Organic Practical Techniques

Question 1

What do you need to carry out an organic preparation?

Answer 1

A basic set of Quickfit apparatus

Question 2

What 5 things does a basic set of Quickfit apparatus contain?

Answer 2

1) round-bottom or pear-shaped flask
2) receiver
3) screw-top adaptor
4) condenser
5) still head

Question 3

Why are many organic reactions heated?

Answer 3

To overcome the activation energy and increase the rate of reaction as many are slow at room temperature

Question 4

What is the process of heating under reflux used for?

Answer 4

To prepare an organic liquid without boiling off the solvent, reactants or products

Question 5

What 5 pieces of apparatus are needed to heat under reflux?

Answer 5

1) round-bottom or pear-shaped flask
2) condenser
3) rubber tubing
4) clamp and stand
5) heat source (bunsen burner with a tripod and gauze)

Question 6

What should be used to heat under reflux if the reaction can be carried out under 100 degrees?

Answer 6

A water bath

Question 7

What does a bunsen burner allow?

Answer 7

A reaction under reflux to be carried out a fixed temperature

Question 8

What should you use when heating flammable liquids under reflux?

Answer 8

A heating mantle so that no naked flame is present, giving an added level of safety should any of the apparatus leak or crack

Question 9

What do you need to add to the reaction mixture before fitting the condenser to the neck of the flask when heating under reflux?

Answer 9

Anti-bumping granules so that the contents will boil smoothly - without the granules, large bubbles form at the bottom of the liquid and make the glassware vibrate or jump violently

Question 10

How do you provide a good seal between a condenser and flask when heating under reflux?

Answer 10

1) apply a thin layer of grease to the ground-glass joint on the condenser
2) place the condenser carefully into the flask and gently rotate the condenser back and forth to provide a good seal

Question 11

What should you do during reflux?

Answer 11

1) keep the condenser in the upright position
2) clamp it loosely to prevent breakage
3) never put the stopper in the top of the condenser bc otherwise you will have a closed system in which pressure would build up as the heated air expanded - the apparatus could explode

Question 12

What is the role of the rubber tubing during reflux?

Answer 12

• It is used to connect the inlet of the condenser to the tap and the outlet into the sink
• Water always enters the condenser at the bottom and leaves at the top to ensure that the outer jacket is full

Question 13

What does heating under reflux enable?

Answer 13

A liquid to be continually boiled whilst the reaction takes place, preventing volatile contents from escaping and the flask from boiling dry

Question 14

What happens in a condenser during reflux?

Answer 14

Vapour from the mixture rises up the inner tube of the condenser until it meets the outer jacket containing cold water, when it condenses and drips back into the flask

Question 15

What needs to happen when reflux is complete and why?

Answer 15

The crude product needs to be purified to remove any by-products or reactants as the reaction may not have gone to completion or produced by-products

Question 16

What is distillation?

Answer 16

The method used to separate pure liquid from impurities

Question 17

What 9 pieces of apparatus are needed for distillation?

Answer 17

1) round-bottom or pear-shaped flask
2) condenser
3) rubber tubing
4) heat source
5) clamp and stand
6) screw-cap adaptor
7) receiver adaptor
8) still head
9) thermometer

Question 18

Describe the set-up of a distillation

Answer 18

1) the flask is clamped by its neck and the still head is connected to the flask
2) the still head adaptor is T shaped and has two ground-glass joints, one to fit the screw-cap adaptor and one for the condenser
3) a second clamp is placed around the receiver adaptor/condenser joint, removing the need to clamp the condenser, as it will be supported sufficiently at both ends
4) a flask is used to collect the distillate so that the distillation apparatus is not completely airtight

Question 19

Why are the joints greased in a distillation set up?

Answer 19

So that the apparatus comes apart easily after the experiment

Question 20

Where does water enter the rubber tubing connected to the condenser?

Answer 20

At the lowest point (the point closest to the receiver adaptor)

Question 21

Describe what happens during a distillation

Answer 21

1) the flask is heated and the mixture in the flask starts to boil
2) the different liquids in the mixture will have different boiling point - the liquid with the lowest boiling point is the most volatile and will boil first
3) the vapour moves out of the flask up into the other parts of the apparatus, leaving behind less volatile components of the mixture
4) when the vapours reach the cold condenser, they condense and this liquid then drips into the collecting flask
5) collect the liquid in the the known boiling point range of the desired product and discard any products below the known boiling point

Question 22

What happens if water has also been obtained when preparing samples of organic liquids?

