Organic Synthesis (OCR AS Chemistry A): Exam Questions

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.

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

i) Name ester A.

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

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

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

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.

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.

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

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

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

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

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.

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

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

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

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.

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.

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

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.

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.

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

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

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

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.

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.

This question is about organic pathways. 

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

i) Draw the skeletal formula for compound A.

[1]

ii) Name and draw the mechanism for step 1. 

[3]

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

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

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

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

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

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

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

In a separate reaction involving similar conditions as part (a) tetrachloromethane was produced from trichloromethane. Outline both propagation steps for this reaction.

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).

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

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

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. 

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

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.

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

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

Reaction step 2 is carried out by heating under reflux.

Identify Compound H.

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

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.