MCAT Organic Chemistry - Separations and Purifications

543

extraction

transfer of a dissolved compound (the desired product) from a starting solvent into an immiscible solvent in which the product is more soluble

Like dissolves like

polar substances will associate with other polar substances, and nonpolar with nonpolar

immiscible

fluids that do not dissolve in the other and instead form two layers that do not mix,

ex. water and oil

aqueous phase (layer)

contains water and other polar or charged compounds

organic phase (layer)

contains oils and other nonpolar or uncharged compounds

separatory funnel

special tool for extraction; gravitational forces cause the denser layer to sink to the bottom of the funnel, where it can then be removed by turning the stopcock at the bottom

three intermolecular forces that affect solubility

1. Hydrogen bonding: Compounds that can do this, such as alcohols or acids, will move most easily into the aqueous layer

2. Dipole–dipole interactions: These compounds are less likely to move into the aqueous layer

   1. Van der Waals (London) forces: With only these interactions, compounds are least likely to move into the aqueous layer

acid-base extraction

move the desired compound into a different phase using acid-base chemistry

rotary evaporator (rotovap)

evaporates the solvent to isolate the desired product

wash

reverse of extraction; small amount of solvent is used to extract and remove impurities, rather than the compound of interest

Filtration

isolates a solid from a liquid; pour a liquid–solid mixture onto a filter that allows only the solvent to pass through

residue

solid of filtration

filtrate

liquid that passed through the filter

Gravity filtration

the solvent’s own weight pulls it through the filter; more commonly used when the product of interest is in the filtrate

hot filtration

type of gravity filtration where the solvent is kept hot to keep the desired compound dissolved; product of interest is in the filtrate

Vacuum filtration

the solvent is forced through the filter by a vacuum connected to the flask; more often used when the solid is the desired product

Recrystallization

method for further purifying crystals in solution; dissolve product in a minimum amount of hot solvent and let it recrystallize as it cools; solvent chosen one in which the product is soluble only at high temperatures so it will precipitate out as the solvent cools

Distillation

separate two liquids by evaporation and condensation; liquid with the lower boiling point will vaporize first, and the vapors will rise up the distillation column to condense in a water-cooled condenser; heating temperature is kept low so that the liquid with the higher boiling point will not be able to boil and therefore will remain liquid in the initial container

condensate

evaporated and cooled liquid from distillation on the condenser

distillate

end product of distillation

Simple distillation

least complex version of distillation; used to separate liquids that boil below 150°C and have at least a 25°C difference in boiling points

distilling flask

contains the combined liquid solution

distillation column

consists of a thermometer and a condenser

condenser

heat exchanger used to condense a gaseous substance into a liquid state through cooling

receiving flask

collects the distillate

Superheating

occurs when a liquid is heated to a temperature above its boiling point without vaporization; gas bubbles within a liquid are unable to overcome the combination of atmospheric pressure and surface tension

can be prevented using boiling chip, ebulliator, or magnetic stirrer

vacuum distillation

distill a liquid with a boiling point over 150°C; lower the ambient pressure, thereby decreasing the temperature that the liquid must reach in order to have sufficient vapor pressure to boil without degrading the product

fractional distillation

separate two liquids with similar boiling points; less than 25°C apart; fractionation column connects the distillation flask to the condenser

fractionation column

a column in which the surface area is increased by the inclusion of inert objects like glass beads or steel wool; lower boiling point heated to stay vapour; only the desired product drips down to the receiving flask

Chromatography

uses physical and chemical properties to separate and identify compounds from a complex mixture; the more similar a compound is to its surroundings (whether by polarity, charge, or other characteristics), the more it will stick to and move slowly through its surroundings

stationary phase / adsorbent

solid medium onto which the chromatogrpahy sample is placed

mobile phase

fluid that is flowed through the stationary phase and carries the sample

elute

displace the sample out of a mixture through chromatography

partitioning

different substances adhere to the stationary phase with differing strengths, causing the different substances to migrate at different speeds; results in separation withinthe stationary phase

partitioning coefficients

unique to each compound; dictates elution rate

thin-layer chromatography

adsorbent = silica/alumina (polar and hydrophilic) adherent on carrier sheet, mobile phase = organic solvent of weak to moderate polarity; solvent will creep up the plate by capillary action, bringing sample with it; more nonpolar the sample is, the further up the plate it will move

paper chromatography

adsorbent = paper (cellulose); solvent will creep up the plate by capillary action, bringing sample with it

spotting

small, well-defined spot of the sample directly onto the silica or paper plate

development

placing the adsorbent upright in a developing chamber, usually a beaker with a lid or a wide-mouthed jar

eluent

solvent system used in chromatography

Reverse-phase chromatography

stationary phase used is nonpolar, so polar molecules move up the plate quickly, while nonpolar molecules stick more tightly to the stationary phase

retardation/retention factor (Rf)

calculated value of relative polarity; relatively constant for a particular compound in a given solvent

Rf = distance of spot / distance of solvent front

ultraviolet light development

many compounds are colorless to the naked eye, so the developed plate can be placed under ultraviolet light, which will show any compounds that are UV-sensitive

TLC stains

shows the compunds on the stationary phase, but destroys the compounds such that they cannot be recovered

ex. iodine, phosphomolybdic acid, vanillin

preparative TLC

larger scale TLC as a means of purification; large plate develops, the larger spot of sample splits into bands of individual compounds, which can then be scraped off and washed to yield pure compounds

column chromatography

uses an entire column filled with silica or aluminum beads as an adsorbent, allowing for much greater separation; uses gravity to move the solvent and compounds down the column; solvent polarity can also be changed to help elute the desired compound

flash column chromatography

force the solvent through the column using gas pressure

ion-exchange chromatography

beads in the column are coated with charged substances so that they attract or bind compounds that have an opposite charge; after all other compounds have moved through the column, a salt gradient is used to elute the charged molecules that have stuck to the column

size-exclusion chromatography

beads used in the column contain tiny pores of varying sizes that allow small compounds to enter the beads, thus slowing them down

affinity chromatography

protein of interest is bound by creating a column with high affinity for that protein; Once the protein is retained in the column, it can be eluted by washing the column with a free receptor (or target or antibody), which will compete with the bead-bound receptor and ultimately free the protein from the column; Eluents can also be created with a varying pH or salinity level that disrupts the bonds between the ligand and the protein of interest

ex. nickel (attracts histidine)

Gas chromatography (GC) / vapor-phase chromatography (VPC)

method that can be used for qualitative separation; eluent is a gas (usually helium or nitrogen) instead of a liquid; adsorbent is a crushed metal or polymer inside a 30-foot coiled column in an oven; gaseous compounds travel through the column at different rates because they adhere to the adsorbent in the column to different degrees and will separate in space by the time they reach the end of the column

volatile

low melting-point, sublimable solids or vaporizable liquids

Mass spectrometry

involves the ionization and fragmentation of compounds; these fragments are then run through a magnetic field, which separates them by mass-to-charge ratio. e total molecular weight can thus be determined, or the relative concentrations of the different fragments can be calculated and compared against reference values to identify the compound

mass spectrometer

tool for mass spectrometry

High-performance liquid chromatography (HPLC)

eluent is a liquid and travels through a column of a defined composition; small sample is injected into the column, and separation occurs as it flows through a detector and is collected as the solvent flows out of the end of the apparatus; sophisticated solvent gradients as well as temperature can be applied to the column to help resolve the various compounds in the sample