Percentage yield

This question is about two metal hydrides, lithium aluminium hydride, LiAl H₄, and sodium borohydride, NaBH₄. Both hydrides are used as reducing agents in organic chemistry.

What is the systematic name of the alcohol C₆H₅CH₂OH?

Calculate the mass of LiAl H₄ that the student should use in Step 2 and the percentage yield of pure C₆H₅CH₂OH obtained in Step 4.

Give the mass of LiAl H₄ and the percentage yield of pure C₆H₅CH₂OH to 3 significant figures.

Percentage yield of pure C₆H₅CH₂OH =........ %

In the preparation, LiAl H₄ behaves as a nucleophile.

What is meant by the term nucleophile ?

The mechanism for Step 2 in the reduction of C₆H₅COOC₂H₅ is shown below. In the mechanism, LiAl H₄ can be represented as H^–.

Complete the mechanism by adding curly arrows to the three boxes.

Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3.

NaBH₄ contains Na⁺ and BH₄^– ions.

Draw a ‘dot‑and‑cross’ diagram for NaBH₄.

Use a different symbol for the electrons from each element.

Show outer electrons only.

NaBH₄/ O₂ cells are being investigated as potential fuel cells.

• The positive electrode contains O₂.

• The negative electrode contains NaBH₄.

The reaction at the positive electrode and the overall cell reaction for a NaBH₄/ O₂ fuel cell are shown below.

Overall cell reaction: NaBH₄ + 2O₂ NaBO₂ + 2H₂O E = 1.64 V

What is the standard electrode potential of the negative electrode in this cell?

Construct the equation for the reaction at the negative electrode of this cell.

Which chemical process is the most sustainable in terms of the atom economy of the iron produced?

Your answer

Oil of wintergreen is a liquid used in medicine to relieve muscle pain.

Compound H is a component in oil of wintergreen and can be synthesised from compound G, as shown below. The boiling point and density of compound H are stated.

Compound G
Compound H boiling point = 222 °C density = 1.174 g cm⁻³

A student prepares a sample of compound H by the method below.

Step 1 Reflux 8.97 g of compound G for 30 minutes with an excess of methanol in the presence of a small amount of sulfuric acid as a catalyst.

Step 2 Add an excess of aqueous sodium carbonate, Na₂CO₃(aq). Two layers are obtained.

Step 3 Purify the impure compound H that forms from the resulting mixture.

The student follows this method and obtains 5.32 g of pure compound H.

Why does the student use reflux in Step 1 ?

In Step 2, Na₂CO₃(aq) removes the sulfuric acid catalyst and any unreacted compound G from the mixture.

Write equations for this removal.

Removal of sulfuric acid

Removal of unreacted compound G

Compound G

Another student suggests that adding aqueous sodium hydroxide would be more effective in removing the sulfuric acid catalyst than Na₂CO₃(aq).

Comment on whether the student’s suggestion is an improvement for the preparation of compound H.

Calculate the percentage yield of compound H.

Give your answer to three significant figures.

Describe how to purify the impure compound H from the two layers in Step 2.

Outline the mechanism for this reaction, including the role of H₂SO₄ as a catalyst.

Determine the percentage yield of 1,3-dinitrobenzene.

Give your answer to 3 significant figures.

Describe how to purify the impure crystals in Step 3.

This question is about compounds that contain the carboxylic acid functional group.

Carboxylic acids react with alkalis, metals and carbonates to form salts.

Write full equations for the following three reactions. Show structures for organic compounds.

• the reaction of propanoic acid with aqueous potassium hydroxide:

• the reaction of aqueous methanoic acid with magnesium:

• the reaction of the –amino acid, aspartic acid (R=CH₂COOH), with an excess of aqueous sodium carbonate, Na₂CO₃:

This part is about polymers derived from carboxylic acid monomers.

Poly(pent-3-enoic acid) is an addition polymer.

Draw the structure of pent-3-enoic acid and two repeat units of this polymer.

Butanedicarboxylic acid and 1,4-dihydroxy-2-methylbenzene react to form a condensation polymer.

Draw one repeat unit of this condensation polymer.

Three repeat units of a condensation polymer are shown below.

Draw the structure of the monomer required to form this polymer.

A polymer is formed from 400 molecules of 2-aminopropanoic acid.

Draw one repeat unit of this polymer.

What is the relative molecular mass, M_r, of the polymer?

Information about 1-bromobutane and butan-1-ol is shown in the table.

A student prepares a sample of 1-bromobutane by refluxing 9.25 g of butan-1-ol with sodium bromide and sulfuric acid.

After reflux, the reaction mixture is purified.

The student obtains 6.10 cm³ of pure 1-bromobutane.

* Draw a diagram to show how the student would have carried out the reflux and calculate the percentage yield of 1-bromobutane that the student obtains.

Describe how the student could have obtained pure 1-bromobutane from the reaction

mixture obtained after reflux.

Butan-1-ol reacts with sodium bromide and sulfuric acid to form 1-bromobutane by nucleophilic substitution.

The mechanism for this reaction takes place by two steps.

Step 1 The oxygen atom of the alcohol group accepts a proton to form a positively-charged intermediate.

Step 2 Bromide ions react with the intermediate from Step 1 by nucleophilic substitution to form 1-bromobutane.

