Preparation of a pure organic solid (and test its purity) Flashcards
How are organic solids prepared?
- solutions of reactants are often added together at room temperature and the product precipitates out
- reactants refluxed together
- anti-bumping granules decanted off and the solid forms on cooling and crystallising
How is the product filtered faster than normal filtration?
- suction filtration/filtration underr redcued pressure
What are the stages in the synthesis of an organic solid?
- preperation
- react suitable quantities of the reactants to produce the product
- seperation of the crude product
- solid seperation from mixture using filtration under reduced pressure
- purification of the product
- removes impurities
- recrystillisation
- drying the product
- sucking air over the solid in the Buchner flask
- drying in a low temp oven
- checking purity
- carrying out a melting point determination
How is the apparatus for suction filtration set up?
- place circle of filter paper in the Buchner funnel and place a stopper in a flask
- connect the flask to a suction pump and pour the mixture into the funnel
- the suction draws the lqiuid into the Buchner flask and leaves the crude product in the filter paper
How is the solvent chosen for recrystallisation?
- solvent chosen in which the desired product dissolves readily at higher temperatures but is only lightly soluble at room temperature
Why is the minimum volume of hot solvent used
to obtain as much of the solute as possible
What happens to the crystals formed? why is this done
- seperated by filtration under reduced pressure
- impurities remain dissolved in solution
Why is slow crystallisation preferable over fast crystillisation?
- fast crystallisation can cause some soluble impurities to be trapped in the crytal
How are insoluble impurities removed?
- gravity filtration
What is the method for recrytalisation?
- dissolve impure crystals in the minimum volume of hot solvent
- filter the hot solution by gravity filtration, using a hot funnel and fluted filter paper, to remove any insoluble impurities
- allow the soln to cool and crytalise
- filter off the crytals using suction filtration
- wash by poruing over some ice cold solvent to remove aqueous impuritieis
- dry by sucking air over the crytals in the Buchner flask and then in a low-temp oven
- or use drying agent
When removing insoluble impurities, why is filtering done througha hto filter funnel and using fluterd paper?
prevents precipitation of the solid
Why is the solution cooled
decreases solubility of the crytal
impurities remain in the solution
How can crytal formation be aided, in addition to cooling
scratching the side of the flask with a glass rod
When removing aqueous impurities from the crytal, why is the water cold
prevents the crytal from dissolving
What is the melting point of a substance? what does it show?
- not the exact point but the range of temperatures when the sample starts to melt to when it is completely melted
- the greeater the range, the more impurities are present
