AQA A level Chem 3.09 Carboxylic Acids and Derivatives

Explain how carbon dioxide causes global warming (2)

C=O bonds in CO₂ absorb infrared radiation (of 2350 cm^-1) (1)

IR radiation emitted by the earth does not escape (from the atmosphere) (1)

Acyl chlorides are useful reagants in synthesis. They react with aromatic compounds also with alcohols.

CH₃CH₂COCl reacts with benzene in the presence of AlCl₃ to form the electrophilic substitution reaction.

Give an equation for the reaction of CH₃CH₂COCl with AlCl₃ to form the electrophile

Outline a mechanism for the reaction of this electrophile with benzene (4)

AlCl₃ + CH₃CH₂COCl → CH₃CH₂-⁺C=O + AlCl₄ (1)

A four-step synthesis of compound T is shown.

Give the reagent and conditions for Step 1. State how you could obtain a sample of the alcohol from the reaction mixture formed in Step 1. (3)

NaOH (1)

Aqueous (1)

Fractional distillation (1)

A four-step synthesis of compound T is shown.

Draw the structure of compound S. For each of the steps 3 and 4, give a reagent and one condition, other than heat (5)

S is CH₃CH(CN)CH₂CH₃ (1)

KCN (1)

Alocholic/(aqueous) (1)

H₂ (1)

Ni (1)

Suggest why aqueous ethanol is a suitable solvent when heating the coconut oil with KOH. Give a safety precaution used when heating the mixture. Justify your choice. (3)

reason - Allow to dissolve both oil and KOH (1)

safety precaution - use a water bath for heating mixture (1)

Prevents risk of fire/ethanol is flammable (1)

The crude aspirin can be purified by recrystallisation using hot ethanol (boiling point = 78^oC) as the solvent.

Describe two importance precautions when heating the mixture of ethanol and crude aspirin (2)

Ethanol is flammable so use a water bath to heat (1)

Heat to temperature below boiling point (1)

The pure aspirin is filtered under reduced pressure. A small amount of cold ethanol is then poured through the Buchner funnel

Explain the purpose of adding a small amount of cold ethanol (1)

To remove soluble impurities (1)

A sample of the crude aspirin is kept to compare with the purified aspirin. Describe one difference in appearance you would expect to see between these two solid samples. (1)

Pure product will have larger crystals (1)

Aspirin can be produced by reacting salicylic acid with ethanoic anhydride. An incomplete method to determine the yield of aspirin is shown.

1. Add about 6 g of salicylic acid to a weighing boat.

2. Place the weighing boat on a 2 decimal place balance and record the mass.

3. Tip the salicylic acid into a 100 cm³ conical flask.

4. Reweigh the empty boat

5. Add 10 cm³ of ethanoic anhydride to the conical flask and swirl.

6. Add 5 drops of concentrated phosphoric acid.

7. Warm the flask for 20 minutes.

8. Add ice-cold water to the reaction mixture and place the flask in an ice bath.

9. Filter off the crude aspirin from the mixture and leave it to dry.

10. Weigh the crude aspirin and calculate the yield.

Suggest two ways in which the melting point of the crude aspirin collected in step 9 would differ from the melting point of pure aspirin (2)

Value is lower (1)

Range of values (1)

This method is used to prepare a sample of ester F.

Step 1

Mix 10 cm³ of propan-2-ol with 10 cm³ of ethanoic acid.

Add 5 drops of concentrated sulfuric acid.

Reflux this reaction mixture for 20 minutes.

Step 2

Transfer the cooled reaction mixture to a separating funnel.

Add 20 cm³ of aqueous sodium carbonate and shake the mixture.

Step 3

Transfer the organic layer to a beaker and add 5 g of anhydrous magnesium sulfate.

Decant off the organic liquid.

Step 4

Collect the ester using simple distillation.

Describe how Step 1 should be done.

In your description you should

• give details of suitable equipment used to add each reagent to the reflux apparatus

• draw a labelled diagram of the apparatus used for refluxing the reaction mixture

• explain any safety precautions needed other than eye protection. (6)

Level 3 5-6 marks - All stages are virtually complete (one from stage 1 and 2 from stages 2 and 3)

stage 1

measuring cylinder(s) for the propan-2-ol and ethanoic acid

(dropping) pipette for sulfuric acid

stage 2

2a Labelled condenser shown vertical and open at top and bottom i.e. in cross section

2b Labelled water in at the bottom and water out at the top of the vertical condenser

2c Labelled reaction flask recognisable as either pear shaped or round bottomed

stage 3

3a use a fume cupboard AND to avoid breathing in harmful/toxic compounds

3b Wear gloves

3c Add glass beads/chips (to the mixture before heating) labelled as anti-bumping granules

3d Use an electric heater/water bath AND as compounds are flammable

This method is used to prepare a sample of ester F.

Step 1

Mix 10 cm³ of propan-2-ol with 10 cm³ of ethanoic acid.

Add 5 drops of concentrated sulfuric acid.

Reflux this reaction mixture for 20 minutes.

Step 2

Transfer the cooled reaction mixture to a separating funnel.

Add 20 cm³ of aqueous sodium carbonate and shake the mixture.

