A LevelChemistryEdexcelExam Questions7. Advanced Organic Chemistry (A Level Only)7.6 Organic Chemistry III: Organic Synthesis

Organic Chemistry III: Organic Synthesis (Edexcel A Level Chemistry)

Exam Questions

2 hours7 questions
Medium
Hard
1a2 marks

This question is about extracting benzoic acid from a mixture of benzoic acid, C6H5COOH, and phenol, C6H5OH.

The following steps were carried out.

Step 1

A suitable mass of the mixture was placed in a separating funnel and some ether added. The funnel was shaken to dissolve the mixture.
Step 2

Aqueous sodium hydrogencarbonate was added to the separating funnel, and the contents shaken.
Step 3 Once the reaction was complete, the two layers were allowed to separate.
Step 4 The lower aqueous layer was removed and placed in a beaker.
Step 5

The ether layer in the separating funnel was washed with deionised water and the washings added to the beaker.
Step 6

Hydrochloric acid was added to the aqueous solution in the beaker to precipitate the benzoic acid.
Step 7 The impure benzoic acid was filtered under reduced pressure.
Step 8 The impure benzoic acid was purified by recrystallization.
Step 9

The melting temperature of the purified benzoic acid was measured and compared with the literature value of 122 oC.

Complete the equation for the reaction between benzoic acid and sodium hydrogencarbonate.

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1b1 mark

In Step 2 there is a pressure build-up in the separating funnel.

Describe how you would lower the pressure.

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1c1 mark

State why, in Step 4, the aqueous layer was the lower of the two layers.

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1d1 mark

Give a reason why, in Step 5, the ether layer was washed with deionised water.

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1e2 marks

Explain why the addition of hydrochloric acid in Step 6 results in the precipitation of benzoic acid.

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1f1 mark

Draw a diagram of the apparatus used in Step 7 to filter under reduced pressure.

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1g
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Benzoic acid can be purified in Step 8 because of its high solubility in hot water and low solubility in cold water.

Calculate the maximum number of benzoic acid molecules that can dissolve in 50.0 cm3 of cold water if the solubility is 1.70 g per 1000 cm3

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1h2 marks

The melting temperature range of the purified benzoic acid in Step 9 was 116 – 121 oC.

Compare this result with the literature value, giving a reason for any differences.

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2a8 marks

Some organic compounds contain metals.

Glycinate ions are formed from the amino acid glycine.

q9a-9cho-al-3-june-2019-qp-edexcel-a-level-chem

glycinate ion

i)
Explain the effect, if any, of an aqueous solution containing glycinate ions on plane-polarised monochromatic light.

(2)

ii)
A hot aqueous solution of glycine is added to a hot solution of copper(II) ethanoate.
When the mixture is cooled, crystals of copper(II) glycinate are formed.

Write the equation for this reaction.
State symbols are not required.

(2)

iii)
In an experiment, the crystals are filtered, weighed and the percentage yield calculated.

Student 1 obtained a yield of 102.6%.

Student 2 obtained a yield of 56.4%.

The expected yield is 82% and the students carried out the calculation correctly.

Discuss possible reasons for the yields obtained by these students.

(4)

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2b2 marks

Haemoglobin is an iron(II) complex. It carries oxygen around the body.
Part of the structure of haemoglobin is shown.

q9b-9cho-al-3-june-2019-qp-edexcel-a-level-chem

The four nitrogen atoms are part of a multidentate ligand in the haem group.

Explain why inhaling carbon monoxide can be fatal.

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2c6 marks

Grignard reagents contain a metal.

Discuss how Grignard reagents are formed and used in adding one or more carbon atoms to the carbon chain in 1-bromopropane to produce primary, secondary and tertiary alcohols and a carboxylic acid.

Include a suitable example for each reaction and give reagents, conditions and products.
You may include equations in your answer.

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3a
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Organic compound D contains the elements carbon, hydrogen, oxygen and nitrogen only.

A sample of D was burned completely in the apparatus shown.
Solid X absorbed the water formed in the combustion.
Solid Y absorbed the carbon dioxide.

q10a-9cho-al-3-june-2019-qp-edexcel-a-level-chem

i)
The masses of solids X and Y increased during the experiment.

Explain the effect, if any, on the changes in mass of X and Y if the oxygen gas was not dry.

(3)

ii)
On combustion in dry oxygen, 3.36 g of D produced 0.72 g of water and 5.28 g of carbon dioxide.
This sample of D also contained 0.56 g of nitrogen.

Use these data to calculate the empirical formula of compound D.

You must show your working.

(5)

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3b
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Part of the mass spectrum of D is shown.

q10b-9cho-al-3-june-2019-qp-edexcel-a-level-chem

Deduce the molecular formula of D. Justify your answer.

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3c6 marks

Compound D contains a benzene ring.

i)
Give the molecular formula of the species that causes the peak at m / z = 76 in the mass spectrum of D.
(1)
ii)
Draw the structures of the three possible isomers of D containing a benzene ring.
(2)
iii)
The 13C NMR spectrum of compound D has four peaks.

Identify the structure of D. Justify your answer by labelling the different carbon environments in all the structures drawn in (c)(ii).
(3)

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1a4 marks

This question is about the preparation and analysis of paracetamol.

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Paracetamol may be prepared from phenol in three stages.

