9.1 Extraction, Distillation & Chromatography

Extraction, Distillation & Chromatography

Content Category 5C — Separation and Purification Methods — tests how chemists physically isolate one component from a complex mixture. This is not lab trivia alone: MCAT passages routinely describe an experiment where a researcher must purify a product, identify an unknown, or track a reaction's progress, and you are expected to know which technique fits which job. Three core methods dominate this section: extraction, distillation, and chromatography. Each exploits a different physical or chemical property difference between the target compound and everything else in the mixture — solubility for extraction, volatility for distillation, and differential partitioning between two phases for chromatography. A fourth classic purification tool, recrystallization, also appears frequently in organic workup passages.

Extraction: Distribution of Solute Between Two Immiscible Solvents

Extraction — more precisely, liquid-liquid extraction — separates a solute by exploiting its different solubility in two immiscible (non-mixing) solvents. In practice this almost always means one aqueous layer (polar, water-based) and one organic layer (typically much less polar), such as diethyl ether, dichloromethane, ethyl acetate, or hexane. The two solvents are shaken together with the mixture in a separatory funnel, allowed to settle into two distinct layers, and then drained apart from each other.

Which layer ends up on top depends on density, not on which solvent is "organic." Diethyl ether (density about 0.71 g/mL) is less dense than water and floats on top; dichloromethane (density about 1.33 g/mL) and chloroform are denser than water and sink to the bottom. A classic MCAT trap is assuming the organic layer is always on top — always reason from density, not from habit.

By the principle that "like dissolves like," a nonpolar or weakly polar solute distributes preferentially into the organic layer, while a polar or ionic solute stays dissolved in the aqueous layer. At equilibrium, this distribution is described quantitatively by the distribution coefficient, also called the partition coefficient (K):

K = [solute]organic / [solute]aqueous

K is a thermodynamic equilibrium constant. For a given solute and a given solvent pair at a fixed temperature, K has one fixed value no matter how much total solute is present or how the volumes of the two layers are chosen. A large K (K >> 1) means the solute strongly favors the organic layer; a small K (K << 1) means it favors the aqueous layer.

Worked Example: Distribution Coefficient and Multiple Extractions

Suppose a solute has a distribution coefficient K = 4 (organic-over-aqueous) between dichloromethane and water, and you start with 100 mg of solute dissolved in 100 mL of water.

Single extraction with 100 mL of dichloromethane. The fraction of solute remaining in the aqueous layer after one extraction is:

fraction remaining = Vaqueous / (K × Vorganic + Vaqueous) = 100 / (4 × 100 + 100) = 100/500 = 1/5 = 20%

So 20 mg remains behind in the water, and 80 mg (80%) has moved into the organic layer.

Two extractions with 50 mL of dichloromethane each (same total organic solvent volume — 100 mL, just split in half). After the first 50 mL extraction:

fraction remaining = 100 / (4 × 50 + 100) = 100/300 = 1/3 ≈ 33%

leaving about 33 mg behind in the water. Extracting that remaining 33 mg with a second 50 mL portion removes another one-third of what is left, leaving roughly 33 × (1/3) ≈ 11 mg behind. That means about 89 mg (89%) has now been extracted — noticeably more than the 80 mg recovered using one large extraction of the same total solvent volume.

MCAT takeaway: for a fixed total volume of extracting solvent, several small extractions always recover more solute than one single large extraction, because the leftover fraction is raised to a power equal to the number of extractions (fraction remaining after n equal extractions is [fraction from one step]^n) rather than shrinking only once.

Acid-Base Extraction

A closely related, high-yield MCAT setup uses pH — rather than an inherent property of the neutral molecule — to control which layer a compound occupies. Ionized (charged) species are far more water-soluble than their neutral forms, so a chemist can manipulate a compound's protonation state to shuttle it between layers on command:

• Adding aqueous sodium hydroxide (NaOH) or sodium bicarbonate to a mixture containing a carboxylic acid deprotonates it into its water-soluble carboxylate salt, pulling it into the aqueous layer while neutral compounds stay in the organic layer. Bicarbonate is selective enough to deprotonate carboxylic acids (pKa ~4–5) without deprotonating most phenols (pKa ~10).
• Adding aqueous hydrochloric acid (HCl) to a mixture containing an amine protonates it into a water-soluble ammonium salt, again pulling it into the aqueous layer.
• A neutral compound unaffected by these pH changes (for example, a simple hydrocarbon, an ether, or a neutral ester) remains in the organic layer throughout.

