), 3. Termination (radicals combine).

What determines the selectivity in free radical substitution?

C-H bond strength (weaker bonds react first), radical stability, and electronic effects.

In chlorination, what is the most stable radical?

Tertiary

In our chlorination lab, C3 and C4 were favored in 1-chloronutane, why?

More stable radicals, less electron-withdrawing effect from Cl at C1

What is the problem with separating enantiomers?

They have identical physical properties, making ordinary separation methods ineffective.

How can enantiomers be separated?

By converting them to diastereomers, which have different properties allowing for separation.

In resolution of enantiomers, how does crystallization separate diastereomers?

One diastereomeric salt is less soluble and crystallizes out first when cooled

Why is optical rotation measured in resolution of enantiomers?

It indicated the enantiomeric purity of the recovered amine by showing whether it rotates plane polarized light and by how much

What distinguishes SN1 from SN2 reactions?

SN1 is a two-step process involving carbocation formation; SN2 is a one-step process with a backside attack.

What conditions favor an SN2 reaction?

Strong nucleophile, primary substrate, and polar aprotic solvent.

What is the stereochemical outcome of an SN2 reaction?

Inversion of configuration due to backside attack.

What conditions favor an SN1 reaction?

Tertiary substrate, weak nucleophile, and polar protic solvent.

Why does SN1 produce a racemic mixture?

The planar carbocation can be attacked from either side, resulting in a mix of enantiomers.

How does boiling point influence the order of distillation?

The liquid with the lowest boiling point vaporizes first and is collected first.

What is the significance of the fractionating column in fractional distillation?

It provides repeated vaporization-condensation cycles, enriching the vapor in the lower-boiling component.

What happens during the initiation step of free radical halogenation?

Radicals are formed, starting the chain reaction.

What is the difference between TLC and column chromatography?

TLC is used to monitor, identify, and check purity of compounds on a small scale whereas

Column chromatography is used to isolate and collect large amounts of purified compounds on a larger scale

Explain the similarities and differences between the phases of TLC and column chromatography

They both use a polar stationary phase and an organic mobile phase

TLC: stationary phase is a thin coating on a flat plate, mobile phases rises by capillary action

Column chromatography: stationary phase is packed into a vertical column, mobile phases rises flows downward by gravity and pressure

How do the less polar compounds move in TLC vs column chromatography?

TLC: travel farther up the plate

Column chromatography: elute faster and come out first

What is the difference in data output for TLC and column chromatography?

TLC: produces Rf value and visual spot patterns (qualitative)

Column chromatography: produces separated, collected fractions (quantitative)

What is the difference of purpose between distillation and steam distillation?

Distillation is used to separate miscible liquids based on boiling point differences

Steam distillation is used to isolate volatile, heat sensitive or high boiling compounds by codistilling them with water

What type of mixture is used in distillation vs steam distillation?

Distillation: miscible liquids (cyclohexane and toluene)

Steam distillation: works on immiscible mixtures (essential oils and water)

When is simple vs fractional distillation used (for what boiling points)

Simple: bp far apart

Fractional: bp close together