AQA A Level Chemistry

Topic Questions

3.4 Alkenes

1a2 marks

A molecule of ethene is shown in Figure 1.

Figure 1

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i)
Describe the shape of the ethene molecule.

ii)

State the angle labelled A in Figure 1.

1b1 mark

A range of diols are shown in Figure 2.

Figure 2

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Suggest why compounds A and B are the same compound but compounds C and D are isomers.

1c2 marks

Name compounds C and D from part (b) using the E / Z naming system.

Compounds C and D are shown again in Figure 2.

Figure 2

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1d1 mark

Compound E, shown in Figure 3, is a derivative of compound C.

Figure 3

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Suggest why the cis / trans naming system fails with compound E.

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2a5 marks

Alkenes are used as building blocks in organic synthesis due to the reactivity of the carbon-carbon double bond. Figure 1 shows some of the compounds that can be formed using an alkene functional group.

Figure 1

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Identify the type of compounds, A - E, shown in Figure 1.

2b2 marks

Alkenes commonly undergo electrophilic addition.

A student suggested that the mechanism for the electrophilic addition of hydrogen bromide to ethene could be as shown in Figure 2.

Figure 2

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Explain why the student’s curly arrows are incorrect.

2c6 marks

The start of the electrophilic addition mechanism for the addition of hydrogen bromide to ethene is shown in Figure 3.

Figure 3

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i)
Add two curly arrows and the partial charges, δ+ / δ-, to the reactants step of Figure 3.

ii)
Add the correct charge to the carbocation intermediate, the anion, including its lone pair, and one curly arrow to the intermediate step.

iii)
Draw the displayed formula of the product.
2d2 marks

Name and draw the displayed formula of the final product in Figure 2, part (c).

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3a1 mark

Electrophilic addition is an organic mechanism used by alkenes in their reactions.

Which of the substances shown in Figure 1 is not a common electrophile in electrophilic addition?

Figure 1

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3b1 mark

Sulfuric acid is another electrophile that can be added across an alkene double bond.

Draw the displayed formula of sulfuric acid.

3c3 marks

The electrophilic addition of hydrogen bromide to but-1-ene results in two isomeric products - one is a major product and one is a minor product. The first steps of the electrophilic addition mechanism are shown in Figure 2.

Figure 2

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i)
Draw the displayed formula of the secondary carbocation intermediate that forms the major product.

ii)
Draw the displayed formula of the primary carbocation intermediate that forms the minor product.

iii)
Explain why the secondary carbocation is more stable than the primary carbocation.
3d2 marks

Name and draw the displayed formula of the minor product of the reaction of hydrogen bromide and but-1-ene.

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4a2 marks

Butene, C4H8, can exist as three isomers.

Describe a chemical test, including the expected observation, that will give a positive result for all isomers.

4b6 marks
i)

Isomer A is a position isomer of isomers B and C and does not show geometric isomerism.

Isomers B and C are geometric isomers of each other.

Isomer B is the Z isomer.

Isomer C  is the E isomer.

Draw the displayed formulae of the three isomers, A, B and C, of butene, C4H8.

ii)
Name all three isomers, A, B and C, of butene, C4H8
4c1 mark

Draw the displayed formula of the addition polymer formed by isomer A.

4d1 mark

A student was asked to draw the displayed formula of the addition polymer of isomers B and C. They suggested that there were two structures as shown in Figure 1.

Figure 1

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State why the student’s two suggested structures are the same addition polymer.

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5a3 marks

Why is the disposal of polymers proving to be a challenge? Give three reasons.

5b1 mark

Which method of recycling heats the polymer to a high temperature so that it breaks down into the original monomer?

5c1 mark

Another recycling method for polymers and plastics involves separating, washing, grinding them into pellets and then melting the pellets.

What happens to the pellets after they have been melted?

5d1 mark

Certain plastics can only be recycled in this way a limited number of times as they deteriorate and degrade. This means that over time, the properties of the plastic are degraded.

What causes plastics such as poly(propene) to deteriorate and degrade when recycled?

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1a1 mark

2-methylprop-2-enenitrile has the molecular formula C4H5N.

Draw the displayed formula for 2-methylprop-2-enenitrile.

1b2 marks

Two isomers of 2-methylprop-2-enenitrile, C4H5N, display geometric isomerism.

Draw and name the isomers.

