Organic Synthesis (OCR A Level Chemistry)

Exam Questions

3 hours31 questions
1a2 marks

This question is about the preparation of organic solids, such as aspirin.

Give two reasons why small quantities of reagents are used in organic preparations.

1b2 marks

The laboratory preparation of aspirin involves reacting salicylic acid with ethanoic anhydride. Suggest why the salicylic acid is used in excess rather than ethanoic anhydride.

1c1 mark

A preparation of methyl 3-nitrobenzoate contains the following instructions: 

2.0 g of methyl benzoate is added to a small conical flask. The flask is immersed in an ice-water bath and 4.0 cm3 of concentrated sulfuric acid is slowly added.
A mixture of 1.5 cm3 of concentrated nitric acid and 1.5 cm3 of concentrated sulfuric acid is made in a separate test tube and immersed in the ice bath.

Suggest how the concentrated sulfuric and nitric acid could be measured safely.

1d2 marks

Organic preparations frequently involved heating under reflux. Outline how this is carried out and explain the reason it is used.

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2a2 marks

Preparations of organic solids will often involve filtration under reduced pressure, recrystallisation and measurement of a melting point.

Explain the purpose of filtration under reduced pressure and state two pieces of apparatus that are needed.

2b3 marks

Outline the technique of recrystallisation and explain its purpose in the preparation of an organic solid such as aspirin.

2c1 mark

Explain why a melting point determination is necessary following the preparation of a solid such as aspirin.

2d2 marks

A student prepared a sample of methyl 3-nitrobenzoate and carried out a melting point determination before and after recrystallisation. The following measurements were taken:

Sample tested Melting point / oC
Sample before recrystallisation 70-75
Sample after recrystallisation 76-78

The accepted melting point of methyl 3-nitrobenzene is 78 oC

Explain why the melting points of the samples are different.

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3a2 marks

This question is about farnesol, a chemical found in essential oils and used in perfumes, especially in lilac perfumes. The structure of farnesol is shown in Figure 1.

Figure 1

6-8-q3a-farnesol

Identify two different functional groups found in farnesol and identify their locations in Figure 1.

3b1 mark

Describe how you would expect farnesol to react if shaken with a little bromine water.

3c1 mark

Farnesol is an important starting compound in organic synthesis. Figure 2 shows a two-step synthesis of a compound made from farnesol.

Figure 2
screenshot-2022-11-03-112824

Suggest a new functional group present in the product of the first step.

3d1 mark

Suggest a functional group present in the final product, but not in farnesol.

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4a2 marks

This question is about the synthesis of propanoic acid from bromoethane, in a two-step pathway:

6-8_q4a-ocr-a-as--a-level-easy-sq

Give the reagents and conditions needed to carry out step 1.

4b2 marks

Give the reagents and conditions needed in step 2.

4c2 marks

Identify the types of reactions taking place in step 1 and 2.

4d2 marks

Outline why step 1 is a particularly useful reaction in organic synthesis.

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5a2 marks

Morphine is a naturally occurring opiate found in opium from poppies. Heroin is a synthetic opiate that can be synthesised from morphine. Figure 1 shows the chemical structure of morphine and heroin.

Figure 1

heroin-corrected-structure

What functional groups are found in morphine?

5b1 mark

Identify the functional group(s) present in heroin, but not in morphine.

5c2 marks

State the reagents and conditions needed to turn morphine into heroin.

5d3 marks

Predict and explain what you would see if morphine and heroin were separately reacted with warm acidified potassium dichromate solution.

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1a4 marks

An organic reaction scheme is shown below.

C4H8 rightwards arrow with Step space 1 on top C4H10O rightwards arrow with Step space 2 on top C4H8O rightwards arrow with Step space 3 on top C4H8O2 

State the IUPAC names of the four substances in the reaction scheme.

1b6 marks

Classify the reactions in (a) and name a pair of reagents required in each step.

1c1 mark

Give the reaction conditions for step 3.

1d1 mark

Write the structural formula of an isomer of C4H10O that will not react according to step 2.

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2a9 marks

Outline how ethanal can be synthesised from ethane in three steps.

State the reaction conditions and reagents and name the type of reaction taking place.

2b3 marks

The following reaction pathway is used to produce Compounds A and B, which when reacted together, form a branched ester molecule, Compound C

Suggest suitable reagents and conditions for the synthesis of Compound A via Step 1 and give the name for this type of reaction.

q2a_20-2_ib_hl_medium_sq

2c3 marks

The ketone in part (a), must be converted to compound B to produce the ester.

i)

Name the molecule that is produced from Step 2

ii)

Name of the type of reaction that is involved in Step 2 and suggest suitable reagents and conditions for this step.

