SNS - Organic Chemistry - Purification and Separation Flashcards
1
Q
Extraction
A
- Transfer of a compound from one solvent to another in which it is more soluble
- The solvents must be immiscible
- For example, isobutyric acid in diethyl etherncan be extracted with water
2
Q
Filtration
A
- Used to isolate a solid from a liquid
- Two main types are gravity filration and vacuum filtration
- The latter is used to isolate relatively large volumes of solid, usually when the solid is the desired product
3
Q
Recrystalisation
A
- Process in which impure crystals are dissolved in a minimum amount of hot solvent. As the solvent is cooled, the crystals reform, leaving the impurities in solution
- To be effective, the solvent must be carefully chosen - must dissolve the solid while it is hot but not while it is cool, and must dissolve the impurities at both temperatures
- In general, polar solvents dissolve polar compounds and vice versa. A solvent with intermediate polarity is desirable for recrystalisation
- A mixed solvent system can be used in some cases - the crude compound is dissolved in a solvent in which it is highly soluble. Another solvent in which the compound is less soluble is added in drops until the solid just begins to precipitate. Solution is heated to redissolve the solid and slowly cooled
4
Q
Sublimation
A
- Heated solid turns directly into a gas
- Used to purify, as the impurities found in most reaction mixtures won’t sublie easily
- Vapors made to condense on a cold finger
- Most performed under vacuum as at higher pressure, more compounds pass through the liquid phase rather that subliming. However, increases the temp required for sublimation and thus the risk of decomposition of the compound
- Optimal conditions therefore depend on the compound to be purifies
5
Q
Centrifugation
A
- Accelerates sedimentation - process by which particles in a solution settle at rates according to their mass, density and shape
6
Q
Distillation
A
- Separation of one liquid from another via vaprorisation and condensation
- Mixture of two or more miscible liquids are heated, and the compound with the lower BP is preferentially vaporised
- Three types:
- Simple - separates liquids that boil below 150oC and at least 25oC apart
- Vacuum - separates liquids that boil above 150oC and at least 25oC apart. Operates under reduced pressure which lowers BPs and thus preventing decomposition due to high temp
- Fractional - separates liquids that boil less than 25oC apart
7
Q
Chromatography
A
- Allows separation and identification of individual compounds from complex mixtures based on differing chemical properties
- Sample is placed onto solid stationary phase, then a mobile phase (liquid or gas) is passed through the stationary phase to elute (displace) the adhered substances
- Different compounds adhere to the stationary phase with different strengths and thus migrate with different speeds
- Four common types:
- TLC
- Column
- Gas
- High performance liquid
8
Q
Chromotography
TLC
A
- Stationary phase is usually silica gel or alumina on a plastic or glass sheet
- Mixture is spotted and the plate placed upright in a developing chamber containing eluant
- Silica gel is very polar and hydrophilic, allowing separation of compounds by polarity
- Distance a compound travels is called its Rf value. Relatively constant for a given compound in a given solvent and can thus be used for identification
9
Q
Chromatography
Column
A
- Principle as for TLC, however silica gel or alumina is used in the form of a column, allowing much more separation
- In TLC, compounds move up the plate by capillary action, whilst in column chromatography, move down the column by gravity or by nitrogen gas (flash column chromatography)
- Particularly useful in boichemistry as can be used to separate macromolecules such as proteins and nucleic acids
10
Q
Chromatography
Column
Techniques
A
- Ino-exchange - beads in the column are coated with charged substances so will bind compounds with opposing charges
- Size exclusion - column contains beads with many tiny pores. Very small molecules can enter the beads, slowing down their progress
- Affinity - columns customised to bind a compound of interest
11
Q
Chromatography
Gas
A
- Eluant is a gas, usually helium or nitrogen
- Stationary phase is a 30 ft column, coiled and lept inside an oven to control temp
- The mixture is injected into the column and vaporised and the gaseous compounds travel allong the column at different rates
- Registered by a detector at the end of the column which records their presence as a peak
12
Q
Chromatography
High Performance Liquid
A
- Eluant is a liquid that travels through a column similar to that for GC, but is under pressure
- A sample is injected into the column and separation occurs as it flows through
13
Q
Electrophoresis
Agarose Gel
A
Used to separate nucleic acids.
As every piece of DNA is highly negatively charged, can be separated easily on the basis of size.
Gels stained with ethidium bromide to visualise
14
Q
Electrophoresis
A
For a macromolecule moving in an electric field, migration velocity is proportional to electric field strength and its net charge and inversely proportional to frictional coefficient which depends on its mass:
V = EZ/f
- Agarose gel
- SDS-PAGE
- Isoelectric focusing
15
Q
Electrophoresis
SDS PAGE
A
Separates proteins by mass.
SDS disrupts non-covalent interactions, binding to proteinsand creating large negative net charges, neutralising the protein’s overall net charge.
As proteins move through the gel, the only variable affecting their velocity is their frictional coeffiecient which depends on mass