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In Stage 1, phenol is nitrated using dilute nitric acid.
The nitration of benzene requires concentrated nitric acid at 55 °C with a catalyst of concentrated sulfuric acid.
Both these reactions are electrophilic substitution.

i)
Explain why phenol can be nitrated using milder conditions than benzene.

(2)

ii)

A mixture of 2-nitrophenol and 4-nitrophenol is produced in Stage 1.
They are separated by steam distillation.

The boiling temperature of 2-nitrophenol is 215 °C and that of 4-nitrophenol is 279 °C.

Explain, in terms of intermolecular forces, why 4-nitrophenol has a higher boiling temperature than 2-nitrophenol.
You may include a diagram in your answer.

(2)

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1b1 mark

State the type of reagent needed to convert 4-nitrophenol into 4-aminophenol in Stage 2.

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1c10 marks

The outline procedure for Stage 3 is:

  • place 1.0 g of 4-aminophenol in a conical flask. Add 9 cm3 of distilled water and stir the mixture
  • add 1 cm3 of ethanoic anhydride to the flask and shake the mixture until a precipitate of impure paracetamol forms
  • remove the paracetamol by filtration under reduced pressure
  • recrystallise the paracetamol using water as the solvent
  • determine the melting temperature of the pure, dry paracetamol.
i)
Draw a labelled diagram of the apparatus used for filtration under reduced pressure.

(3)

ii)

Describe the recrystallisation process to obtain a pure, dry sample of paracetamol.

(5)

iii)
The melting temperature of pure paracetamol is 170°C.

Describe what happens to the melting temperature if the paracetamol is not pure.

(2)

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1d
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The percentage by mass of paracetamol in three brands of paracetamol tablets are shown in the table.

Brand of tablet Percentage by mass of paracetamol
P 92.1
Q 93.6
R 99.7

The amount of paracetamol in a tablet can be determined using a titration with cerium(IV) ions.

The tablets are crushed and then hydrolysed in acid to form 4-aminophenol.

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A tablet from one of the three brands of paracetamol was analysed following the outline procedure.

  • one of the tablets was crushed and 0.500 g of the powder was added to dilute sulfuric acid
  • the mixture was heated under reflux until the hydrolysis was complete
  • the solution was made up to 100.0 cm3 in a volumetric flask
  • 25.0 cm3 portions of the solution were titrated against 0.100 mol dm−3 Ce4+ using ferroin as indicator

Result
The mean titre was 16.50 cm3

i)
Give the molecular formula of paracetamol.

(1)

ii)
Determine, by calculation, which brand of tablet was analysed.

(5)

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2a5 marks

This question is about the synthesis of organic compounds.

A student suggested the following plan for the synthesis of pentanoic acid from but-2-en-1-ol.

q7a-9cho-al-2-nov-2020-qp-edexcel-a-level-chem

i)
LiAlH4 is a source of hydride ions, H−

Give a possible reason why LiAlH4 cannot be used to reduce alkenes.

(1)

ii)
Give a suitable reagent and condition for Step 1.

(2)

iii)
Step 2 is incorrect because alcohols can only be converted to nitriles via an intermediate compound.
Identify a suitable intermediate compound by name or formula.

(1)

iv)
Step 3 involves the hydrolysis of a nitrile.

Give the additional reagent that should be added after heating under reflux with aqueous sodium hydroxide, to produce pentanoic acid.

(1)

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2b7 marks

Devise a four-step synthesis, involving the use of a Grignard reagent, to convert benzene into benzoyl chloride.

q7b-9cho-al-2-nov-2020-qp-edexcel-a-level-chem

Include the reagents and conditions for each step in the synthesis and the structures of the intermediates.

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3a5 marks

Phenylethene, commonly known as styrene, is an important substance in the production of polystyrene which is used for some types of plastic packaging.
Phenylethene can be made from benzene in a three-step synthesis.

q7-9cho-al-2-june-2018-qp-edexcel-a-level-chem

Some of the following compounds can be used to make phenylethene from benzene.

Aluminium chloride Chloroethane Ethanal Ethanol
Ethanoic acid Ethanoyl chloride Ethene Ether
Hydrochloric acid, concentrated Lithium
tetrahydridoaluminate(III)		 Phosphoric acid,
concentrated		 Sulfuric acid,
concentrated

Selecting only from these compounds, devise a synthetic pathway for converting benzene into phenylethene, clearly identifying compounds A and B and stating the appropriate conditions for each step.

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3b1 mark

Which reagent could produce a diol from phenylethene?

  ☐ A acidified potassium dichromate(VI)
  ☐ B acidified potassium manganate(VII)
  ☐ C aqueous sodium hydroxide
  ☐ D steam

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3c1 mark

Draw a section of the polymer, polystyrene, showing two repeat units.

q7c-9cho-al-2-june-2018-qp-edexcel-a-level-chem

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3d2 marks

Give one advantage and one disadvantage of the disposal of polystyrene by incineration.

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3e
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Calculate the percentage by mass of carbon in phenylethene, giving your answer to an appropriate number of significant figures.

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46 marks

The alcohol 2,2-dimethylbutan-1-ol has the structure

q12-9cho-al-2-oct-2021-edexcel-a-level-chem

Devise a reaction scheme for a synthesis of this alcohol starting from 2-bromo-2-methylbutane.

Include in your answer all reagents and conditions and the structures of any intermediate compounds.

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