2d1 mark

Draw the skeletal formula of the ester formed from the reaction scheme.

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3a2 marks

1-(3-Aminophenyl)ethanol can be synthesised according to the following three-step reaction scheme.

q4b_20-2_ib_hl_medium_sq

Step 1 involves a reaction at position 3 of the benzene ring of 1-phenylethan-1-one.

State the reaction type, including suitable reagents, for Step 1.

3b1 mark

Step 2 involves changes to the ketone group.

State a suitable reagent for Step 2.

3c2 marks

In Step 3 of the reaction scheme, compound C is heated with hydrochloric acid in the presence of a tin catalyst to form the final product, 1-(3-aminophenyl)ethanol. 

Explain why Step 3 is a reduction reaction.

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4a3 marks

Suggest a reaction scheme, using displayed formulae, that could be used to prepare a sample of propyl propanoate from one reactant. 

Conditions and reagents are not required.

4b2 marks

One of the intermediates in the reaction scheme has a molecular mass of 74.0 g mol-1.

i)

State suitable reagents and conditions required to form this intermediate. 

ii)

Describe how you could test that this intermediate has been formed.

4c2 marks

Propanal and the intermediate (Mr = 74.0) in the reaction scheme are to be separated by distillation.

Explain which chemical will distil off first.

4d2 marks

One of the intermediates in the reaction scheme has a molecular mass of 60.0 g mol-1. 

i)

State suitable reagents and conditions required to form this intermediate. 

ii)

Describe how you could prove that this intermediate has been formed without reversing the reaction.

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5a1 mark

Aspirin is a medicine used to relieve pain and reduce fever.

A student prepares a sample of aspirin, HOOCC6H4OCOCH3, from salicylic acid, HOOCC6H4OH.

NgLY2ZDQ_salicylic-acid

The student makes aspirin by warming 7.5 g of salicylic acid with 15.0 cm3 of ethanoic anhydride, (CH3CO)2O, in the presence of concentrated sulfuric acid.

Write a balanced equation, using structural formulae, for this reaction.

5b
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4 marks

The density of ethanoic anhydride is 1.10 g cm–3.

Deduce whether the salicylic acid or ethanoic anhydride is in excess. 

5c1 mark

The student transfers their hot solution into water and impure aspirin crystallises out as the water cools.

The student then purifies the aspirin by recrystallisation.

State the properties of a suitable solvent for recrystallisation. 

5d
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2 marks

After recrystallisation, the student obtained 5.91 g of aspirin.

Calculate the student's percentage yield. 

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1a4 marks

Shikimic acid is a cyclohexene derivative found in Japanese star anise and is used as a base for the synthesis of Tamiflu, an important influenza treatment. The structure of shikimic acid is shown below

6-8_q1a-ocr-a-as--a-level-hard-sq

Identify three functional groups found in shikimic acid and suggest how each may be identified.

1b6 marks

Shikimic acid can be converted into other useful intermediates. A three step synthesis is shown below:

6-8_q1b-ocr-a-as--a-level-hard-sq

i)
Give the reagents and conditions needed for steps 1, 2 and 3.
[3]
ii)
State the type of reactions and mechanisms taking place.
[3]
1c
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2 marks

Determine the mass of shikimic acid (Mr =174.15 g mol-1) that must be used in step 1 to produce 10.0 g of compound P(Mr = 255.05 g mol-1), assuming a yield of 65%.

1d1 mark

The conversion of compound P to Q occurs in a single step and requires heating. Heating substances in the laboratory can be carried out using the apparatus below.

6-8_q1d-ocr-a-as--a-level-hard-sq

Give one reason why this apparatus would not be suitable.

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2a3 marks

This question is about a drug called mesalamine (also known as 5-aminosalicylic acid) that is used to treat bowel disease. It can be synthesised from salicylic acid, the chemical structure of which is shown below:

6-8_q2a-ocr-a-as--a-level-hard-sq

State the systematic IUPAC name for salicylic acid and deduce how many positional isomers are possible, giving their names and structures.

2b2 marks

A possible two step synthesis of mesalamine is shown below.

jBY71EV-_ocr-6-8h-q2b

Complete the flowchart showing the structures of compound B and mesalamine.

