Practical Techniques in organic chemistry

Question 1

State the apparatus needed to prepare an organic liquid

Answer 1

Quickfit apparatus

1. Round-bottom/ pear shaped flask
2. Receiver
3. Screw tap adaptor
4. Condenser
5. Still head

Question 2

Why do you need to heat under reflux

Answer 2

1. Many organic reactions occur slowly at room temperature, so is common for them to be heated to overcome activation energy
2. Heating under reflux is common procedure used to prepare an organic liquid without boiling off the solvent, reactants, or products

Question 3

What do you need to heat under reflux

Answer 3

1. Round-bottom/ pear-shaped flask
2. Condenser
3. Rubber tubing
4. Stand and clamp
5. Heat source e.g bunsen burner

Question 4

What should be used if you are heating flammable liquids to reflux

Answer 4

1. Heating mantle so there is no naked flame present

2. Provides extra added layer of safety should nay of the apparatus leak or crack

Question 5

Why are anti-bumping granules used

Answer 5

1. Added to liquid before the flask is heated so that the contents will boil smoothly
2. If the granules are not used, large bubbles form at the bottom of the liquid that make the glassware vibrate or jump violently

Question 6

Why should you not put a stopper in the top of the condenser

Answer 6

1. You would create a closed system
2. Pressure would build up inside as the heated air expanded
3. Could result in apparatus exploding

Question 7

State the steps needed to prepare an organic liquid

Answer 7

1. Heating under reflux
2. Distillation
3. Purifying organic products
4. Drying the organic product
5. Redistillation

Question 8

Describe the process of heating under reflux

Answer 8

1. Have a clamp attached to a pear-shaped flask above a bunsen burner.
2. A water bath can be used if the reaction is below 100 degrees.
3. The pear-shaped flask should be clamped by its neck and anti-bumping granules should be added
4. Put the condenser in the flask and apply a thin layer of grease to the joint of the condenser to provide a good seal.
5. The condenser should be kept upright and clamped loosely as it is fragile.
6. Rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink. Water always enters at the bottom and leaves at the top to ensure the outer jacket is always full
7. The vapour from the mixture rises up the inner tube of the condenser until it meets the outer jacket containing cold water. The vapour then condenses and drips back into the flask.

Question 9

Why do you need to do distillation

Answer 9

1. Chemical reactions may not go to completion or may produce by-products as well as the desired product.
2. Once reflux is complete the crude liquid or solid product present in the flask needs to be purified to remove any by-products and remaining reactants
3. Distillation is a common method used to separate a pure liquid from its impurities

Question 10

What equipment is needed for distillation

Answer 10

1. Round-bottom or pear-shaped flask
2. Condenser
3. Rubber tubing
4. Heat source
5. Stand and clamp
6. Screw-cap adaptor
7. Receiver adaptor
8. Still head
9. Thermometer

Question 11

Describe the set-up used for distillation

Answer 11

1. The flask is clamped by its neck and the still head is connected to the flask.
2. The still head adaptor has two joints, one to fit the screw-cap adaptor and one to fit the condenser. You should grease the joints so the apparatus comes apart easily after the experiment
3. A second clamp is placed round the receiver adaptor at the point which it is attached to the condenser. This removes the need to clamp the condenser.
4. Rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink. Water always enters the condenser at the lowest point.
5. A flask is used to collect the distillate so that the distillation apparatus is not completely airtight.

Question 12

Describe what happens during distillation

Answer 12

1. The flask is heated and the mixture in the flask will start to boil.
2. The different liquids in the mixture will have different boiling points. 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 the less volatile components of the mixture
4. When vapours reach the cold condenser, they condense and become liquid
5. The liquid then drips into the collecting flask

Question 13

When do you need to purify organic products

Answer 13

1. When preparing samples of organic liquids, water may be obtained along with the product.
2. If this happened you will see two liquid layers inside your collection flask, one the organic layer and one the aqueous or water layer.
3. To identify which is which add some water to your mixture and the layer that gets bigger is the aqueous layer

Question 14

How do you purify an organic product

Answer 14

1. Use a separating funnel
2. Ensure the tap of the funnel is closed
3. Pour mixture of liquids into the separating funnel, place a stopper in the top of the funnel and invert to mix the contents
4. Allow the layers to settle and identify which is the aqueous layer
5. Place a conical flask under the separating funnel, remove the stopper and open the tap until the whole of the lower layer has left the funnel
6. Place a second conical flask under the separating funnel to collect the other layer
7. You will now have one conical flask containing the organic layer and one containing the aqueous layer.

Question 15

How do you purify when preparing acids

Answer 15

1. In preparation using acids, your impure product may contain acid impurities.
2. These can be removed by adding aqueous sodium carbonate and shaking the mixture in the separating funnel
3. Any acid present will react with the sodium carbonate releasing CO2 gas.
4. The tap needs to be opened slowly, holding the stoppered separating funnel upside down, to release any gas pressure that may build up
5. The aqueous sodium carbonate layer is removed and the organic layer washed with water before running both layers off into two separate flasks

Question 16

When do you need to dry the organic product

Answer 16

1. There may be some water left in the organic product

2. Traces of water are removed by adding a drying agent to the organic liquid

Question 17

What is a drying agent and give three common examples

Answer 17

1. An anhydrous inorganic salt that readily takes up water to become hydrated
2. Anhydrous calcium chloride CaCl2 - used to dry hydrocarbons
3. Anhydrous calcium sulfate CaSO4 - general drying
4. Anhydrous magnesium sulfate MgSO4 - general drying

Question 18

Describe the process for drying organic products

Answer 18

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 together
3. Place a stopper on the flask to prevent your product from evaporating away. leave for about 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 the solid into another flask
6. If the liquid is dry it should be clear.