Step 3

Transfer the organic layer to a beaker and add 5 g of anhydrous magnesium sulfate.

Decant off the organic liquid.

Step 4

Collect the ester using simple distillation.

In Step 2 the reaction mixture from Step 1 is shaken with aqueous sodium carbonate.

State the purpose of the sodium carbonate

Suggest a precaution that should be taken while this mixture is shaken in the separating funnel

Give a reason for your suggested precaution (3)

To neutralise/remove (excess) acid (1)

Remove stopper/bung (1)

There will a build up of pressure/gas/CO₂ (1)

Aspirin is produced from 2-hydroxybenzenecarboxylic acid by reaction with

ethanoic anhydride in the presence of concentrated phosphoric acid.

Method

1. Add 2-hydroxybenzenecarboxylic acid to a conical flask.

2. Add excess ethanoic anhydride.

3. Add a few drops of concentrated phosphoric acid.

4. Heat the flask to 85 °C for 10 minutes.

5. Cool the flask and pour the contents into 150 cm3 of cold water.

6. Filter and wash the impure solid aspirin.

7. Recrystallise the aspirin using a 50:50 mixture of water and ethanol.

8. Check the purity of the aspirin.

Suggest why reflux is not essential when the flask is heated to 85^oC for 10 minutes (1)

Boiling points are above 85^oC (1)

Complete and label the diagram below to show how the impure solid is filtered. (2)

(2)

Describe the recrystallisation process used to purify the aspirin. Include the method and apparatus used. (6)

Dissolve crude product in hot solvent (1)

of minimum volume (1)

Filter (hot to remove insoluble impurities) (1)

cool (to recrystallise) (1)

filter under reduced pressure/ with Buchner apparatus (1)

Wash (with cold solvent) and dry (1)

State the physical property that is measured to check the purity of the aspirin. (1)

Melting point (1)

Ethyl ethanoate can be made by reacting ethanol with ethanoic acid in the presence of concentrated sulfuric acid.

Method

1. A mixture of ethanol, ethanoic acid, and concentrated sulfuric acid, with anti-bumping granules, is heated under reflux for 10 minutes.

2. The apparatus is rearranged for distillation.

3. The mixture is heated to collect the liquid that distils between 70 and 85 °C

4. The distillate is placed in a separating funnel. Aqueous sodium carbonate is added, and a stopper is placed in the funnel. The mixture is shaken, releasing pressure as necessary.

5. The lower aqueous layer is removed and the upper organic layer is placed in a small conical flask.

6. Anhydrous calcium chloride is added to the sample in the conical flask. The flask is shaken well and left for a few minutes.

7. The liquid from the flask is redistilled and the distillate is collected between 74 and 79 °C

The figure below shows how a student set up the apparatus for reflux in step 1. You should assume that the apparatus is clamped correctly.

State why sodium carbonate is added to the distillate in step 4. Explain why there is a build-up of pressure in the separating funnel. (2)

to neutralise/react with/remove the acid (1)

CO₂ is produced (1)

Ethyl ethanoate can be made by reacting ethanol with ethanoic acid in the presence of concentrated sulfuric acid.

Method

1. A mixture of ethanol, ethanoic acid, and concentrated sulfuric acid, with anti-bumping granules, is heated under reflux for 10 minutes.

2. The apparatus is rearranged for distillation.

3. The mixture is heated to collect the liquid that distils between 70 and 85 °C

4. The distillate is placed in a separating funnel. Aqueous sodium carbonate is added, and a stopper is placed in the funnel. The mixture is shaken, releasing pressure as necessary.

5. The lower aqueous layer is removed and the upper organic layer is placed in a small conical flask.

6. Anhydrous calcium chloride is added to the sample in the conical flask. The flask is shaken well and left for a few minutes.

7. The liquid from the flask is redistilled and the distillate is collected between 74 and 79 °C

Give a reason why two layers form in the separating funnel. Suggest why ethyl ethanoate forms the upper layer. (2)

ethyl ethanoate/it is immiscible with/insoluble in water (1)

ethyl ethanoate is less dense (than water) (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

What happens in step 1? (1)

Aspirin dissolves (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

Why is a minimum volume of hot water used? (1)

To obtain a saturated solution (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

What is the purpose of step 2? (1)

To remove insoluble impurities (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

What is the purpose of step 3 (1)

Aspirin will form crystals whereas soluble impurities continue to stay in solution (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

Draw a diagram of vacuum filtration apparatus (1)

filtrate labelled too (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

Give two reasons why the yield could be less than 100%

loss of aspirin in the apparatus (1)

some aspirin dissolves in the water with all reactants reacted (1)

A solvent is chosen where the organic product is very soluble in hot solvent and insoluble(or less soluble) in cold solvent e.g aspirin is very soluble in hot water and not very soluble in cold water

Method

Step 1 : The sample is dissolved in a minimum volume of hot water

Step 2 : The solution is filtered hot

Step 3 : The filtrate is cooled in ice to form crystals

Step 4 : The crystals are collected by vacuum filtration, washed with cold water and left to dry

Give one reason why the yield could be more than 100% (1)

Not dry/water still present (1)