52: Tests for Functional Groups, Separation and Purification of Compounds

Question 1

Describe the test for carbon-carbon double bond (unsaturation) in organic compounds and its expected result.

Answer 1

Add a few drops of aqueous bromine to the organic compound under test and shake the mixture. When added to an unsaturated carbon compound, the aqueous bromine changes from brown to colourless quickly.

Question 2

Describe the test for primary or secondary alcohols and its expected result.

Answer 2

Mix acidified potassium dichromate solution with the compound under test and warm the mixture. A primary or secondary alcohol turns the acidified potassium dichromate from orange to green.

Question 3

Describe another test other than using acidified potassium dichromate for primary or secondary alcohols and its expected result. Explain why the test is not preferred.

Answer 3

Mix acidified potassium permanganate solution with the compound under test and warm the mixture. A primary or secondary alcohol turns the acidified potassium permanganate from purple to colourless.
However, if excess potassium permanganate is added, the colour change is not as obvious as the reaction mixture remains purple at the end of the reaction.

Question 4

Describe another test without involving acidified solutions for alcohols and its expected result.

Answer 4

Add phosphorus pentachloride to the compound under test. White mist of hydrogen chloride gas is released and the alcohol becomes chlorinated.

Question 5

Acidified potassium dichromate solution is mixed with an organic compound under test and heated under reflux. Explain whether pH testing ensures that the original compound under test is a primary alcohol.

Answer 5

No, because the reaction mixture has already been acidified, and the pH will always be lower than 7 regardless of the products produced after reaction (if any).

Question 6

Describe the test for carbonyl functional groups in organic compounds and its expected result.

Answer 6

Add a solution of 2,4-dinitrophenylhydrazine to the carbon compound under test. A carbonyl compound gives an orange or yellow precipitate.

Question 7

Draw the structure of 2,4-dinitrophenylhydrazine.

Answer 7

Question 8

Name the products after ethanal reacts with 2,4-dinitrophenylhydrazine.

Answer 8

ethanal-2,4-dinitrophenylhydrazone

Question 9

Name two tests for aldehyde functional group and its expected result.

Answer 9

1. The aldehyde can be oxidised by acidified potassium dichromate, turning it from orange to green.
2. The aldehyde produces a silver mirror when treated with Tollens’ reagent.

Question 10

Describe the steps to produce Tollens’ reagent.

Answer 10

1. Add sodium hydroxide solution to silver nitrate solution, producing a brown precipitate.
2. Add dilute aqueous ammonia dropwise until the brown precipitate re-dissolves.

Question 11

Name and state the chemical formula of the ion present in Tollens’ reagent.

Answer 11

Diamminesilver(I) ion,
[Ag(NH₃)₂]⁺

Question 12

State the products after reacting an aldehyde with Tollens’ reagent.

Answer 12

Carboxylate ion, ammonia, water, silver mirror

Question 13

Write the ionic equation of the reaction between an aldehyde (RCHO) and Tollens’ reagent.

Answer 13

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH^- -> RCOO^- + 2Ag + 4NH₃ + 2H₂O

Question 14

Describe the steps in conducting Tollens’ test and its positive result.

Answer 14

Add a few drops of the carbon compound under test to around 2cm³ of freshly prepared Tollens’ reagent in a test tube. Warm the mixture in a hot water bath. A silver mirror forms on the inside of the test tube if the carbon compound is an aldehyde.

Question 15

Describe the test for carboxylic acids and its expected result.

Answer 15

Add the carbon compound to some hydrogen carbonate or carbonate solution (or solid). Effervescence occurs if the carbon compound if a carboxylic acid. The gas can be collected and bubbled into limewater which turns milky, proving that the gas is carbon dioxide.

Question 16

Describe the test for carboxylic acids or alcohols and its expected result.

