Kaplan MCAT Organic Chemistry chapter 12: Separations and Purifications

vacuum distillation

-Used whenever liquid has a boiling point above 150 degrees C -Vacuum lowers the ambient pressure, which subsequently lowers the boiling points -Higher temperature may degrade the product

Recrystallization

-used to further purify crystals in solution -product is dissolved in a minimum amount of hot solvent and is cooled down to let it recrystallize -solvent should be chosen so that the solute (product) is only soluble at high temperatures

If we are given a solution of ether, with a boiling point of 308 K and methylene chloride, with a boiling point of 313K which type of distillation should be used to separate them?

A solution of ether and methylene chloride, which have very close boiling points, can be separated by using fractional distillation.

in what way is gas chromatography distinct from all of the other techniques we have discussed?

As the name suggests, gas chromatography is simply the same technique of mobile and stationary phases performed with a gaseous eluent (instead of liquid). The stationary phase is usually a crushed metal or polymer.

retardation factor

R_f=(distance spot moved)⁄(distance solvent front moved)

vacuum filtration

Solvent is forced through the filter by a vacuum connected to the flask, used when the solid is the desired product

superheating

When a liquid is heated to a temperature above its boiling point without vaporization. gas molecules stay trapped within liquid due to surface tension and atmospheric pressure

separatory funnel

equipment used to isolate the two phases after they have been given time to spread out again

filtration

isolates a solid from a liquid --solid is called the residue and the flask full of liquid is called the filtrate

reverse-phase chromatography

opposite of TLC, Stationary phase is instead nonpolar so that polar molecules move up the most.

ion exchange chromatography

-Beads in the column are coated with charged substances so that they attract or bind compounds. --A positively charged compound will attract and hold a negatively charged backbone of DNA or protein as it passes through the column. -Salt gradient can be used to elute the charged molecules that have stuck

size exclusion chromatography

-Beads used in the column contain tiny pores of varying sizes. --Pores allow small compounds to enter the beads and thus slows them down. -Small compounds are slowed down and retained longer

Gas Chromatography

-GC is also known as vapor-phase chromatography (VPC). Main difference between it and other methods is that the eluent is a gas. Instead of a liquid. --Adsorbent is crushed metal or polymer which is coiled into a column and placed in an oven to control the temperature. -Gaseous compounds travel through the column at different rates since they adhere to the adsorbent in the column at different degrees. -Injected compounds must be volatile: low melting-point, sublimable solids or vaporizable liquids. -Common to separate molecules using GC and then inject the pure molecules into a mass spectrometer for molecular weight determination.

High Performance Liquid Chromatography

-Liquid eluent travels through a column of defined composition. -Needed high pressure before but not anymore. -Small sample is injected into the column and separation occurs as it flows through. The compound then passes through a detector and is collected as the solvent flows. -Difference is that the process is computerized by the detector --Allows for sophisticated solvent/temperature gradients can be applied to the column to help resolve the various compounds in the sample. used when sample size is small or if forces such as capillary action will affect results

Chromatography

-The more similar a compound is to its surroundings, the more it will stick to and move slowly through its surroundings. -Sample is placed onto a solid medium *stationary phase or adsorbent*, a liquid or a gas +mobile phase* will then run through the stationary phase. --Displaces (*elutes*) the sample and carries it through the stationary phase. -*Partitioning* then occurs after the sample is eluted. The mobile phase will adhere to the stationary phase with differing strengths which causes the substances to move at different speeds. --Represents an equilibrium between two phases --Different compounds will have different *partitioning coefficients* and will elute at different rates. ---Results in the separation within the stationary phase and allows for the isolation of each substance individually -Different media can be used for the stationary phase and the property most commonly exploited on the MCAT is polarity

Fractional distillation

-Used to separate liquids with similar boiling points -Fractionation column connects the distillation flask to the condenser. --Increases the net surface area by including inert objects on the column --Vapor condenses on these surfaces and refluxes back down --Each time the condensate evaporates, the vapor consists of a higher proportion of the compound with a lower boiling point -Only the desired product drips down to the receiving flask once it goes through the "rungs" of the column.

column chromatography

-Uses an entire column filled with silica or aluminum beads as a adsorbent, which allows for greater separation. -Uses gravity as opposed to capillary action to move the solvent and compounds down the column. -Flash Column: solvent can be forced through the column using gas pressure. -Solvent eventually drips out of the end of the column and the different fractions that leave the column can be collected. --After collection, the solvent can be evaporated to leave behind the substance of interest. -Can be used to separate and collect macromolecules such as proteins and nucleic acids (biochemistry)

extraction

-two solvents must be immiscible --two layers are temporarily mixed by shaking so that the solute can pass from one solvent to another --aqueous phase (polar layer, usually water) --organic phase (nonpolar layer) -separatory funnel --denser layer sink to the bottom due to gravity, and can be removed --more common for organic layer to be on top, but ultimately depends on density --process is repeated several times to ensure all of the desired product is extracted -once the desired product has been isolated, it can be obtained by evaporating the solvent by using rotary evaporator -wash --small amount of solute is used to extract and remove impurities

Distillation

-used when product is a liquid that is soluble in the solvent -takes advantage of differences in boiling point to separate two liquids by evaporation and condensation --liquid with the lower BP will vaporize first, and the vapors will rise up into the distillation column to condense in a water cooled condenser --forms a condensate which drips down into the vessel and forms the distillate

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

Would acid dissolve better in aqueous acid or aqueous base?