By alternating extractions with acidic, then basic, then neutral aqueous washes — and re-extracting after back-neutralization — a chemist can sequentially pull an acid, then a base, then a neutral compound out of the same organic solution. This sequential acid-base extraction is a favorite MCAT passage design because it forces you to track protonation state, charge, and layer preference step by step.

Recrystallization and Filtration

After extraction and solvent removal, a solid organic product is often purified by recrystallization: dissolve the impure solid in a minimum of hot solvent in which the desired compound is highly soluble when hot and much less soluble when cold, then cool slowly so pure crystals form while impurities remain dissolved in the mother liquor. The crystals are collected by filtration — often vacuum filtration on a Büchner funnel for speed — washed with cold solvent, and dried. Recrystallization improves purity but typically reduces yield; on the MCAT, expect questions that ask why a solvent must dissolve the compound when hot but not when cold, or why rapid cooling can trap impurities inside poorly formed crystals.

Distillation

Distillation separates the components of a liquid mixture based on differences in boiling point (and therefore vapor pressure): the mixture is heated, the more volatile (lower-boiling) component vaporizes preferentially, and that vapor is condensed and collected separately from the less volatile residue. Boiling occurs when a liquid's vapor pressure equals the surrounding pressure — so changing ambient pressure changes boiling temperature for every component.

• Simple distillation works well when the boiling points of the components differ by more than roughly 25°C, or when separating a volatile solvent from a nonvolatile solute (for example, removing water from a dissolved salt, or recovering ether from a dried extract).
• Fractional distillation is required when boiling points are closer together. A fractionating column packed with glass beads, steel wool, or structured packing creates a large surface area for many repeated small vaporization–condensation cycles — informally called theoretical plates — as vapor rises through it, progressively enriching the vapor in the more volatile component before it reaches the condenser. Industrial petroleum refining is large-scale fractional distillation.
• Vacuum distillation lowers the pressure above the liquid. Because a liquid boils once its vapor pressure equals the surrounding pressure, reducing that surrounding pressure lowers the boiling point of every component. This protects compounds that would decompose if heated all the way to their normal (1 atm) boiling point.
• Steam distillation co-distills a volatile organic compound together with water at a temperature below the compound's own normal boiling point. Because the two immiscible liquids each contribute their own independent vapor pressure, their combined vapor pressure reaches atmospheric pressure — and the mixture boils — at a lower temperature than either pure liquid would need alone. Heat-sensitive natural products such as essential oils (terpenes from plants) are classic steam-distillation targets.

Azeotropes are mixtures that boil at a constant composition and temperature, so they cannot be fully separated by ordinary distillation (ethanol–water at ~95% ethanol is the classic example). On the MCAT, if a passage mentions an azeotrope, ordinary fractional distillation will not yield a pure component past the azeotropic composition.

Chromatography: Basic Principles

Chromatography separates the components of a mixture by passing a mobile phase (a gas or liquid that carries the sample) over or through a stationary phase (a solid or liquid held fixed in place). Each component of the mixture continuously partitions between the two phases according to its own affinity for each one. Components that interact more strongly with the stationary phase are held back and move slowly; components that favor the mobile phase are carried along quickly. This difference in travel rate is exactly what achieves separation — components exit (or are visualized) in a predictable order rather than all at once.

Open-Column Chromatography

In classical column chromatography (including flash chromatography under modest pressure), a vertical glass column is packed with a solid adsorbent — usually polar silica gel or alumina — and the sample is loaded at the top. An organic solvent (the eluent / mobile phase) flows downward by gravity or gentle pressure, carrying compounds at different rates. Polar compounds stick more tightly to polar silica and elute later (or require a more polar eluent); nonpolar compounds elute first. Collecting sequential fractions and checking them by TLC lets you pool pure product-containing fractions. This is the preparative workhorse that sits between analytical TLC and high-pressure methods.

