Separation Methods

Question 1

What do separation methods generally rely upon?

Answer 1

differences in chemical or physical properties - usually they rely upon physical movement and no substantial chemical modification

Question 2

Filtration: how does it work? what are the different types?

Answer 2

• mixture of solid and liquid pass through porous material which traps the solid while the liquid passes through

Isolation of solid: Suction (vacuum) filtration
Isolation of liquid: Gravity filtration

Question 3

Why use fluted filter paper for gravity filtration?

Answer 3

It minimises contact between paper and funnel

Question 4

What does gravity filtration isolate?

Answer 4

Liquid from unwanted solid

Question 5

Tips for gravity filtration

Answer 5

• swirl the suspension before transfer
• do not overfill the filter cone
• wash flask and solid with small amount of filtered solution then a little pure solvent

Question 6

What is the reason for hot filtration?

Answer 6

Removal of solvent-insoluble impurities from hot solution before allowing filtrate to cool and form crystals of purified product

Question 7

Tips for hot filtration?

Answer 7

• keep glassware hot and insulated
• wash filter paper with a little hot solvent to dissolve any product that has crystallised out before the receiving flask

Question 8

What does suction filtration isolate?

Answer 8

solid from unwanted liquid

Question 9

Tips for suction filtration

Answer 9

• swirl mixture to form suspension
• slowly pour into funnel so filtration is rapid
• rate of filtration starts to slow
• wash flask and filter cake with some filtrate
• remove tubing before turning off vacuum
• wash filter cake with some cold solvent to wash solid

Question 10

What characteristics must a filter aid have?

Answer 10

• having rigid intricately shaped, porous individual particles
• forms a highly permeable, stable, incompressible cake
• remove the finest solids at high rates of flow
• chemically inert and essentially insoluble in the liquid being filtered

Question 11

Example of a filter aid?

Answer 11

celite

Question 12

What does Soxhlet Extraction isolate?

Answer 12

partially soluble component of solid mixture into liquid phase using continuous extraction process - impurities remain in paper thimble

Question 13

Tips for Soxhlet Extraction

Answer 13

• Soxhlet thimble should be higher than siphon outlet
• half fill thimble with solid to be extracted and plug with white cotton wool to prevent any solid from being transferred into solvent

Question 14

What does centrifugation isolate?

Answer 14

separation of particles in a solid-liquid mixture - centrifugation force promotes accelerated settling of particles

Question 15

What does recrystallisation remove?

Answer 15

Three types of impurities
1. insoluble material (hot filtration)
2. small quantities (10-15%) of unreacted starting materials and/ or byproducts
3. small amounts of coloured products resulting from oxidation or polymerisation

Question 16

What are requirements for the solvent in recrystallisation?

Answer 16

• solvent dissolves the solid while hot
• solvent should not dissolve desired solid while cold
• cold solvent must keep impurities dissolved

Question 17

What do solvent likely dissolve?

Answer 17

‘like dissolves like’ - consider polarity and hydrogen bonding

Question 18

Desirable solvent characterisitics

Answer 18

• high dissolving power for solute at high temperature, low dissolving power at room temp or below
• high or negligible dissolving power for solid impurities so can be removed by filtration
• boiling point >60ºC
• boiling point should be at least 10ºC lower than that of the compound to be crystallised

Question 19

Essential properties of solvent pair in mixed-solvent recrystallisation

Answer 19

• Miscible in all proportions over temperature range used
• Solute must be insoluble in one solvent and soluble in the other
• Ideally solvents have similar boiling points (within 20-30ºC)

Question 20

What happens if a compound doesn’t recrystallise?

Answer 20

• too much solvent (evaporate)
• supersaturated (seed or scratch)

Question 21

Top tips for recrystallisation

Answer 21

• cover flask with watch glass to prevent solvent evaporation and stop dust getting in
• clamp flask in ice-water bath so it doesn’t fall over as ice melts and volume of water increases

Question 22

What type of isolation is sublimation used for?

Answer 22

fairly volatile organic solids - solids are heated and one turns directly into a gas (on cooling the gas converts back to a solid)

Question 23

What are the four types of drying methods for solids?

Answer 23

1. Heat - oven
   (only if they’re thermally stable and oven has to be at least 30ºC below mp of solid)
2. Desiccator
   (drying agent/ desiccant)
3. Vacuum desiccator
4. Reduced atmospheric pressure
   (Schlenk line, rotatory evaporator)

Question 24

How to differ between pure and impure compounds regarding melting point?

Answer 24

Pure covalent organic compounds have definite melting points.

If a compound is impure, the melting range will increase significantly and the upper end of the melting range will be lowered.

Question 25

What are the errors involved in measuring melting points?

Answer 25

• Most common error is heating the sample too quickly
• Decomposition may occur during the melting point determination - do not remelt samples

Question 26

What is a reaction work-up?

Answer 26

It refers to the series of manipulations required to isolate and purify the product(s) of a chemical reaction

Question 27

What can reaction work-ups include?

Answer 27

• quenching
• cooling
• precipitation
• filtration or centrifugation
• liquid-liquid extraction
• drying agent to remove water from organic layer
• removal of solvents (evaporation)

Question 28

What is quenching?

Answer 28

stopping a reaction and deactivation of any reactive reagents

• avoid side reactions or decomposition of product
• makes reaction safe to handle

Question 29

What does choice of quenching depend upon?

Answer 29

• the reagents used
• product stability

Question 30

How would you quench a strongly basic non-aqueous (e.g. BuLi, LDA)?

Answer 30

cool to 0ºC and add ammonium chloride solution

Question 31

How would you quench a strongly acidic non-aqueous (e.g. TiCl ₄ )

Answer 31

Cool to 0ºC and add sodium bicarbonate or sodium carbonate solution

Question 32

How would you quench a neutral reaction?

Answer 32

Add water

Question 33

How would you quench a slightly acidic reaction?

Answer 33

add water or sodium bicarbonate solution

Question 34

How would you quench a slightly basic reaction?

Answer 34

add water or ammonium chloride solution

Question 35

What is liquid-liquid extraction?

Answer 35

separation of compounds based on their relative solubility in two different immiscible liquids

Question 36

Tips for liquid-liquid extraction

Answer 36

• remove particulate matter before
• remove any water miscible solvent (e.g. THF, MeOH, acetone) and redissolve in suitable solvent
• use low b.p. solvent (e.g. ethyl acetate or diethyl ether)

Question 37

Extraction tips

Answer 37

• test pH of aqueous layer, not organic
• several smaller extractions are better than a big one

Question 38

What to do if organic compound is slightly soluble in water?

Answer 38

wash with saturated sodium chloride solution

Question 39

What to do if an emulsion forms in liquid-liquid extraction?

Answer 39

• add base, or acid or ethanol
• stir with glass rod or gravity filter (check product stability in acids/ bases before)

monitor with TLC

Question 40

Examples of analytical techniques and their limitations

Answer 40

Melting point - needs material to be solid and have a reference m.p.
TLC - requires all materials to be visible using same visualisation technique
GC - only for volatile materials, not good for insoluble materials
HPLC - not every analyse is detected by every type of detector
NMR - only shows NMR active impurities