Answer 22

There will be two layer in the collection flask - one organic layer and one aqueous layer

Question 23

How do you identify the organic layer from the aqueous layer?

Answer 23

Add some water to the mixture and the layer that gets bigger is the aqueous layer

Question 24

What is used to separate the aqueous and organic layers?

Answer 24

A separating funnel

Question 25

How would you use a separating funnel to separate the aqueous and organic layer?

Answer 25

1) ensure that the tap of the separating funnel is closed
2) pour the mixture of liquids into the separating funnel, place a stopper in the top of the funnel and invert it to mix the contents
3) allow the layers to settle
4) place a conical flask under the separating funnel, remove the stopper and open the tap until the whole of the lower (aqueous) layer has left the funnel
5) place a second conical flask under the separating funnel to collect the other (organic) layer

Question 26

Which layer is on top?

Answer 26

The organic layer

Question 27

What else might an impure product contain?

Answer 27

Acid impurities

Question 28

How do you remove acid impurities from an organic liquid?

Answer 28

1) add aqueous sodium carbonate and shake the mixture in the separating funnel
2) any acid present will react with sodium carbonate, releasing CO2 gas
3) slowly open the tap, holding the stoppered separating funnel upside down, to release any gas pressure that may build up - stop when effervescence stops
4) remove the aqueous sodium carbonate layer and wash the organic layer with water before running both layers off into two separate flasks

Question 29

What can be used to remove water left in an organic product?

Answer 29

A drying agent

Question 30

What is a drying agent?

Answer 30

An anhydrous inorganic salt that readily takes up water to become hydrated e.g. CaCl2, CaSO4, MgSO4

Question 31

How do you use a drying agent to dry an organic liquid?

Answer 31

1) add the organic liquid to a conical flask
2) using a spatula, add some of the drying agent to the liquid and gently swirl the contents to mix them together
3) place a stopper on the flask to prevent the product from evaporating away and leave it for roughly 10 minutes
4) if the solid has all stuck together in a lump, there is still some water present ∴ add more drying agent until some solid is dispersed in the solution as a fine powder
5) decant the liquid from solid into another flask (clear liquid = dry)

Question 32

Describe redistillation

Answer 32

1) the sample may still contain organic impurities bc sometimes organic liquids have very similar boiling points
2) ∴ carry out a second distillation, only collecting product with the boiling point of the compound you are trying to make
3) the narrower the boiling range, the purer the product

Question 33

What are two examples of experiments to prepare an organic solid

Answer 33

1) preparation of aspirin

2) preparation fo benzoic acid by alkaline hydrolysis

Question 34

Describe briefly the preparation of aspirin

Answer 34

Reflux salicylic acid, ethanoic anhydride and glacial ethanoic acid with Quickfit apparatus

Question 35

Describe briefly the preparation of benzoic acid by alkaline hydrolysis

Answer 35

Reflux methyl benzoate with aqueous sodium hydroxide, flowed by acidification

Question 36

What is the technique used to separate a solid product from a solvent or liquid reaction mixture?

Answer 36

Filtration under reduced pressure

Question 37

What are the 5 pieces of apparatus needed for filtration under reduced pressure?

Answer 37

1) Buchner flask
2) Buchner funnel
3) pressure tubing
4) filter paper
5) access to a filter or vacuum pump

Question 38

How do you filter a sample using a Buchner funnel to obtain an organic solid?

Answer 38

1) to filter the sample, slowly pour the reaction mixture from a beaker into the centre of the filter paper
2) rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel
3) rinse the crystals in the Buchner funnel with more solvent and leave them to dry under suction for a few minutes so that the crystals start to dry

Question 39

How do you set up a Buchner funnel for filtration under reduced pressure to obtain an organic solid?

Answer 39

1) connect one end of the pressure tubing to the vacuum outlet or filter pump whilst attaching the other end of the rubber tubing to the Buchner flask
2) fit the Buchner funnel to the Buchner flask ensuring that there is a good tight fit, using a Buchner ring or rubber bung
3) switch on the vacuum pump or tap to which the filter pump is attached
4) check for good suction by placing hand across the top of the funnel
5) place a piece of filter paper inside the Buchner funnel and were with the same solvent used to prepare the solid - should see the paper being sucked down against the holes of the funnel

Question 40

How can impurities be removed from organic solid products?

