Separations and Purifications

Question 1

Give the basic of a organic extraction

Answer 1

A process by which a solute is transferred (extracted) from one solvent to another. This is possible if the two solvents used cannot mix (immiscible) and if the solute is more soluble in the solvent used for the extraction.

Better to use a small amount of extractant multiple times, rather than a lot at once.

Question 2

What is chromatography?

Answer 2

Separation of a mixture of compounds by their distribution between two phases: one stationary and one moving. Molecules are separated based on differences in polarity and molecular weight.

Question 3

What is gas-liquid chromatography?

Answer 3

Where the stationary phase is a liquid absorbed to an inert solid.

The mobile phase is a gas (ie. He, N2), which is unreactive both to the stationary phase and to the substances being separated. The sample being analyzed can be injected in the direction of gas flow into one end of a column packed with the stationary phase.

As the sample migrates through the column, certain moleucles will move faster than others. As mentioned, the separation of the different types of molecules is dependent on size (molecular weight) and charge (polarity). Once the molecules reach the end of the column, special detectors signal their arrival.

Question 4

What is thin-layer chromatography?

Answer 4

Stationary phase is thinly coated on a plate, it is a finely divided polar material (eg. silica).

The ‘possible’ interactions between sample and silica gel are (weakest to strongest):

• Van der Waals forces
• dipole-dipole interactions
• Hydrogen bonding
• Coordination (lewis bases)

Question 5

What type of molecules will not go far up the stationary phase on a glass plate, in thin layer chromatography?

Answer 5

Molecules with highly polar functional groups

These will also dissolve more readily in polar solvents (eg. acetone, ethanol)

Question 6

What is distillation (in organic chemistry)?

When is fractional distillation necessary?

Answer 6

The process to separate compounds based on differences in boiling points.

Eg. salty water boiling away and leaving salt behind and ‘distilled’ water condensed on a collecting surface (or in the air..)

Fractional distillation is used when the difference in boiling points is small. It increases surface area in the fractionating column (eg. with glass beads) so that the less volatile substance can condense and drip back down into the distilling flask. Repeated distillations improve efficiency of distillation process.

Question 7

How can overheating during a distillation be avoided?

Answer 7

• Boiling slowly
• Boiling chips (ebuilators)
• Vacuum (decreases vapour pressure and thus boiling point)

Question 8

What is the principle behind recrystallization?

How should a solvent be chosen for recrystallization?

Answer 8

Recrystallization is a method where a solid is heated in a solvent and then cooled slowly to allow the solid to crystallize and leave impurities in the solvent. Impurities do not normally fit within the crustal structure of the compound.

Solubility data (eg. Ksp) at various temperatures regarding the compound to be purified and impurities should be known. The solvent should:

• Dissolve the compound near its boiling point but not near room temperature
• Impurities should be soluble at room room temperature (or colder)
• Have a low boiling point so that it can be removed from solid in drying process
• Not react with the solid

Question 9

An ebulliator introduces small air bubbles into a system.

What might the function of an ebulliator be in a distillation flask below a fractionating column and distillation head during vacuum fractional distillation. (the

Answer 9

To prevent superheating of the liquid to be distilled.

This is the same function of a boiling chip at atmospheric pressure. The air bubbles break the surface tension of the liquid being heated and prevent superheating and bumping.

Question 10

If you are trying to separate two substances with vacuum fractional distillation, which of the following modifications will likely improve the degree of separation?

1. Heating the distillation flask at a slower rate
2. Using a vacuum source that can achieve a lower pressure inside the distillation apparatus.
3. Cooling the condenser with ice water .
4. Using a shorter fractionating column

Answer 10

1. Heating the distillation flask at a slower rate.

Heating the distillation flask at a slower rate will allow both liquids more time in the fractionating column (inrease the number of theoretical plates, allowing liquid and vapour to equilibrate); therefore, of the four options, this one is most likely to improve the degree of separation between the two compounds.

Question 11

If distilling with vacuum fractional distillation, will shortening the fractionating column improve or worsen the degree of separation?

Answer 11

It will worsen it, because the liquids will vaporize and condense fewer times (ie. there will be fewer theoretical plates).

Question 12

During vacuum fractional distillation, if a leak develops in the vacuum distillation apparatus, the boiling points of the two components being separated will ___?

Answer 12

Both increase

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the surface pressure. The normal boiling point is measured at 1 atm pressure. The vapour pressure of a liquid increases with increasing temperature. Hence, the boiling point of a liquid decreases as the pressure on the surface of the liquid is decreased. If a leak develops in the apparatus, the surface pressure will increase, as will the boiling points of both liquids.

