Question

MCAT Tutorial: Extraction, Distillation & Chromatography

Answer 1

\({\bf{Extraction:}}\) the process by which a substance is partitioned and removed from other components in the mixture, typically liquid-liquid immiscible phase
> Basic procedure: place solution into a separatory funnel, add the extracting compound, mix, wait for layers to form, separate, drain
> both components start in the organic phase and some will enter the aqueous phase; separation of layers depends on the relative density of the organic solvent compared to water
> extracting compound must have higher affinity for the substance being extracted, high partition coefficient desirable
> the components involved cannot react irreversibly

Answer 2

\({\bf{Distillation:}}\) is a technique to separate miscible liquids.
> basic procedure: one liquid is vaporized and condensed back into a liquid for collection. > liquids are separated based on the difference between boiling point, the higher the separation is, the easier it will be
> different differentials in boiling points will require different distillation techniques

\({\bf{Simple~Distillation:}}\) for low BP compounds and high BP differentials (~30degrees C)
> contains two flasks, one with the heat source and thermometer, another as the non-heated source. condensing tube between them.
> liquids are placed in the first flask and heated, then when the first component reaches its boiling point its vapors enter the condenser



\({\bf{Sub-types~of~Distillation:}}\)
vacuum: used for liquids with BP's above 150C (too high for the normal method, would denature the compound)
> lowers the pressure with a vacuum tube, thus lowering the BP

fractional: if the BP's are very close, add a fractionating column between the first flask and the condenser
> the vapor will condense and revaporize multiple times, increasing the amount of the lower BP liquid that enters vapor phase

Answer 3

\({\bf{Chromatography:}}\) technique used to separate and analyze mixtures. two main subtypes: analytical and preparative
> stationary phase: the substance which has affinity for all the compounds in the mixture
> mobile phase: the substance which has affinity for the desired components
> elution: process through which the mobile phase moves through the stationary phase

Basic mechanism: the mixture is added to the stationary phase; the mobile phase carries it along the medium depending on its affinity for the stationary phase. low affinity = moves quickly and moves farther. the separation is measured and used to compare the components of the mixture.

\({\bf{Thin-Layer~Chromatography:}}\) uses a thin medium (duh) like silica gel coated onto a plate as the stationary phase.
> origin: the location at the bottom of the plate/tube where the mixture starts, then moves up through the medium w/ capillary action
> solvent front: line formed by the solvent at its edge
> retention factor: distance the solute moved from the origin/distance the solvent moves
note: this depends greatly on experimental conditions so they cannot be compared unless the conditions are identical

\({\bf{Gas-Liquid~Chromatography:}}\) separates components using a gaseous mobile phase
> uses a glass tube coated with oil; affinity of gas for the oil is the basis for separating the components
> analyzed with a computer

Answer 4

\({\bf{Preparative~Chromatography:}}\) main purpose is to separate compounds, not necessarily get information about their components

> uses long glass/quartz column with silica/alumina as the stationary
> mobile phase is used to wet the walls of the column
> the sample mixture is added slowly and flows through the mixture
> can form colored bands under UV radiation (quartz) based on affinity
> bands keep flowing, reach the bottom, and are collected as solution fractions
> each band is allowed to evaporate and isolated in its pure, unsolvated form

Answer 5

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT