Organic Synthesis (OCR A Level Chemistry)

Exam Questions

3 hours39 questions
1a2 marks

This question is about the two-step synthesis of an alcohol using a 2-bromopropane intermediate.

Alkene Q is used as the starting chemical in the following synthetic route to alcohol R

     Alkene Q rightwards arrow with Step space 1 on top 2-bromopropane begin mathsize 14px style rightwards arrow with Step space 2 on top end style alcohol R

State the identity of alkene Q and alcohol R.

1b3 marks

This question is about alkene Q and Step 1.

i)
Give the empirical formula of Q.

[1]

ii)
Give the reagents and conditions needed for Step 1.

[2]

1c2 marks

State the mechanism involved in Step 1 and explain why it might not be effective in producing 2-bromopropane.

1d2 marks

Give the reagents and conditions needed for Step 2.

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2a2 marks

This question is about the reactions of butan-1-ol.

Butan-1-ol can be oxidised with acidified potassium dichromate to form two different chemicals. 

Name the two possible products formed by the oxidation of butan-1-ol.

2b3 marks

One of the oxidation products of butan-1-ol with acidified potassium dichromate requires the use of distillation apparatus. 

Draw the distillation apparatus for this reaction.

2c2 marks

One of the oxidation products of butan-1-ol with acidified potassium dichromate requires heating under reflux.

Draw the apparatus for heating under reflux.

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3a6 marks

Different alcohols can undergo oxidation reactions to produce a number of different products. These products not only depend on the type of alcohol used, but also on the reagents and the conditions used during the oxidation.

A teacher uses samples of different alcohols with the formula C4H9OH to illustrate some of the different reactions.

Name and draw the displayed formula of a primary alcohol, secondary alcohol and tertiary alcohol with the formula C4H9OH.

3b1 mark

The teacher states that some alcohols will oxidise when heated under reflux with acidified potassium dichromate. 

State the colour change that would be observed for an alcohol that will oxidise under these conditions.

3c2 marks

The teacher mixes the primary and secondary alcohol together before heating them under reflux with acidified potassium dichromate.

Complete the table with the organic products from the reaction of each alcohol with the acidified potassium dichromate. 

 
Reactant Organic product
Primary alcohol  
Secondary alcohol  

3d1 mark

The two organic products of the teacher's reaction using a primary and secondary alcohol have boiling points of 163.5 oC and 79.6 oC.

State, with a reason, which technique can be used to separate the two organic products.

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4a1 mark

This question is about three different methods to purify organic liquids.

  • The use of a separating funnel
  • The use of drying agents
  • Redistillation

Name the two layers that form in a separating funnel. 

4b2 marks

Suggest two ways that the position of the organic layer, in a separating funnel, can be determined. 

4c1 mark

Sodium carbonate solution is sometimes added to the contents of a separating funnel to neutralise any unreacted acid. 

During shaking, the separating funnel is inverted and the tap is opened to release the pressure that builds up due to the formation of a gas. 

Name the gas that builds up inside the separating funnel.

4d2 marks

Drying agents can be added to remove traces of water from organic liquids. A drying agent is added until it remains dispersed in the organic liquid.

State the type of chemicals that are commonly used as drying agents and how they are removed from the organic liquid. 

4e1 mark

A mixture of propanal (boiling point = 49 oC) and propanoic acid (boiling point = 141 oC) can be separated by distillation. 

A crude distillate of propanal has a boiling point range of 46 oC to 53 oC. State what would happen to the boiling point range of the distillate after redistillation.

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5a3 marks

Alcohols and alkenes are versatile chemicals that are often involved in many organic synthetic reaction schemes.

Butan-1-ol can be converted directly into 1-bromobutane. 

State the reagents and conditions required for this conversion.

5b2 marks

Heating butan-1-ol with concentrated sulfuric acid causes the loss of a small molecule.  

State the mechanism involved and name the resulting alkene.

5c1 mark

The alkene formed in part (b) can react with hydrogen bromide to form 1-bromobutane, however there will be a second organic product.

Name the second organic product formed.

5d1 mark

Butan-1-ol can be distilled with acidified potassium dichromate to form butanal according to the following equation.

C4H9OH + [O] → C4H8O + H2O

 Suggest why this is an oxidation reaction.

