Separation Methods Flashcards
What do separation methods generally rely upon?
differences in chemical or physical properties - usually they rely upon physical movement and no substantial chemical modification
Filtration: how does it work? what are the different types?
- 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
Why use fluted filter paper for gravity filtration?
It minimises contact between paper and funnel
What does gravity filtration isolate?
Liquid from unwanted solid
Tips for gravity filtration
- 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
What is the reason for hot filtration?
Removal of solvent-insoluble impurities from hot solution before allowing filtrate to cool and form crystals of purified product
Tips for hot filtration?
- 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
What does suction filtration isolate?
solid from unwanted liquid
Tips for suction filtration
- 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
What characteristics must a filter aid have?
- 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
Example of a filter aid?
celite
What does Soxhlet Extraction isolate?
partially soluble component of solid mixture into liquid phase using continuous extraction process - impurities remain in paper thimble
Tips for Soxhlet Extraction
- 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
What does centrifugation isolate?
separation of particles in a solid-liquid mixture - centrifugation force promotes accelerated settling of particles
What does recrystallisation remove?
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
What are requirements for the solvent in recrystallisation?
- solvent dissolves the solid while hot
- solvent should not dissolve desired solid while cold
- cold solvent must keep impurities dissolved
What do solvent likely dissolve?
‘like dissolves like’ - consider polarity and hydrogen bonding
Desirable solvent characterisitics
- 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
Essential properties of solvent pair in mixed-solvent recrystallisation
- 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)
What happens if a compound doesn’t recrystallise?
- too much solvent (evaporate)
- supersaturated (seed or scratch)
Top tips for recrystallisation
- 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
What type of isolation is sublimation used for?
fairly volatile organic solids - solids are heated and one turns directly into a gas (on cooling the gas converts back to a solid)
What are the four types of drying methods for solids?
- Heat - oven
(only if they’re thermally stable and oven has to be at least 30ºC below mp of solid) - Desiccator
(drying agent/ desiccant) - Vacuum desiccator
- Reduced atmospheric pressure
(Schlenk line, rotatory evaporator)
How to differ between pure and impure compounds regarding melting point?
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.
What are the errors involved in measuring melting points?
- Most common error is heating the sample too quickly
- Decomposition may occur during the melting point determination - do not remelt samples
What is a reaction work-up?
It refers to the series of manipulations required to isolate and purify the product(s) of a chemical reaction
What can reaction work-ups include?
- quenching
- cooling
- precipitation
- filtration or centrifugation
- liquid-liquid extraction
- drying agent to remove water from organic layer
- removal of solvents (evaporation)
What is quenching?
stopping a reaction and deactivation of any reactive reagents
- avoid side reactions or decomposition of product
- makes reaction safe to handle
What does choice of quenching depend upon?
- the reagents used
- product stability
How would you quench a strongly basic non-aqueous (e.g. BuLi, LDA)?
cool to 0ºC and add ammonium chloride solution
How would you quench a strongly acidic non-aqueous (e.g. TiCl 4 )
Cool to 0ºC and add sodium bicarbonate or sodium carbonate solution
How would you quench a neutral reaction?
Add water
How would you quench a slightly acidic reaction?
add water or sodium bicarbonate solution
How would you quench a slightly basic reaction?
add water or ammonium chloride solution
What is liquid-liquid extraction?
separation of compounds based on their relative solubility in two different immiscible liquids
Tips for liquid-liquid extraction
- 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)
Extraction tips
- test pH of aqueous layer, not organic
- several smaller extractions are better than a big one
What to do if organic compound is slightly soluble in water?
wash with saturated sodium chloride solution
What to do if an emulsion forms in liquid-liquid extraction?
- add base, or acid or ethanol
- stir with glass rod or gravity filter (check product stability in acids/ bases before)
monitor with TLC
Examples of analytical techniques and their limitations
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
