Purifying and Organic liquid Flashcards
what could a desired product from a reaction be contaminated with
- unreacted starting materials
- other organic products
- the inorganic reagents used or formed
- water
what type of product do we need to know the separation techniques of
organic liquids
what are the four techniques we need to know of
- simple distillation
- fractional distillation
- solvent extraction
- drying
what is a problem that comes with handling organic compounds in experiments and separations
- they are flammable and sometimes toxic
- they also attack corks and bungs which increases the risk of leaks and contamination
what is done to prevent those things from happening
- use a type of apparatus made mainly from glass
- which can be fitted together tightly using ground-glass joints
what are better options in terms of safety than a bunsen burner when wanting to heat something in these experiments
heating mantle, hot water bath or hot oil bath
what does simple distillation simply involve
heating something in a flask connected to a condenser
how does simple distillation work
- the liquid with the lowest boiling temperature evaporates or boils off first
- it then passes into the condenser first
- this means that it can be collected in the receiver separately from any other liquid that evaporates later
why do you place a thermometer in the set-up right above the reaction mixture
- to monitor the temperature of the vapour as it passes into the condenser
- if the temp remains steady it indicates that one compound is distilling over
- if the temp begins to rise it indicates that a different compounds is distilling over
what are the advantages of using simple distillation over fractional distillation
it is easier to set up and is quicker to do
what is a disadvantage of simple distillation
it doesnt separate the liquids as well as fractional distillation
when should simple distillation be used
- when the boiling temp of the liquid being purified is very different from the other liquids in the mixture
- ideally a difference of 25C
what is the only difference between how fractional distillation is set up from simple distillation
there is a fractionating column between the heating flask and the still head
what is the column usually filled with and why
- glass beads or pieces of broken glass
- because it acts as a surface in which the vapour can condense
- theen be evaporated again as hot vapour passes up
what is the reason why fractional distillation separates the liquids out more effectively
- the vapour should undergo several repeated distillations as it passes up the column
- which would provide a better separation
when should fractional distillation be used
- when the difference in the boiling temperatures between the substances in the mixture is small
- and when there are several compounds to be separated from a mixture
what does solvent extraction involve`
using a solvent to remove the desired product from the other substances in the reaction mixture
what two requirements does the solvent need to meet
- it should be immiscible (not mix) with the solvent containing the desired product
- the desired product should be much more soluble in the solvent than in the reaction mixture
what is the process of solvent extraction
- place the reaction mixture in a separating funnel and then add the chosen solvent (it should from a separate layer)
- place the stopper in the neck of the funnel and gently shake the contents of the funnel for a while
- allow the contents to settle into two layers
- remove the stopper and open the tap to allow the lower layer to drain into a flask
- pour the upper layer into a separate flask
why is it better to add the solvent small portions rather than in a single large volume
- because it is more efficient
- as using more portions of solvent with the same total volume removes more of the desired product
what then needs to be done in order to get the desired organic product on its own as it is now mixed with the solvent
simple distillation or fractional distillation needs to be used to separate them
why is drying a separation technique in the first place
- a liquid organic product may partially or completely dissolve in water
- so water might be an impurity that needs to be removed by a drying agent
what feature does the drying agent need to have
that it doesnt react with the organic liquid
how is drying done
- the drying agent is added to the organic liquid and the mixture is swirled or shaken ten left for a bit
- before use the agent is powdery but after it looks more crystalline
- if a bit more is added and it remains powdery it indicates that the liquid is dry
- the drying agent is removed either by decantation or by filtration
what is decantation
pouring the organic liquids off the solid drying agent
what is the easiest way to test whether a liquid is pure
- measure its boiling temperature
- as impurities raise its boiling temp
how would you practically do this with a liquid organic compound
- you would measure its temperature
- and compare it to the carefully measured boiling temps of the same compound that are online or in data books
- you can then make a decision about how pure it is
why is using a thermometer quite inaccurate when it comes to measuring boiling temps and determining whether something is pure
- the thermometer might read too high or low
- so even if your measured temp exactly matches the one on the data book or online, you may wrongly assume that your compound is pure
what is another thing that you need to be aware of when doing this
that some organic compounds have the same boiling temp
What are drying agents usually
- anhydrous metal salts
- like calcium sulfate
Why do do drying agents work
- they form hydrated salts when they come into contact with water
- so when they come into contact with water in an organic liquid they absorb the water as water of crystallisation