2c5 marks

What reagents and conditions are needed to convert:

i)
Salicylic acid into compound B?
[2]
ii)
Compound B into mesalamine?
[3]
2d2 marks

Give the structures of compound C and the compound D(repeat unit).

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3a2 marks

Chloroquine is antimalarial drug that was briefly trialed during the Covid-19 pandemic, but not found to be effective in treating SARS-CoV-2 infections. It can be synthesised from 4,7-dichloroquinoline. The structures of the two compounds are shown below.

6-8_q3a-ocr-a-as--a-level-hard-sq

Determine the molecular formula of 4,7-dichloroquinoline and chloroquine.

3b2 marks

Deduce the type of reaction taking place and write the formula of the co-product of the reaction that converts 4,7-dichloroquinoline into chloroquine.

3c2 marks

Organic molecules have to be highly purified for use as pharmaceutical drugs. Suggest how the purity of a sample of chloroquine can be determined after synthesis from 4,7-dichloroquinoline.

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4a2 marks

Ethyl 4-aminobenzoate, shown in Figure 1, is more commonly known as benzocaine. It is an ester based compound that is used medicinally as a local anaesthetic in pain relievers and cough drops.

Figure 1

IMtF8JjJ_q4a-hard-6-8-organic-synthesis-ocr-a-level-chemistry

One of the first industrial processes to manufacture benzocaine was the multi-step reaction shown in Figure 2.

Figure 2

q4a-2-hard-6-8-organic-synthesis-ocr-a-level-chemistry

The current cost of >99.7% pure benzene is around £10.00 per litre compared to > 99.5% pure methylbenzene at a cost of around £7.00 per litre.

Explain why it would be beneficial to start from methylbenzene instead of benzene in the manufacture of benzocaine. Your answer should not include economic considerations.

4b3 marks

Concentrated sulphuric acid and concentrated nitric acid are used to form the nitronium ion required for step 2 of the reaction scheme, in part (a).

The following equilibrium is the first step in the formation of the nitronium ion.

H2SO4 + HNO3 ⇋ HSO4- + H2NO3+

i)
State the implications of this reaction in terms of the roles played by the acids. 

 

ii)
Use your answer to part (i) to compare the strengths of the acids.
4c3 marks

Give the reagents and conditions necessary for steps 4 and 5 of the reaction scheme shown in part (a).

4d2 marks

A student suggests the reaction scheme shown in Figure 3 as an alternative to the original industrial reaction scheme shown in part (a) for the production of benzocaine.

Figure 3

q4d-hard-6-8-organic-synthesis-ocr-a-level-chemistry

i)
State why the student’s reaction scheme is an improvement.

ii)
Suggest why the student’s reaction scheme may not work.

 

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5a2 marks

A student is asked to prepare a sample of propyl propanoate using propanal.

Suggest a reaction scheme, using displayed formulae, that the student could use to prepare their sample of propyl propanoate.

Conditions and reagents are not required.

5b2 marks

Use your answer from part (a) to help answer this question.

One of the intermediates in the reaction scheme, from part (a), has a molecular mass of 74.0 g mol-1.

i)
Give the reagents and conditions required to form this intermediate.

ii)
Describe how you could test that this intermediate has been formed.
5c2 marks

Propanal and the intermediate (Mr = 74.0) in the reaction scheme, from part (a), are to be separated by distillation.

Explain which chemical will distill first.

5d4 marks

Use your answer from part (a) to help answer this question.

One of the intermediates in the reaction scheme, from part (a), has a molecular mass of 60.0 g mol-1

i)
Give the reagents and conditions required to form this intermediate.

ii)
Describe how you could prove that this intermediate has been formed without reversing the reaction.

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6a3 marks

Propanone can be synthesised from 2-bromopropane according to the reaction scheme shown in Figure 1

Figure 1

q6a-hard-6-8-organic-synthesis-ocr-a-level-chemistry

Step 1 is not completed in acidic conditions.

i)
Give the reagents and conditions for step 1.

ii)
Outline the mechanism for step 1
6b3 marks

Give the reagents and conditions for steps 2 and 3 of the reaction scheme in part (a).

6c2 marks

Suggest one reaction to replace steps 1 and 2 in the part (a) reaction scheme.

6d3 marks

To prepare a sample of ethanal, a student adapts the reaction scheme from part (a). The student’s adapted reaction scheme is shown in Figure 2.

Figure 2

q6d-hard-6-8-organic-synthesis-ocr-a-level-chemistry

Explain one improvement the student should make to their adapted reaction scheme.

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