Answer 40

By recrystallisation

Question 41

What does purification by recrystallisation depend on?

Answer 41

The desired products and the impurities having different solubilities in the chosen solvent

Question 42

Describe how you would carry out a recrystallisation

Answer 42

1) pour a quantity of the chosen solvent into a conical flask and warm it
2) tip the impure sample into a second conical flask or beaker
3) slowly add the minimum volume needed of the solvent to the impure sample until it dissolves in the solvent
4) once the solid has dissolved, allow the solution to cool
5) crystals of the desired product should form in the conical flask or beaker
6) when no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid

Question 43

How should you warm a solvent if it is flammable?

Answer 43

Over a water bath

Question 44

How should you warm a solvent if the solvent is water?

Answer 44

Using a bunsen burner, tripod and gauze

Question 45

Why can knowing the melting point of an organic solid help identify whether it is pure?

Answer 45

Bc a pure organic substance usually has a very sharp melting range of 1 to 2 degrees

Question 46

What is the melting range?

Answer 46

The difference between the temperature at which the sample starts to melt and the temperature at which melting is complete

Question 47

How can measuring the melting point of an organic solid show that it is impure?

Answer 47

1) the solid melts over a wide range of temperatures

2) the sample has a lower melting point than a pure sample

Question 48

What two methods are used to determine the melting point of an organic solid?

Answer 48

1) using an electrically heated melting point apparatus

2) using an oil bath/Thiele tube method

Question 49

How do you prepare a sample before measuring its melting point?

Answer 49

1) ensure the sample is completely dry and free flowing
2) take a glass capillary tube or melting point tube and hold one end of the capillary tube in the hot flame of a Bunsen burner
3) rotate the tube in the flame until the end of the tube is sealed
4) allow the capillary tube to cool and then fill it with crystals to a 3mm depth - usually done by pushing the open end of the capillary tube into the solid sample to force some of the solid into the tube

Question 50

Describe the process of using electrically heated melting point apparatus to determine the melting point of an organic solid

Answer 50

1) place the capillary tube containing the sample into a sample hole and place a 0-300 degrees thermometer in the thermometer hold of the melting point apparatus
2) using the rapid heating setting, start to heat up the sample whilst observing it through the magnifying window
3) once the solid is seen to melt, record the melting point and allow the melting point apparatus to cool
4) prepare a second sample in a new capillary tube and place it in the melting point apparatus and again heat up the sample
5) as the melting point is approached, set to low and raise the temperature slowly whilst observing the sample to obtain an accurate determination of the melting point

Question 51

Describe the process of using an oil bath or Thiele tube method to determine the melting point of an organic solid

Answer 51

1) set up an oil bath or Thiele tube
2) attach a capillary tube containing the sample to a thermometer using a rubber band
3) insert the thermometer through a hole in the cork if using a Thiele tube or clamp the thermometer if using an oil bath so that the end of the thermometer and end of the capillary tube should dip into the oil
4) using a micro-burner, slowly heat the side arm of the Thiele tube or oil bath whilst observing the solid
5) when the solid starts to melt remove the heat and record the temperature at which all of the solid has melted

Question 52

What two things should you do when using a Thiele tube or oil bath to determine the melting point of an organic solid?

Answer 52

1) heat the oil slowly win approaching the melting point

2) repeat the melting point determination a second time to ensure that you obtain an accurate value

Question 53

How do you prepare cyclohexane from cyclohexane-1-ol?

Answer 53

Add phosphoric acid slowly to the alcohol and reflux for 15 minutes - slowly adding the acid avoids a dramatic temperature ruse which could cause side reactions

Question 54

How do you synthesise a haloalkane?

Answer 54

Mix the alcohol and acid (HCl) in a separating funnel and shake for 20 minutes

Question 55

Describe how you carry out the hydrolysis of haloalkanes (used to show the relative rates of haloalkanes)?

Answer 55

1) pre-mix haloalkane and ethanol (solvent and nucleophile) for 5 minutes
2) warm in a 50 degree water bath and add the pre-warmed AgNO3
3) time the formation of the precipitate

Question 56

What are the products of the hydrolysis of haloalkanes?

Answer 56

Carbon chain with oxygen in the middle, X- and H+