Question 19

When do you need to redistill an organic product

Answer 19

1. Sometimes organic liquids have boiling points that are relatively close together, so your prepared sample may still contain some organic impurities.

Question 20

Describe how to carry out redistillaiton

Answer 20

1. The distillation apparatus is cleaned and dried and set up again so that a second distillation can be carried out
2. This time, only collect the product with the boiling point of the compound you are trying to make.
3. The narrower the boiling range, the purer the product
4. You now will have separated you product from any impurities

Question 21

State the steps needed to prepare an organic solid

Answer 21

1. Filtration under reduced pressure
2. Recrystallisation
3. Melting point determination

Question 22

What are the 2 different organic solids you can prepare and state the chemicals and outline the method used in each case

Answer 22

1. Aspirin- reflux salicylic acid and ethanoic acid and glacial ethanoic acid using Quickfit apparatus
2. Benzoic acid by alkaline hydrolysis- Use Quickfit apparatus to reflux methyl benzoate with aqueous sodium hydroxide, followed by acidification
3. In both preparations a solid forms which has to be separated from the reaction mixture before it can be purified.

Question 23

When do you do filtration under reduced pressure

Answer 23

1. It is a technique for separating a solid product from a solvent or liquid reaction mixture

Question 24

What apparatus do you need to carry out filtration under reduced pressure

Answer 24

1. Buchner flask
2. Buchner funnel
3. Pressure tubing
4. Filter paper
5. Access to filter or vacuum pump

Question 25

Describe the set up for filtration under reduced pressure

Answer 25

1. Connect one end of the pressure tubing to the vacuum outlet or to the 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. This is usually obtained using a Buchner ring or a rubber bung
3. Switch on the vacuum pump, or the tap, to which your filter pump is attached and check for good suction by placing your hand across the top of the funnel

Question 26

Describe how to carry out filtration under reduced pressure

Answer 26

1. Place a piece of filter paper inside the Buchner funnel and wet this using the same solvent used in preparing your solid.
2. You should see the paper being sucked down against the holes in the funnel
3. To filter your sample, slowly pour the reaction mixture from a beaker into the centre of the filter paper
4. Rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner flask
5. Rinse the crystals in the Buchner funnel with more solvent and leave them under suction for a few minutes so that the crystals start to dry

Question 27

When do you need to do recrystallisation when preparing an organic solid

Answer 27

1. The solid product obtained after filtration will contain impurities which can be removed by carrying out recrystallisation.
2. Purification by recrystallisation depends upon the desired product and the impurities having different solubilities in the chosen solvent

Question 28

What are the steps in recrystallisation for the preparation of an organic solid

Answer 28

1. Pour a quantity of the chosen solvent into a conical flask.
2. If the solvent is flammable, warm the solvent over a water bath. If the solvent is water, place the conical flask on a tripod and gauze over a bunsen and warm the solvent
3. Tip the impure sample into a second conical flask or beaker
4. Slowly add the solvent to the impure sample until it dissolves in the solvent. You should add the minimum volume of solvent needed to dissolve the solid
5. Once the solid has dissolved allow the solution to cool
6. Crystals of the desired product should form in the conical flask or beaker.
7. When no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid.

Question 29

When do you carry out melting point determination in the preparation of an organic solid

Answer 29

1. Chemists determine the melting point of solids to identify whether a solid compound is pure.
2. A pure organic substance usually has a very sharp melting range of one or two degrees
3. The melting range is the difference between the temperature at which the sample starts to melt and the temperature at which melting is complete.
4. If the compound contains impurities, the solid melts over a wide range of temperatures.
5. An impure sample has a lower melting point than a pure sample

Question 30

Describe the process of preparing a sample of an organic solid product to determine the melting point its melting point

Answer 30

1. Before taking the melting point of a solid you should ensure that the sample is completely dry and free flowing
2. Take a glass capillary tube or melting point tube. Hold one end of the capillary tube in the hot flame of a Bunsen burner. Rotate the tube in the flame until the end of the tube is sealed
3. The capillary tube is allowed to cool and is then filled with crystals to around 3mm depth. This is usually carried out by pushing the open end of the capillary into the solid sample to force some of the solid into the tube.
4. Once you have prepared you sample you will need to take its melting point. By one of two methods.

Question 31

State the two methods used to determine the melting point of an organic solid product

Answer 31

1. Using electrically heated melting point apparatus

2. Using an oil bath or Thiele tube method

Question 32

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

Answer 32

1. Place the capillary tube containing the sample into a sample hole and a 0-300 degrees thermometer in the thermometer hole of the melting point apparatus
2. Using the rapid heating setting, start to heat up the sample whilst observing the sample through the magnifying window
3. Once the solid is seen to melt, record the melting point. Allow the melting point apparatus to cool
4. Prepare a second sample in a new capillary tube and place 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. An accurate determination of the melting point can then be obtained.

Question 33

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

Answer 33

1. Set up the Thiele tube or oil bath- It is filled with oil and clamped in place
2. Attach the capillary tube containing the sample to a thermometer using a rubber band.
3. Insert the thermometer through a hole in the cork if using Thiele tube or clamp the thermometer if using an oil bath. The end of the thermometer should dip in the oil
4. Using a micro-burner, slowly heat the side arm of the Thiele tube or the oil bath whilst observing the solid.
5. When the solid starts to melt, remove the heat and record the temperature at which all the solid has melted
6. It is important to heat the oil slowly when approaching the melting point, and it is advisable to repeat the melting point determination a second time to ensure that you obtain an accurate value.