Show both steps in this mechanism.

This question is about organic synthesis.

Prenal, shown below, is used in the synthesis of some pharmaceuticals.

What is the systematic name for prenal?

* A student intends to synthesise compound A using the two-stage route below.

(chloromethyl)benzene Compound A

Plan a two-stage synthesis to prepare 5.44 g of compound A starting from (chloromethyl)benzene, C₆H₅CH₂Cl. Assume that the overall percentage yield of compound A from (chloromethyl)benzene is 25%.

In your answer, include the mass of C₆H₅CH₂Cl required, reagents, and equations where appropriate.

Purification details are not required.

Cyclopentanol can be reacted to form cyclopentene. Cyclopentene is a liquid with a boiling point of 44 °C and a density of 0.74 g cm⁻³.

A student plans to prepare 4.00 g of cyclopentene by reacting cyclopentanol (boiling point 140 °C) with an acid catalyst.

The expected percentage yield of cyclopentene is 64.0%.

Method The student carries out the preparation using apparatus set up for distillation, as shown below.

1 The reaction mixture is heated gently, and a distillate containing impure cyclopentene is collected.

2 The distillate has an aqueous layer and an organic layer. The student purifies the cyclopentene from the distillate.

* Calculate the mass of cyclopentanol that the student should use and explain how pure

cyclopentene could be obtained from the distillate.

A student reacts 4.50 g of C₆H₅NH₂ with excess CH₃COCl in the reaction below.

C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl

M_r = 93.0 M_r = 135.0

The reaction produces 3.25 g of C₆H₅NHCOCH₃.

What is the percentage yield of C₆H₅NHCOCH₃?

A 49.8

B 68.9

C 72.2

D 95.4

This question is about esters.

The structure of ester A is shown below.

What is the systematic name of ester A ?

Name the type of reactions of ester A shown in (ii).

A polyester is formed from 200 molecules of 4-hydroxybenzoic acid.

What is the relative molecular mass, M_r, of the polyester?

M_r =........ g mol⁻¹

* A student intends to synthesise ester C.

Plan a two-stage synthesis to prepare 12.75 g of ester C starting from 2-methylpropanal, (CH₃)₂CHCHO. Assume the overall percentage yield of ester C from 2-methylpropanal is 40%.

In your answer include the mass of 2-methylpropanal required, reagents, conditions and equations where appropriate.

Purification details are not required.

In Step 1, sodium carbonate, Na₂CO₃, makes the reaction mixture alkaline.

Write an ionic equation to show how carbonate ions form an alkaline solution in water.

In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear.

Write the overall equation for the preparation of benzoic acid from phenylmethanol.

Use [O] for the oxidising agent.

Calculate the percentage yield of benzoic acid.

Give your answer to 3 significant figures.

In Step 3, describe how the student can recrystallise the impure crystals to obtain pure benzoic acid.

Ethanol can be prepared by different reactions.

Which reaction has the lowest atom economy?

A C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

B C₂H₄ + H₂O → C₂H₅OH

C C₂H₅Br + H₂O → C₂H₅OH + HBr

D CH₃COOC₂H₅ + H₂O → C₂H₅OH + CH₃COOH

This question is about two compounds used in medicine.

Cis-platin, PtCl₂(NH₃)₂, is a complex of platinum which is used in cancer treatment.

What is the oxidation number of platinum in cis-platin?

Cis-platin is prepared in a ligand substitution reaction which takes place in multiple steps.

The equation for the final step forming cis-platin is shown below.

cis-platin

In the box, outline the mechanism for the formation of cis-platin from [PtCl₃(NH₃)]^–. Use curly arrows and lone pairs where appropriate.

cis-platin

Paracetamol is a solid organic compound used in tablets as a painkiller.

Name the functional groups present in paracetamol.

Which type of reaction has the greatest atom economy?

A Substitution

B Hydrolysis

C Elimination

D Addition

This question is about the chemistry of aromatic compounds.

Benzoic acid can be nitrated by concentrated nitric acid in the presence of concentrated sulfuric acid as a catalyst, as shown in Equation 17.1.

The organic product of this reaction is 3-nitrobenzoic acid.

Outline the mechanism for this nitration of benzoic acid.

Show how H₂SO₄ behaves as a catalyst.

* A chemist carries out the reaction in Equation 17.1 using 4.97 g of benzoic acid.

The chemist obtains 3-nitrobenzoic acid as an impure solid.

The chemist purifies the solid to obtain 4.85 g of 3-nitrobenzoic acid.

Describe a method to obtain a pure sample of 3-nitrobenzoic acid from the impure solid, determine the percentage yield and check its purity.

A student investigates the relative ease of nitration of phenol, benzene, and benzoic acid.

The student finds that the conditions required for the nitration of each compound are different, as shown in Table 17.1.

State the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.

Apply your knowledge of the bonding in arenes to explain the trend in part (b)(i).

A student synthesises 3-bromophenylamine, shown below, starting from nitrobenzene.

Complete the flowchart showing the structure of the intermediate and the formulae the reagents for each stage.

Another student attempts the same synthesis but carries out reduction before bromination. The student was surprised to find that two structural isomers of 3-bromophenylamine had been formed instead of the desired organic product.

Explain this result and suggest the structures of the two isomers that formed.

Explanation........

Structures