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1a4 marks

Esters are prepared by heating the carboxylic acid and alcohol under reflux in the presence of a concentrated sulfuric acid catalyst.

Draw a labelled diagram of the equipment required for heating under reflux.

1b2 marks

A student intends to synthesise an ester, A, below:

methylpropanoate-displayed-formula

i)

Name ester A.

ii)

Identify a suitable carboxylic acid and alcohol that could be used to synthesise ester A.

1c3 marks

The student intends to synthesise 15.00 g of ester A, below:

methylpropanoate-displayed-formula

Calculate the number of moles of carboxylic acid required to make 15.00 g of ester A.

1d5 marks

A saturated solution of sodium carbonate is added to the reaction mixture and then placed into the following piece of equipment.

Label the diagram of the equipment that is used to separate ester A from the reaction mixture.

separating-funnel-unlabelled

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2a2 marks

Propanal is a versatile organic building block used in the synthesis of plastics and rubber chemicals.

Propanoic acid can be produced from propanal in the following two-step reaction.

bold Propanal bold space bold rightwards arrow with bold Step bold space bold 1 on top bold Propan bold minus bold 1 bold minus bold ol bold rightwards arrow with bold Step bold space bold 2 on top bold Propanoic bold space bold acid

State the reaction type and suitable reagents for Steps 1 and 2.

2b1 mark

Suggest why it is not possible to convert propanoic acid directly to propanal using the reagent you identified for Step 1 in (a).

2c1 mark

Explain why Step 2, in (a), is completed by distillation.

2d2 marks

Identify which of the three organic compounds, from the reaction scheme in (a), would be distilled first. Explain your reasoning.

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3a2 marks

A student is given the following reagents and instructions to prepare a sample of 1-bromobutane, C4H9Br, (boiling point 102 oC).

Reagents

  • Concentrated sulfuric acid
  • Sodium bromide
  • Butan-1-ol, C4H9OH

Procedure 

  1. Place 5 g of crushed sodium bromide in a 100 cm3 round-bottomed flask containing 10 cm3 of distilled water and 5 cm3 of butan-1-ol (boiling point 117 oC). Swirl gently until the sodium bromide is dissolved.
  2. Carefully add 6 cm3 of sulfuric acid. The sulfuric acid reacts with the sodium bromide to form hydrogen bromide. If the reaction mixture becomes too hot then cool the flask with cold water.
  3. Fit a condenser, in reflux, to the flask and boil the reaction mixture for 30 minutes.
  4. Allow the equipment to cool and rearrange it for distillation.
  5. Gently heat the flask to distil the reaction mixture into a small conical flask. Collect the fraction that boils between 95 oC and 125 oC.
  6. Add sodium hydrogen carbonate solution to the distillate and shake this mixture until no more gas is evolved. Two layer of liquid will form inside the conical flask.
  7. Separate the two layers of liquid, collect the impure 1-bromobutane and wash it with water. Separate the water layer before drying the 1-bromobutane.
  8. Re-distil the 1-bromobutane and collect the fraction that boils between 101 oC and 103 oC.

Suggest hazards for butan-1-ol and concentrated sulfuric acid.

3b4 marks

Draw a diagram to show how you would set up the apparatus for distillation, required in step 4.

3c4 marks

Suggest what impurities might be present within the fraction collected in step 5.

3d2 marks

Write a balanced equation to explain the formation of the gas, in step 6.  

3e
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3 marks

An equation for the reaction is shown below:

C4H9OH + HBr → C4H9Br + H2O

Calculate the mass of 1-bromobutane that would be formed from 5.55 g of butan-1-ol, assuming a 50% yield.

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4a3 marks

But-2-ene can be converted into butan-2-ol by two different synthetic routes. One route can be carried out as a one-step process whilst the other requires two steps.

State reagents and provide an equation, using structural formulae, for the one-step process.

4b3 marks

Draw a reaction scheme, including reagents and conditions for the two-step process.

4c2 marks

But-1-ene can be used to form butan-2-ol by both the one-step and two-step reaction schemes.

Explain why using but-1-ene gives a lower yield of butan-2-ol.

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5a1 mark

A student was asked to prepare a sample of 2-chloro-2-methylpropane using the following instructions.