Answer 16

Mix the carbon compound (suspected to be carboxylic acid) under test with an alcohol, or mix it (suspected to be alcohol) with a carboxylic acid.
Use concentrated sulphuric acid as a catalyst and heat the mixture under reflux. A sweet, fruity smell from an ester can be detected if the carbon compound is a carboxylic acid (or alcohol).

Question 17

Name 3 separation and purification methods for liquid products.

Answer 17

Distillation, fractional distillation, liquid-liquid extraction

Question 18

Name 2 separation and purification methods for solid products.

Answer 18

Recrystallisation, chromatography

Question 19

Describe the steps in conducting a simple distillation.

Answer 19

Heat the mixture until it boils gently.
When the vapour temperature is approximately 2ºC below the boiling point of the desired liquid, put the collecting flask in place.
Collect the distillate until the temperature of the apron rises above the boiling point of the desired liquid and stop heating.

Question 20

Explain the difference between fractional distillation and simple distillation.

Answer 20

A fractionating column is used in fractional distillation while it is not in simple distillation. It is filled with glass beads, which greatly increases the surface on which the vapour from the mixture can condense and the condensed liquid can vaporise when meeting hot vapour passing up the column. This allows the vapour to become richer and richer in the most volatile liquid in the mixture as it rises up the column.

Question 21

List two features which are important for the choice of solvents in liquid-liquid extraction.

Answer 21

1. The solvent added should be immiscible with the solvent containing the desired product.
2. The desired product would have a higher solubility in the solvent added than in the reaction mixture.

Question 22

Describe the steps in conducting a liquid-liquid extraction.

Answer 22

1. Place the reaction mixture in a separating funnel, and the add the chosen solvent, which should form a separate layer.
2. Place the stopper and gently shake the contents of the funnel. During shaking, open the tap of the separating funnel regularly to release any pressure that may build up.
3. Allow the contents to settle into two layers.
4. Remove the stopper and open the tap to drain the lower layer into a flask. Then pour the upper layer into a separate flask.

Question 23

List one feature which is important for the choice of solvents in recrystallisation.

Answer 23

The solid product is soluble when the solvent is hot, but insoluble or much less soluble when the solvent is cold.

Question 24

Describe the steps in conducting recrystallisation.

Answer 24

1. Dissolve the impure solid in a minimum volume of hot solvent.
2. Filter the hot mixture to remove any insoluble impurities, retaining the filtrate.
3. Leave the filtrate to cool until crystals form, leaving the soluble impurities in solution.
4. Collect the crystals by filtration.
5. Wash the crystals with a small amount of cold solvent to remove any of the filtrate which contains dissolved impurities.
6. Allow the solvent to evaporate from the crystals in the air or in a desiccator.

Question 25

Explain a choice of a specialised funnel used for hot filtration in recrystallisation.

Answer 25

The solution may cool down quickly on the stem of the funnel to form crystals which may block the funnel stem. As a solution, **stemless funnels** can be used so any solution passing through the filter paper can directly be collected as filtrate in the container below, reducing the loss of product.

Question 26

Explain the importance of using a minimum amount of hot solvent in recrystallisation.

Answer 26

This ensures that as much of the solid product is obtained as possible.

Question 27

List 4 usages of chromatography.

Answer 27

1. To separate and identify the components of a mixture of chemicals 2. To check the purity of a chemical 3. To identify the impurities in a chemical preparation 4. To purify a chemical product.

Question 28

Name three types of chromatography.

Answer 28

Paper chromatography, thin layer chromatography, column chromatography

Question 29

Describe the difference between the stationary phase and the mobile phase in chromatography.

Answer 29

The stationary phase does not move and is normally a solid or liquid supported on a solid. Meanwhile, the mobile phase moves and is normally a liquid or a gas.

Question 30

Name the stationary phase and mobile phase in paper chromatography.

Answer 30

Stationary phrase: water trapped in the fibres of the chromatography paper Mobile phase: developing solvent

Question 31

Describe the steps in producing a paper chromatogram.

