8.3 Organic Chemistry Practicals (A Level only)

Before a student can prepare aspirin, the salicylic acid used must be pure. Describe how the student can purify the salicylic acid and explain why each step is required.

The student prepared a sample of aspirin by following the two stages outlined as follows:

Stage 1

Add a known mass of the purified salicylic acid into a dry pear shaped flask and add an excess of 5 cm³ of ethanoic anhydride followed by 8 drops of concentrated phosphoric acid. Attach a condenser to the flask and in the fume cupboard warm the mixture with swirling until the solid has dissolved and warm for another 5 minutes

Stage 2

Carefully add 5 cm³ of cold water to the solution and stand the flask in cold water for precipitation to occur. Filter off the product and wash the product with a little cold water. Transfer to a watch glass and leave to dry overnight. Once dry, weigh your product and record the mass.

Suggest four improvements to the method the student followed.

Draw a labelled diagram of the apparatus used to filter the mixture in Stage 2 outlined in part (b). 

The reaction for the preparation of aspirin is shown in Figure 1.

Figure 1

The average percentage yield for the industrial synthesis of aspirin is around 79.2%. If student uses 1.05 g of salicylic acid with an excess of ethanoic anhydride for their aspirin synthesis, calculate the minimum mass of aspirin that they would need to produce to meet industrial standards

Tollens’ reagent or diammine silver hydroxide, [Ag(NH₃)₂]⁺OH^-, is a mixture of liquid ammonia and silver nitrate. Tollens’ reagent is prepared in the laboratory in two steps

The first step is to react silver nitrate and sodium hydroxide to form a brown precipitate of silver(I) oxide. This precipitate is then dissolved in concentrated ammonia to form the reagent.

Write two equations, including state symbols, to show the laboratory formation of Tollens’ reagent.

Write an equation, including state symbols and give the observation for the reaction of Tollens’ reagent with benzaldehyde.

Suggest why the student should only use clean and dry glassware for this practical.

Methyl benzoate is an ester that can be formed from benzoic acid. Outline the mechanism for the alkaline hydrolysis of this ester showing the formation of the salt and alcohol produced.

Methyl salicylate, C₆H₄(OH)COOCH₃, shown in Figure 1 is also known as oil of Wintergreen. It is a natural product of many species of plants. Methyl salicylate may be used by plants as a pheromone to warn other plants of pathogens such as tobacco mosaic virus. Methyl salicylate is also used in deep heating liniments and in small amounts as a flavoring agent.

Figure 1

Methyl salicylate was formed by the following method

• 6 g of salicylic acid and 15 cm³ of methanol were added to a pear shaped flask
• 2 drops of sulfuric acid were added dropwise
• When all of the acid is complete a reflux condenser was fitted forming an airtight seal and the reaction is heated using an electric heating mantle at 60 ℃ for 30 minutes
• Once cool 30 cm³ of distilled water and 15 cm³ of ethyl ethanoate were added
• The mixture is transferred to a separating funnel and sodium carbonate solution is added
• Calcium chloride solution was added and the mixture was shaken
• The pressure was released 15-20 times
• The lower layer was discarded
• The ester was run into a clean, dry flask containing anhydrous calcium chloride and swirled
• The ester was filtered into a clean, dry flask

Explain the importance of the parts of the method underlined.

Suggest how an airtight seal could be formed between the reflux condenser and the round bottom flask

A student performed the synthesis of wintergreen oil and recorded the data in Table 1.

Table 1

Methanol has a density of 0.792 g cm^-3

Find the limiting reagent for the reaction and therefore the percentage yield for this reaction.

Aspirin is also known as acetylsalicylic acid and can also be produced from salicylic acid.

Outline a method for the nitration of methyl benzoate. Include safety considerations and equations for the reaction. You do not need to discuss purification of the crude product.

Describe the purification process of the product using a suitable solvent.

A student outlines the mechanism for the electrophilic substitution for the formation of 4-nitrotoluene

Figure 1

Identify the errors that the student has made

According to a data book, the melting point of methyl 3-nitrobenzoate is 78 ℃.

A student tested the product produced in part (a) and found the melting point to be lower than expected.         

Explain how the student can obtain a more accurate value.

In a multi-step reaction, benzoic acid can be prepared by hydrolysing ethyl benzoate using sodium hydroxide.

Write an equation for this reaction.

A student used the following method to make benzoic acid.