Acid dissolves better in aqueous base because it will dissociate to form the conjugate base and, being more highly charged, will become more soluble. Note that like dissolves like applies to polarity; acids and bases dissolve more easily in solutions with the opposite acid-base characteristics.

what properties of molecules do thin-layer chromatography, paper chromatography, and standard column chromatography take advantage of to separate compounds?

Each of these methods separates compounds using charge and polarity.

What is the major historical distinction between HPLC and column chromatography? What is the major distinction now?

Historically, HPLC was performed at high pressures, whereas column chromatography uses gravity to pull the solution through the column. Now, HPLC is performed with sophisticated and variable solvent and temperature gradients, allowing for much more specific separation of compounds than column chromatography; high pressures are no longer required.

when doing an extraction, would it be better to do three extractions with 10 ml of solvent, or one extraction with 30 ml?

It is better to do three washes with 10 mL than to do one with 30 mL; more of the compound of interest would be extracted with multiple sequential extractions than one large one.

Preparative TLC

Preparative TLC is a larger scale means to purify substances. the larger spot of sample splits into bands of individual compounds, which can then be scraped off and washed to yield pure compounds

wash

The reverse of extraction, in which a small amount of solvent is poured over the compound of interest to dissolve and remove impurities.

what must be true about the two solvents used for an extraction to work?

The two solvents must be immiscible and must have different polarity or acid-base properties that allow a compound of interest to dissolve more easily in one than the other.

If we are given a solution of bromobenzene, with a boiling point of 156 degrees C and camphor; with a boiling point of 204 which type of distillation should be used to separate them?

Vacuum distillation would be the best technique to separate two chemicals with such high boiling points because the decreased ambient pressure will allow them to boil at a lower temperature.

Distillation separates compounds based on what?

Vapor pressure. If two volatile liquids have a large separation of boiling point (usually > 20C), a slow boil will cause the compound with a lower boiling point to boil off and be captured. This can then be condensed into a cooler tube.

Immiscible

form two layers and do not mix

flash column chromatography

solvent can be forced through the column using gas pressure

Gravity Filtration

solvents own weight pulls through the filter and is most commonly used when the filtrate is the product of interest

mass spectrometry

the ionization and fragmentation of compounds which then runs the fragments through a magnetic field which separates the masses based on charge. ---Relative concentration of the different fragments can be calculated and compared against reference values to identify the compounds.

Extraction Definition

the transfer of a dissolved compound (desired product) from a starting solvent into a solvent in which the product is more soluble -like dissolves like

affinity chromatography

-Protein of interest is bound be creating a column with high affinity for that protein. --Accomplished by coating beads with a receptor that binds to the protein or a specific antibody to the protein. -Protein can be eluted by washing the column with a free receptor --Or Eluents can be created to alter the pH or salinity levels that disrupts the bonds between the ligand and the protein of interest ---Recovered substance can be bound to the eluent, which can be difficult to remove.

Simple distillation

-Same technique as above and should only be used to separate liquids that boil below 150 degrees Celsius and have at least a 25 degree' difference. -Apparatus consists of a: distilling flask (contains the combined liquid), distillation column (thermometer and a condenser), receiving flask (collects distillate). -Superheating

Thin-Layer And Paper Chromatography

-Two above only vary in the medium used for the stationary phase. *Thin-layer uses silica gel that is highly polar or an alumina adherent to an inert carrier sheet is used. Paper chromatography uses paper (cellulose)* -this means that any polar compound will adhere to the gel well and thus move through (elute) slowly -*Spotting*: sample that is to be separated is placed directly onto the adsorbent itself -*Developed*: adsorbent is placed upright in a developing chamber (usually a jar or beaker) --*Eluent*: shallow pool of solvent at the bottom of the jar. SAMPLE MUST BE ABOVE ELUENT SO IT DOESNT DISSOLVE INTO SOLVENT -Solvent will then creep up the plate by capillary action and carry various compounds in the sample with varying rates. When the solvent reaches the top of the plate, the plate is removed for drying. -In TLC, silica gel is polar and hydrophilic while mobile phase is usually a moderately polar organic solvent. --Non-polar compounds dissolve in the organic solvent and move quickly as the solvent moves up the plate --The more nonpolar the sample is, the further up the plate it will move. -spots of individual compounds are usually white, which makes them difficult or impossible to see on the white paper or TLC plate so you can place under UV light - can use iodine phosphomolybdic acid or vanillin to stain but ten it destroys compounds so they can't be recovered -Retardation factor: compounds are identified using this factor. Relatively constant for a particular compound in a given solvent -Most frequently performed on a small scale.