Question 13

Compound C is soluble in both dilute acid and base. Of the following, what two functional groups are found in compound C?

A. Carboxylic acid and amide
B. Amine and amide
C. Amide and nitro
D. Amine and carboxylic acid

Answer 13

D. Amine and carboxylic acid

Carboxylic acids are soluble in dilute base (eg. 5% NaOH), and amines are soluble in dilute acid (eg. 5% HCl).

Amides and nitro compounds are not soluble in dilute acid or base.

Question 14

The most effective way to remove triethylamine during the workup of an organic reaction would be to extract the reaction mixture with:

A. Sodium bicarbonate
B. Sodium bisulfite
C. Sodium sulfate
D. Hydrochloric acid

Answer 14

D. Hydrochloric acid

Triethylamine would most likely be soluble in an organic solvent. Washing with aqueous HCl acid will result in the formation of triethylamine HCl. This salt is water-soluble and will be removed with the water wash.

Question 15

Uric acid is formed by the breakdown of purines to xanthine, a uric acid precursor.

What nitrogenous base would promote the formation of uric acid crystals in the gut?

A. Cytosine
B. Uracil
C. Guanine
D. Thymine

Answer 15

Guanine

Guanine is one type of purine that is found in cells.

Question 16

A common method of preparing alkenes, is accomplished by heating the alcohol in the presence of an acid, such as H3PO4.

A distillation apparatus is assembled and the solution is distilled. A mixture, including the product, is removed by distillation and then the distillate is poured into a separatory funnel and washed with aqueous Na2CO3 and NaCl. The The aqueous washes are discarded and the organic layer is decanted and dried.

What is the reason for washing the distillate with aqueous Na2CO3?

Answer 16

To neutralize any remaining acid (H3PO4). It will react with the acid but not the alkene.

Question 17

Aspirin is partially purified of polymer contaminants by dissolving it in aqueous sodium bicarbonate solution to produce a water-soluble sodium salt. Salicylic acid, which also contains a strongly acidic CO2H group, will dissolve as well in aqueous basic solution by forming the corresponding water-soluble sodium salt.

What is accomplished by filtering after purification of polymer contaminants? How?

Answer 17

Solid polymer impurities are separated from the water-soluble sodium salts of salicylic acid and aspirins.

The fact that aspirin can be separated from polymer contaminants in this fashion suggests that the polymer remains insoluble in this medium and can be separated simply by filtration.

Question 18

True or false? In a chair conformation, the molecule is most stable when the largest substituent is in a equatorial position.

Answer 18

True

The consequences for having a large substituent in the axial position is a higher energy state (less stable) than having a smaller substituent in the axial state

Question 19

You have a compound in a chloroform (type of organic) solution. You want to extract this compound out of the organic solution with HCl. In order for this to be possible it must contain:

A. An amine
B. An acid
C. A ketone
D. An aldehyde

Why?

Answer 19

A. An amine

Amines are basic and will react with strong acids to produce ‘water soluble’ ammonium salts. These salts are more soluble in water than they are in chloroform and will partition into the aqueous layer during an extraction procedure.

Question 20

An old bottle of an aldehyde is obtained from a chemical storeroom. It has undergone some oxidation. Which method would be best to separate the aldehyde from the carboxylic acid?

A. Dissolve in water and filter
B. Dissolve in ether and filter
C. Dissolve in ether and extract with 5% HCl
D. Dissolve in ether and extract with 5% NaHCO3

Answer 20

D. Dissolve in ether and extract with 5% NaHCO3

The best strategy to separate a mixture containing a neutral compound and an acidic compound is to convert the acid into a salt by reaction with a base. Since the salt is ionic and will be much more soluble in water than in an organic solvent, an extraction procedure in which a solution of the mixture in ether is extracted with dilute aqueous base will remove the acidic component.

The aldehyde can then be recovered by evaporating the ether solvent.

Question 21

How does branching effect boiling point? (eg isopentane vs. a n-pentane)?

Answer 21

Branching in a molecule DECREASES boiling point. This is because branching disrupts the spatial packing of molecules in the condensed phase, and hence, compromise intermolecular attractions.

Question 22

Define amphiphilic and amphophilic.

Answer 22

Amphiphilic: From amphipathic, both hydrophilic and hydrophobic.

Amphophilic: From amphoteric, both acid and base.