Topic 18: Organic III (c), Organic Synthesis

Question 1

What is refluxing

Answer 1

The continuous evaporation and condensation of a chemical

Question 2

When do you use purification by washing?

Answer 2

When you have a liquid product which is insoluble in water

Question 3

Describe the steps involved in purification by washing

Answer 3

1. The distillate obtained during separation is washed with sodium carbonate solution in a separating funnel to remove acidic impurities (SO2, HBR, Br2)
2. The aqueous (bottom) layer is discarded and the organic layer is washed with water to remove unreacted sodium salts and any soluble organic substances such as ethanol
3. The new aqueous layer is discarded and the organic layer is dried
4. Decant off the organic liquid and check for purity

Question 4

What drying agents are used for purification by washing

Answer 4

anhydrous CaSO4, MgSO4, or Na2SO4

Question 5

Describe the process for solvent extraction of a liquid product which is slightly soluble in water

Answer 5

1. The distillate is transferred to a separating funnel
2. A suitable solvent is added in which only the organic product will be soluble in
3. Two layers form - allow them time to settle then open the tap to discard the bottom layer

Question 6

When do you use solvent extraction

Answer 6

When the liquid product is slightly soluble in water

Question 7

What quality of the organic compound does solvent extraction rely on

Answer 7

Solvent extraction relies on the organic compound being soluble in a specific solvent more so than the contaminates present in the distillate

Question 8

Describe the process of solid product recrystallisation

Answer 8

1. Solid dissolved in hot solvent.
2. Hot solution fitered to remove insoluble impurities
3. Solution cooled to allow the organic compound to recrystallise
4. Mixture filteres under reduced pressure using a Buchner funnel
5. Crystals washed with ice-cold solvent and then removed from the filter paper

Question 9

How do you choose the solvent for solid product recrystallisation

Answer 9

The organic product should dissolve in the solvent when hot but be insoluble when cold.

Question 10

Why do you use the minimum amount of hot solvent

Answer 10

So that the solution is as saturated as possible, making it more likely for crystals to form

Question 11

Why do you wash with the minimum amount of ice-cold solvent

Answer 11

So that the solid product crystal doesnt dissolve in the solvent when you’re washing and decrease yield

Question 12

How do drying agents remove traces of water

Answer 12

The anhydrous drying agent is added and is powdery to begin with. The drying agent absorbs the water to form the water of crystallisation.

Question 13

Describe how to use a drying agent to remove traces of water

Answer 13

1. Add the drying agent continuously until some of it remains powdery indicating that the liquid is dry
2. The drying agent is removed by decantation (pouring off the organic liquid)

Question 14

what can you NOT use anydrous calcium chloride to dry?

Answer 14

amines or alcohols

Question 15

What drying agents can you use for amines or alcohols?

Answer 15

Anhydrous potassium hydroxide
Anhydrous potassium carbonate

Question 16

When can you use salting to separate organic and aqueous layers?

Answer 16

When the organic (liquid) product is slightly soluble in water

Question 17

Describe the process of salting

Answer 17

1. If the organic compound is soluble in water then it can be made less soluble by adding salt to the distillate.
   * The salt has a greater solubility than the organic chemical. The solubility of the organic is reduced.
2. The aqueous layer is then removed using a separating funnel.

Question 18

When should you use fractional distillation instead of simple distillation?

Answer 18

When the liquids you want to separate have similar boiling temperatures - less than 25K difference

Question 19

When is steam distillation used?

Answer 19

Steam distillation is used to extract:
* Scented oils from solvents
* Phenylamine from the reaction mixture after reduction of nitrobenzene with Sn and conc HCl

Question 20

What is the purpose of adding anti-bumping granules?

Answer 20

Promote smoother boiling - prevents large bubbles forming or exploding of liquid down the condensor during distillation for example

Question 21

What can be done to the joints of Quickfit apparatus to give them a better seal?

Answer 21

The joints often have a thin layer of silicon grease smeared over them to give them a better seal (as well as to make it easier to disassemble the equipment afterwards)

Question 22

Where should the bulb of the thermometer sit during distillation?

Answer 22

Where the vapours will pass into the condenser

Question 23

Where does the water enter and exit the condenser jacket?