Gas-Liquid Chromatography (GC)

In gas-liquid chromatography (GLC / GC), the mobile phase is an inert carrier gas (commonly helium, nitrogen, or hydrogen), and the stationary phase is a thin liquid film coating the inside of a long, narrow capillary column (or packing a packed column). Because the sample is vaporized before it enters the column, it must be volatile and thermally stable. Components separate primarily according to boiling point and polarity relative to the stationary phase, and a detector at the far end of the column reports each compound's retention time — how long it took to travel through the column. GC is excellent for small organic volatiles (solvents, fragrance compounds, fatty acid methyl esters) but cannot handle large proteins or nonvolatile salts.

High-Performance Liquid Chromatography (HPLC)

High-performance (or high-pressure) liquid chromatography (HPLC) pumps a liquid mobile phase at high pressure through a column packed with a fine solid stationary phase. In normal-phase HPLC, the stationary phase is polar and the mobile phase is nonpolar, so polar analytes are retained longest. In reverse-phase HPLC (the more common setup in practice), the stationary phase is nonpolar (e.g., C18 alkyl chains on silica) and the mobile phase is polar (water/acetonitrile or water/methanol), so nonpolar analytes are retained longest and polar analytes elute earlier. Because HPLC does not require the sample to be volatile, it can separate a much wider range of compounds — including large or heat-sensitive biomolecules, peptides, and metabolites — at higher resolution than simple gravity column chromatography.

Paper Chromatography and Thin-Layer Chromatography (TLC)

Paper chromatography uses a strip of cellulose (ordinary paper) as the physical support; the water trapped within the paper's fibers is technically the true polar stationary phase, while an organic solvent serves as the mobile phase and climbs the paper by capillary action, carrying the sample's components with it at different rates.

Thin-layer chromatography (TLC) replaces paper with a thin, uniform layer of an adsorbent — almost always polar silica gel or alumina — coated onto a rigid backing of glass, metal, or plastic. A small spot of sample is applied near the bottom of the plate (the origin), and the plate is stood upright in a shallow pool of solvent, which climbs the plate by capillary action, carrying each component of the mixture upward at its own characteristic rate. Because spots are often colorless, they must typically be visualized under a UV lamp or by chemical staining (for example, iodine vapor or potassium permanganate).

Once a TLC plate has been developed, each spot's migration is quantified by its retention factor (Rf):

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

Rf values always fall between 0 and 1. Because a standard TLC plate uses a polar stationary phase, a more polar compound binds more tightly to the silica and travels a shorter distance, giving it a lower Rf; a less polar (more nonpolar) compound interacts more weakly with the silica, travels farther, and has a higher Rf. TLC is fast, cheap, and requires only a tiny sample, which makes it the standard bench technique for monitoring whether a reaction has gone to completion or for quickly comparing the relative polarity of several compounds side by side. Running a co-spot of pure standard next to an unknown is a common identity check when Rf values match under the same conditions.

Comparing the Chromatography Types

Common MCAT Traps

• Don't assume the organic layer is always on top in a separatory funnel — compare densities directly. Diethyl ether floats; dichloromethane and chloroform sink.
• A larger K does not mean a solute is "more soluble" in some general sense — it specifically compares that one solute's organic-phase concentration to its aqueous-phase concentration for that solvent pair.
• Several small extractions beat one large extraction for a fixed total solvent volume; K itself does not change with volume.
• Rf values are only meaningful for comparison when the same solvent system, plate, and conditions are used — the identical compound will show a different Rf in a different mobile phase.
• In reverse-phase HPLC, the polarity rules flip relative to normal silica TLC: nonpolar analytes stick to the nonpolar stationary phase and elute later.
• GC requires volatility and thermal stability; HPLC does not — choose the method that matches the sample.