Purifying an organic liquid Flashcards
when producing organic compounds in the lab, the product is often far from pure as it could be contaminated with
unreacted starting materials
other organic products
the inorganic reagents used, or the inorganic product
water
the techniques used to separate the intended products from a reaction mixture depend on whether the intended product is a
gas, liquid or solid
all of the following techniques are used to purify
organic liquids only
the four techniques for separating in tended products from reaction mixtures are:
simple distillation
fractional distillation
solvent extraction
drying
to prevent leakages of gas we use glassware with……..joints
ground-glass joints
the eight ‘specialist’ pieces of glass apparatus used for purification are:
pear-shaped flask receiver round-bottomed flask still head condenser receiver adaptor thermometer & adaptor separating funnel fractionating column
simple distillation involves the distillation of
an impure liquid and heating it in a flask connected to a condenser
the liquid with the lowest boiling temperature evaporates or boils
first and passes through the condenser for collection in the receiver separately from any other liuqid which might evaporate later
the thermometer in simple distillation is used to
see if a constant temperature is present in the pear shaped flask.
a constant temperature in simple distillation indicates that
one compound is distilling over
a change in temperature after a while in simple distillation indicates that
a different compound is distilling over
a rise in temperature in simple distillation indicates
a new compound is distilling over
an advantage of using simple distillation is
that is is quick and easy to set up
a disadvantage of simple distillation is that
it does not separate the liquids as well as fractional distillation
simple distillation should only be used is
the difference in the boiling temperature of the liquid being purified is more than 25° C than the other liquids in the mixture
fractional distillation uses what apparatus?
pear-shaped flask, condenser, fractionating column, receiver round-bottomed flask, thermometer & adaptor
simple distillation uses what apparatus?
pear-shaped flask, condenser, receiver round-bottomed flask, thermometer & adaptor
the fractionating column usually contains
glass beads or pieces of broken glass
the glass beads/ broken glass in the fractionating column act as a
surface on which vapour leaving the column can condense and then be evaporated again as more hot vapour passes up the column
in fractional distillation, the vapour undergoes
several repeated distillations as it passes up the column
a disadvantage of fractional distillation is that
it takes longer than simple distillation
an advantage of fractional distillation is that
it can be used when the difference in boiling temperatures is small and when there are several compounds to be separated.
the solvent used in solvent extraction depends on two main features:
the solvent added should be immiscible (not mix) with the solvent containing the desired organic product
and
the desired organic product should be much more soluble in the solvent added than in the reaction mixture
for solvent extraction, we use………..as apparatus
a separating funnel
the steps for solvent extraction are
placing the reaction mixture in a separating funnel and adding the solvent
place the stopper in the neck and gentle shake the contents
allow the contents to form two separate layers
remove the stopper and open the tap to drain the lower layer into a flask, then pour the upper layer into a separate flask
in solvent extraction, the desired organic product moves from
the reaction mixture to the solvent
for solvent extraction, it is better to use the solvent in
small portions than in a single large volume because it is more efficient
after the desired organic product has been separated from the reaction mixture using solvent extraction, it has to be removed from the solvent through
simple or fractional distillation
many organic liquids are prepared using
inorganic reagents in aqueous solution
an important feature of drying agents is that
it does not react with the organic liquid
a liquid organic product may partially or even completely
dissolve in water
the most common drying agents are
anhydrous metal salts
the commonest anhydrous metal salts used as drying agents are
calcium sulfate, magnesium sulfate and sodium sulfate
anhydrous metals from………….when they come into contact with water
hydrated salts
anhydrous metal salts absorb water as
water of crystallisation
anhydrous calcium chloride can also be used for some organic compounds but it
does react with other organic compounds and is soluble in alcohols
anhydrous calcium chloride is soluble in
alcohols
the steps for drying are
the drying agent is added to the organic liquid and the mixture is shaken, then left for some time
more drying agent is added until it no longer turns crystalline and stays powdery
the drying agent is removed by either decantation or by filtration
decantation is when
the organic liquid is poured off the solid drying agent