1. Place 3.0 cm³ of ethyl benzoate (density = 1.05 g cm⁻³, M = 150.0 g mol^-1) in a pear-shaped flask
2. Add 25 cm³ (an excess) of sodium hydroxide solution.
3. Set the pear-shaped flask up for reflux.
4. Heat the mixture, under reflux, for approximately 30 minutes.
5. Leave the mixture to cool to room temperature.
6. Add 50.0 cm³ of 2 mol dm⁻³ hydrochloric acid
7. Collect the crude product using suction filtration.

The crude product mixture from part b will be purified by recrystallisation.

Describe how to identify a suitable solvent for the recrystallisation process.

After successful recrystallisation, the melting point of the benzoic acid was determined to assess the purity of the sample. The expected melting point was 122.3 ^oC.

A student has two unlabelled samples of colourless liquid. The samples are known to be
cyclohexanol, C₆H₁₁OH, and t-butanol, (CH₃)₃COH.

Describe a chemical test to distinguish between cyclohexanol and t-butanol. Your answer should explain the expected observations.

Cyclohexanol and t-butanol both undergo dehydration reactions by heating in the presence of sulphuric acid.

Identify the carbocations formed during the dehydration reactions.

Use the carbocations to explain why t-butanol requires a lower temperature for the reaction to occur.

A student has correctly identified the cyclohexanol sample.

They set up the following equipment, Figure 1, to determine the boiling point of the cyclohexanol.

Figure 1

Explain three ways that the student could improve their equipment.

The purity of a liquid can be determined by measuring its boiling point. An alternative method to distillation would be to boil a tube of the sample in an oil bath.

The expected boiling point of a cyclohexanol sample is 161^oC but following distillation it was found to be 168^oC.

Suggest why the boiling point was higher than expected and why measuring the boiling point is not the most accurate method to confirm the identity of a known sample.

Propyl butanoate can be made in the laboratory by the reaction of an appropriate acid and alcohol. The ester formed is distilled off as a measure to increase the yield.

Ethyl ethanoate can be made in a similar fashion using ethanol and ethanoic acid. Part of the method that a student followed for this practical is given below:

• 50 cm³ of ethanoic acid and 50 cm³ ethanol are mixed thoroughly in a 250 cm³ round bottomed flask.
• 10cm³ of concentrated sulphuric acid is added dropwise to the mixture.
• The mixture is well mixed and kept cool in an ice-bath.
• When all of the acid has been added, a reflux condenser is fitted to the flask.
• The mixture is then gently boiled over an electric heating mantle for around 30 minutes.
• The mixture is then cooled and rearranged for distillation.
  

Parts of the method are in bold text. For each part, decide if it is essential and justify your answer.

After distillation of approximately 50 cm³ of their crude ethyl ethanoate, the distillate is shaken in a separating funnel with 25 cm³ of sodium carbonate solution. At intervals, the pressure inside the separating funnel is released. After shaking, the lower aqueous layer is discarded.

25 cm³ of calcium chloride solution is then added to the separating funnel and the mixture is shaken. The lower layer is again discarded after shaking.

Suggest why sodium carbonate and calcium chloride were used to wash the distillate.

Another student prepared ethyl ethanoate using the method outlined in parts b and c. They reacted 10 cm³ of ethanol (density = 0.79 g cm^-3) with excess ethanoic acid.

After purification, the student had obtained 6.90 g of ethyl ethanoate.

Calculate the student’s percentage yield.

A student prepared a sample of aspirin by refluxing salicylic acid with an excess of acetic anhydride in the presence of a phosphoric (V) acid catalyst. The reaction was quenched with water, cooled and crystallised. The crude product was collected by suction filtration, washed and then purified by recrystallisation. Thin-layer chromatography was then used to ascertain whether the product had been sufficiently purified by recrystallisation.

Outline the method that the student would use to determine whether the product was pure, including the results that the student would expect to see.
You should include a labelled diagram to support your answer.

Outline how the student could identify the positions of any compounds on the TLC plate and describe the expected results to show that the crude product had been purified by recrystallisation.

There is an extensive range of over the counter and prescription medicines, that contain a  variety of chemicals. TLC is often one of the first methods used to analyse medicines.

Give three advantages of using TLC in the analysis of such medicines.

The structure of aspirin is shown in Figure 1.

Figure 1

Standard over the counter aspirin tablets are around 85% pure. One of the common impurities found in aspirin is ethanoic acid from the manufacturing process.

A student analysed a 325 mg aspirin tablet and found it to contain 1.56 x 10^-3 moles of aspirin.

Evaluate the safety of the tablet for human consumption.