RP 10 - organic

what is RP10

preparation of a pure organic solid, test of its purity, and preparation of a pure organic liquid

steps

1. preparation - react suitable quantities of the reactants to produce the product

2. separation of the crude (impure) product - solid is separated from the reaction mixture by suction filtration

3. purification - removes impurities and carried out by recrystallisation

4. drying the product - by sucking air over the solid in a buchner flask and drying in a low temp oven

5. checking purity - by melting point determination

what is reflux

continuous boiling & condensing of a mixture

whens reflux used

to heat an organic mixture without losing reactants or products

purpose of anti bumping granuels

prevent vigorous or uneven boiling - by making small bubbles form instead of large ones

separating funnel purpose

to separate into two layers

1. higher density liquid (typically aqueous) = bottom layer

2. organic product layer = top layer

purifying organic liquid steps

1. put distillate of impure product in separating funnel

2. wash product

3. allow layers to separate, then run and discard aqueous layer

4. run organic liquid into conical flask

5. add 3 spatulas of drying agent

6. decant into distillation flask

7. distil to collect pure product

ways to wash the product (step 2)

1. sodium hydrogencarbonate solution, shaking and releasing pressure from CO2 produced - removes acid impurities by neutralisation

2. saturated NaCl solution - helps separate organic from aqueous layer, and removes any remaining alcohol

what should the drying agent be (step 5)

insoluble in the organic liquid and not react with it

what happens when the drying agent is added and examples

examples = anhydrous sodium sulphate, anhydrous calcium chloride

• absorbs remaining water

• liquid remains cloudy until all water drops have been removed

testing with melting point

pure = sharp, same as in quoted data books

impure = lower and melt over a range of degrees

measuring melting point ways

1. electronic melting point machine

2. practical set up - capillary tube strapped to a thermometer immersed in some heating oil

what happens in both cases

1. small amount of the salt put into capillary tube

2. heated up - slowly near melting point

3. compare experimentally determined value with one quoted in a data source

where can an error occur

if the temperature on the thermometer is not the same as the temperature in the sample tube

whats the most likely impurity to have caused a lower mp and how to prevent

water - press sample of crystals between filter papers

when preparing and carrying out a melting point test what must be done

1. correctly prepare the tubes

2. heat slowly

3. repeat multiple times

what does a melting point of less than 2 degrees mean

indicates a fairly pure substance

measuring boiling point to determine purity

1. distillation or boiling tube of sample in a heating oil bath

2. note pressure - changing pressure can change the boiling point

whys it not as accurate

several substances may have the same boiling point

how to get accurate measure of boiling point

thermometer should be above the level of the surface of the boiling liquid, and be measuring the temperature of the saturated vapour

recrystallisation method

1. dissolve impure compound in minimum volume of hot solvent

2. filter hot to remove insoluble purities

3. cool by inserting beaker in ice to recrystallise

4. suction filtrate with buchner flask to separate crystals

5. wash crystals with distilled water

6. dry crystals between absorbent paper

whats an appropriate solvent (step 1)

1. dissolves substance and impurities when hot, but only impurities when cold

2. allows substance to crystallise as it cools

why is the minimum volume added (step 2)

to obtain saturated solution, and enable crystallisation on cooling

• means as much of the solute is obtained as possible

what does both do

means the desired compound is pure in the crystals formed

what does hot filtering do (step 2)

1. removes insoluble impurities

2. the heat prevents crystals forming during filtration

whys it cooled (step 3)

to increase the yield by ensuring all of the compound crystallises

• yield is lower when warm

what does the water pump in a buchner flask do (step 4)

reduces pressure and speeds up filtration

why is it filtered this way

air passes through the sample, not just around it

why are the crystals washed with distilled water (step 5)

to remove soluble impurities

why is water removed (step 6)

it would affect % yield

what does recrystallisation do

remove impurities

whys slow crystallisation preferred

because fast can cause some soluble impurities to become trapped in the crystals

alternative methods of drying

• low temperature oven

• a desiccator with a drying agent

what can be done to compress the crystals in a buchner funnel

a clean cork - ensures air passes through the sample of crystals and not just round it, giving better drying

buchner flask diagram

buchner flask set up

1. place circle of filter paper into a buchner funnel and place in a stopper in a buchner flask

2. connect buchner flask to a suction pump

3. pour mixture into funnel

suction draws the liquid through into the buchner flask and leaves the crude product in the filter paper

how is yield lost

1. crystals lost when filtering or washing

2. some product stays in solution after recrystallisation

3. other side reactions occurring

4. sample still wet

two ways the melting point shows a product is impure

1. it melts slower

2. its broad

why do you cool flask to room temp before the crystals are filtered

yield is lower when warm/solubility higher when warm

why are crystals compressed in the funnel ie by cork

so the air passes through the sample, rather than around it

assuming reaction goes to completion, practical reasons why % yield not 100%

1. sample still wet

2. product lost in beakers/via filtration

whys the sample dissolved in hot water

1. to increase yield

2. to ensure cyrstallisation on cooling

why is the sample dissolved hot

to remove insoluble impurities

whys it cooled in ice

to increase amount of crystals formed

role of H3PO4 and how can it be added safely

• catalyst

• added slowly, with stirring, to safely distribute any heat produced

what happens to the ethanoic acid formed by hydrolysis of ethanoic anhydride

ethanoic acid is miscible with water, so it will be in the filtrate

preparing aspirin equation

preparing aspirin

1. reflux - both reactants and H3PO4

2. put in beaker with cold water and cool

3. suction filtration

4. recrystallisation

5. record solid mass

selecting a heating oil

in exam, can say oil bath

• needs to have a bp higher then the samples mp, and low flammability

benefits of an electric heater

• allows for controlled heating

• stops flammable vapour lighting

whys pressure noted when measuring bp to determine liquid purity

because changing pressure can change the boiling point of a liquid