6. Organic synthesis - practical techniques Flashcards
how does reflux work
mixture heated in a flask (which is fitted with a vertical liebig condenser)
when mixture boils vapours are condensed and recycled back into the flask
why use reflux
reactions often need to be heated to work
reflux heats reactants whilst stopping them from being lost, giving time for them to react
- what is a problem of reflux
2. how to get round this problem
- it can cause desired product to react further
2. carry out reaction in a distillation apparatus instead
how does distillation work
mixture gently heated
substances evaporate out of mixture in order of increasin boiling point
if you know bpt of pure product use thermometer to tell you when it is evaporating and condensing
if product has lower bpt than starting materials reaction mixture can be heated so product evaporates as it forms. starting materials stay in reaction mixture aslong as temperature is controlled
which process purifies an organic solid
recrystallisation
hiw is recrystallisation done
very hot solvent added to impure solid until it just dissolves
- giving a saturated solution of the impure product
- solution left to cool slowly. crystals of the product form. impurities stay in solution.
- crystals removed by filtration under educed pressure and washed with ice cold solvent . dried. left with crystals much purer than original solid
why is the choice of solvent for recrystallisation very important?
only works if solid is very soluble in hot solvent but nearly insoluble in cold solvent
if product isnt soluble enough in hot solvent wont be able to dissolve it at all
if product too soluble in cold solvent most will stay in solution even after cooling. when you gfilter youll loose most of product giving a very low yield.
how can you filter an organic solid
under reduced pressure
pour reaction micture into buchner funnel with a peice of filter paper in it
buchner funnel is connected to vacuum line causing it to be under reduced pressure
reduced pressure causes suction through funnel causing liquid to pass quickly into flask leaving behind dry crystals of solid
how to accuratley measure melting point
put small amount of solid in capillary tube
place in a beaker of oil with a very sensetive thermometre
slowly heat with constant stirring until solid just melts.
read temperature on thermometre
what are melting and boiling points good indicators of
purity
pure substances have specific mpt and bpt
impure = melting point lowered + bpt raised
very impure= ,pt and bpt occur at wide range of temperatures
why are mpt and bpt useful
can compare to known mpt/bpt of substanse to determine purity.
mpt/bpt similar sample is quite pure.
mpt/bpt lower sample contains impurities.