Answer 23

It enters at the bottom and leaves at the top so that there is a steady and constant stream of water which fills the whole water jacket

Question 24

What should you use to heat the reaction mixture if the chemicals are flammable?

Answer 24

Electric heating mantles

Question 25

What temperatures should the distillate be collected at?

Answer 25

+/- 2 K of the boiling point of the desired product

Question 26

What is the purpose of steam distillation

Answer 26

To separate an insoluble liquid from an aqueous solution.

Question 27

Describe the process of steam distillation

Answer 27

Steam is bubbled through a reaction mixture containing the aqueous solution and the insolube liquid that forms a separate layer.
As the steam bubbles through the reaction mixture, it mixes the layers so that they form part of the evaporating liquid.

Question 28

What are the advantages of steam distillation?

Answer 28

• The insoluble liquid distils at a temperature below its usual boiling point
• It reduces the chances of thermal decomposition of the insoluble liquid

Question 29

How can you determine which layer is aqueous and which layer is organic?

Answer 29

Add water and the aqueous layer increases in volume.

Question 30

What should you do before using a separating funnel to remove the aqueous/organic layer?

Answer 30

Once the solution is added to the separating funnel, a stopper is added. The separating funnel should be inverted and then the stopcock opened to release the pressure. This is repeated 15-20 times.
The two layers are then allowed to separate.

Question 31

What should you do as you are drying if the product has a low boiling point?

Answer 31

A lid/stopper should be added to reduce the potential evaporation of any product.

Question 32

What does it mean if the drying agent clumps together?

Answer 32

There is still water in the organic liquid.

Question 33

What does an impurity do to the melting point of a solid?

Answer 33

Imputrities tend to decrease the melting point of a solid.

Question 34

What is the difference in the melting point of a pure and an impure solid?

Answer 34

Pure substances have sharp well defined melting points whereas impure substances have a broad melting point range

Question 35

What must the sample solid be before melting point determination?

Answer 35

Totally dry and finely powdered - this can be achieved by crushing it with the back of a spatula onto some filter paper or the back of a white tile

Question 36

How do you determine the boiling point of a liquid?

Answer 36

Using distillation

Question 37

How might impurities affect the boiling point of a liquid?

Answer 37

The boiling point may appear higher than the database values or it may boil over a range of temperatures instead of a single temperature

Question 38

What is a non-experimental (kind-of) method of identification and assessment of purity?

Answer 38

Infrared fingerprint comparison (the region of IR spectra below 1300cm-1)

Question 39

Describe the process of boiling point determination

Answer 39

1. Liquid organic sample placed in small ignition tube and capillary tube placed inside the ignition tube
2. Ignition tube attached to thermometer and placed in a water bath and slowly heated
3. Record the temperature when a stream of bubbles comes out of the capillary tubing
4. Allow water bath to cool and record temperature when the organic liquid sucks back into the tubing
5. The average of the two temperatures is the boiling temperature

Question 40

Describe the process of melting point determination

Answer 40

1. Small amount of solid added into a capillary tube
2. Tube attached to a thermometer and placed in water bath
3. Water bath slowly heated and temperature recorded at the moment the solid melts inside the capillary tube

Question 41

Hazard type: Organic chemicals

Answer 41

Flammable

Question 42

Precautions: Flammable organic chemicals

Answer 42

No direct heating or naked flames, only use water baths or electric heaters

Question 43

Hazard: Chemical

Answer 43

• Harmful, can be absorbed through skin
• Harmful, irritating or poisonous vapour
• Corrosive, harmful, irritating and poisonous (toxic)
• Carcinogen

Question 44

Precautions?: harmful chemical which can be absorbed through skin

Answer 44

No bare hands, always wear appropriate protective gloves

Question 45

Precautions?: Harmful, irritating or poisonous vapour

Answer 45

Do not use in populated laboratory spaces or confined spaces; use only in fume cupboard

Question 46

Precautions?: Corrosive, harmful, irritating, and poisonous (toxic)

Answer 46

No bare hands, always wear appropriate protective gloves

Question 47

Precautions: Carcinogen

Answer 47

Not to be used in laboratory