Procedure

  1. Measure out 20.0 cm3 of concentrated hydrochloric acid and transfer to a separating funnel.
  2. Measure out 4.7 cm3 of 2-methylpropan-2-ol and add to the separating funnel.
  3. Insert the stopper and shake the reaction mixture.
  4. Leave for ten minutes. After this time, two separate layers should start to form.
  5. Separate the organic layer and treat it with a dilute solution of sodium hydrogen carbonate, NaHCO3 (aq) until no further gas is evolved.
  6. Separate then distil the new organic layer. Record the temperature range for the distillation of the organic product.
  7. Add excess anhydrous calcium chloride to the new organic layer and filter. 
  8. Re-distil the organic product. Record the temperature range for this distillation.

Using molecular formulae, write a balanced symbol equation for the reaction.

5b2 marks

In step 5, the organic layer is separated and purified.

Explain why any unreacted 2-methylpropan-2-ol is in the aqueous layer.

5c2 marks

State and explain the difference in the boiling point ranges measured in steps 6 and 8.

5d
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3 marks

The density of 2-methylpropan-2-1ol is 0.78 g cm-3.

Calculate the amount, in mol, of  2-methylpropan-2-ol used in the experiment.

5e
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2 marks

After step 8, 1.60 g of 2-chloro-2-methylpropane was produced.

Calculate the percentage yield of 2-chloro-2-methylpropane in this experiment.

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1a4 marks

This question is about organic pathways. 

Propanone can be synthesised from 2-bromopropane according to the reaction scheme below. 

4-6_q1a-ocr-a-as--a-level-hard-sq

i)
Draw the skeletal formula for compound A.
[1]
ii)
Name and draw the mechanism for step 1. 
[3]

1b4 marks

Give the reagents and conditions required for steps 1 and 2. 

1c3 marks

To prepare a sample of ethanal, the student adapted the reaction scheme from part (a). 

4-6_q1c-ocr-a-as--a-level-hard-sq

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

1d2 marks

Explain the observations the student would make when bromoethane is correctly oxidised to ethanal in part (c).

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2a6 marks

This question is about the synthesis of organic compounds. 

Outline the mechanism and give the reagents and conditions for the reaction shown in Figure 1.

Figure 1

free-radical-substitution

In your answer include the name and conditions required for the mechanism

2b2 marks

Explain how the conditions of this mechanism could be altered to produce a high yield of trichloromethyl benzene.

2c2 marks

In a separate reaction involving similar conditions as part (a) tetrachloromethane was produced from trichloromethane.

Outline both propagation steps for this reaction.

2d3 marks

Draw the skeletal formula of all possible products of mono-substituted halooalkanes when 2-methylbutane undergoes a reaction with bromine in the same conditions asked for in part (a).

2e3 marks

Using information from the question, describe the limitations of free radical substitution. 

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3a6 marks

This question is about a reaction pathway to produce 1-bromo-1-methylcylcohexane.

1-bromo-1-methylcyclohexane can be produced by the following reaction steps, as shown in Figure 1 below.

Figure 1

compound space bold D bold space rightwards arrow with step space 1 on top bold space compound space bold E space rightwards arrow with HBr on top 1 minus bromo minus 1 minus methylcyclohexane

Name and outline the mechanism for the reaction between compound E and hydrogen bromide, HBr. In your answer, draw the major product of the reaction. 

3b3 marks

Explain why a major product and minor product are produced in the reaction outlined in part (a)

3c2 marks

Suggest a possible starting molecule for the reaction pathway in part (a) and give the reagents required for step 1 based on this starting molecule.

3d2 marks

Suggest an alternative reaction pathway to produce 1-bromomethylcyclohexane and give a potential disadvantage of this pathway.

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4a2 marks

This question is about reaction profiles using chloroalkanes

For the reaction profile outlined in Figure 1, state the mechanism or type of reaction for each step.

Figure 1

4-6_q4a-ocr-a-as--a-level-hard-sq

4b1 mark

Reaction step 2 is carried out by heating under reflux.

Identify Compound H.

4c2 marks

1-chlorobutane can be produced from an alkene.  Identify the alkene and give the reagent required.

4d2 marks

Explain how the functional group of the final compound in the reaction scheme in part (a) would be different if 2-chlorobutane was used as the starting compound.

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