Answer 31

1. Spot the solution under test on a pencil line 1cm from the base of the chromatography paper. 2. Add the developing solvent to a chromatography tank and cover it with a lid. Let the tank stand for a while to allow the air inside to be **saturated** with the solvent vapour. 3. Place the prepared paper in the tank with the spot **above** the level of the solvent. 4. The solvent rises up the paper, carrying the components of the mixture with it. 5. Take the paper from the tank when the solvent gets near the top. Mark the position of the solvent front. A **chromatogram** is produced on the resulting paper.

Question 32

Explain the importance of using pencil to mark the starting and finishing lines of the solvent.

Answer 32

Pencil is used because pencil lines (graphite) will not run into the solvent.

Question 33

Explain the relationship between the height of different dyes and their solubilities in a chromatogram.

Answer 33

Dyes that are more soluble in the mobile phase than in the stationary phase move more **rapidly** up the paper and rise to a **higher position** compared to the starting position. Meanwhile, those that are more soluble in the stationary phase than the mobile phase move **slower** up the paper and are **not carried as far up** the paper, resulting in a lower final position.

Question 34

State the definition of the retention factor.

Answer 34

The retention factor is the distance a component travels relative to the solvent. R_f = distance travelled by the component / distance travelled by the solvent

Question 35

Name a substance which can make amino acid spots in chromatograms visible.

Answer 35

Ninhydrin (forming a purple compound with amino acids)

Question 36

Name a substance which can make carbon compounds in chromatograms visible.

Answer 36

Iodine (turning the carbon compounds brown)

Question 37

Name the stationary phase in thin layer chromatography.

Answer 37

A thin layer of alumina (**aluminium oxide**) or silica gel (**silicon dioxide**), which is supported on a glass or plastic plate.

Question 38

List two reasons why components in the mixture can be separated using thin layer chromatography.

Answer 38

1. They differ in the extent to which they are **soluble in the mobile phase**. 2 .They are **absorbed by the stationary phrase** to different extents.

Question 39

Name the stationary phase in column chromatography.

Answer 39

Alumina (**aluminium oxide**) or silica gel (**silicon dioxide**) in a vertical glass column.

Question 40

List two advantages of thin layer chromatography compared to paper chromatography.

Answer 40

1. It is quicker than paper chromatography. 2. It can be used to analyse smaller samples.

Question 41

Describe the steps in conducting column chromatography.

Answer 41

1. The mixture to be separated is dissolved in a suitable solvent and placed on the top of the column. 2. Fresh solvent is added to the column. The solvent moves through the column and the components in the mixture separate depending on their **different solubilities in the solvent** and their **different absorption by the stationary phase**.

Question 42

State the meaning of elution in column chromatography.

Answer 42

Elution refers to adding a solvent in the column and washing the mixture so that different components are separated.

Question 43

State one advantage of column chromatography over other chromatography methods.

Answer 43

It can be used to **collect fairly large amounts** of substances for further use.

Question 44

Describe the steps in determining the melting point of a substance.

Answer 44

1. Insert some of the solid product into a **capillary tube** and then attach the tube open end upwards to a thermometer with a rubber band. 2. Place the thermometer into a warm liquid bath with boiling point **higher** than the melting temperature of the solid product tested. 3. Slowly heat the liquid bath with **constant stirring**. Notice the temperature at which the soldi product starts and finished melting.

Question 45

Describe the steps in determining the boiling point of a small amount of liquid product (boiling point lower than 100ºC).

Answer 45

1. Place a small amount of the liquid product in the **ignition tube** and attach it to the thermometer using a rubber band. 2. Insert a capillary tube sealed at 0.5cm form. one end into the liquid. 3. Clamp the thermometer in the beaker of water. 4. Slowly heat the water, stirring all the time. Stop heating when the stream of bubbles coming out of the capillary tube is **rapid and continuous**. 5. Allow the beaker of water to cool while stirring continuously. The boiling point of the liquid product is reached when the bubbled stop.