1c2 marks
i)
Draw the repeating unit of the polymer formed by addition polymerisation of (E)-but-2-ene.

ii)
Explain why the polymer formed by (E)-but-2-ene is the same as the polymer formed by (Z)-but-2-ene.
1d6 marks

Addition polymerisation is normally an exothermic reaction. In industry, a compromise temperature of 200 degreeC is typically used to complete the addition polymerisation process.

LDPE is produced at high temperatures and pressures while HDPE is produced at lower temperatures and pressures in the presence of a heterogeneous catalyst.

Explain how the catalyst works to reduce the reaction conditions for HDPE formation. Explain the effects of changing temperature and pressure on the yield and rate of reaction for the polymer produced from the ethene monomer.

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2a1 mark

A student completed an investigation into the amount of C=C bonds in a molecule of cooking oil, with a density of 0.526 g cm-3, by titrating samples with 0.02 mol dm-3 bromine water.

As part of the practical procedure, the student was advised not to perform more than 2 titrations in a 45 minute period. Suggest why.

2b2 marks

The student plans to use the following procedure for their investigation.

  1. Measure 5.0 cm3 of solvent into a conical flask.
  2. Add five drops of cooking oil to the solvent, using a dropping pipette.
  3. Swirl to ensure mixing.
  4. Note the initial reading of the bromine water in the burette.
  5. Add the 2 x 10-2 mol dm-3 bromine water from the burette to the solution in the conical flask slowly.
    • Shake vigorously after each addition until the bromine colour disappears.
  6. As the bromine colour takes longer to fade with each addition, add less bromine water each time until there is just an excess of bromine in the flask.
    • This will be shown by a permanent yellow tint.
  7. Note the final reading of the burette.

Suggest two improvements that the student could make to this procedure to obtain more consistent results.

2c6 marks

The student’s titration results are shown in Table 1.

Table 1

Experiment

Initial volume / cm3

Final volume / cm3

Rough

0.00

26.35

1

0.00

26.25

2

0.10

26.45

3

0.00

26.10


The approximate Mr of the cooking oil is 1002 and it’s density is 0.526 g cm–3.

Assume each drop of cooking oil has a volume of 5.0 × 10–2 cm3.

The concentration of bromine water used was 2.0 × 10–2 mol dm–3.

Use these data and the titration results from Table 1 to deduce the number of C=C double bonds in a molecule of the oil. Show your working.

2d2 marks

The student researched other titration methods after observing that the colour change at the end point with bromine water was not obvious.

The student discovered that they could use 0.005 mol dm-3 acidified potassium manganate (VII) solution.

Use your answers from part (c) to evaluate if the use of 0.005 mol dm-3 acidified potassium manganate (VII) solution would be suitable for the titration with the cooking oil.

(If you have been unable to calculate an answer for part (c), you may assume the moles of bromine = 5.02 x 10-4. This is not the correct answer.)

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3a2 marks
The following scheme shows the reactions of Compound A. 

10-2-ib-sl-sq-hard-q4a-reaction-scheme
i)
Deduce the structural formula of compound A.

ii)
State the IUPAC name of compound B.
3b2 marks

Reaction 1 forms an alcohol when reacted with concentrated sulfuric acid, H2SOand steam.

i)
State the conditions required for this reaction.

ii)
Deduce the structure of the intermediate in this reaction.
3c3 marks

Butan-2-ol can also be directly formed from a halogenoalkane.

i)
State the type of reaction occurring in this conversation.

ii)
State the conditions for this reaction.
3d1 mark

Identify the structure of the repeating unit of poly(but-2-ene).

3e1 mark

Compound A reacts with hydrogen bromide to form compound C. A student suggested a possible formula of compound C is CH2(Br)CH2CH2CH3.

State whether the student is correct and justify your answer.

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4a2 marks

Explain why Figure 1 is not the repeating unit of poly(ethene).

Figure 1

40

4b3 marks

Poly(ethene) exists in two forms, high density poly(ethene) or HDPE and low density poly(ethene) or LDPE.

Explain why HDPE is used for milk crates, buckets and bottles while LDPE is used for plastic bags and sheeting.

4c2 marks

The structure of poly(ethene) and polypropene are shown in Figure 2.

Figure 2

41

Give two reasons why these polymers are unreactive.

4d3 marks

Amorphous polymers tend to be more flexible polymers because the forces of attraction between the chains are usually weaker. Poly(ethene) and polyvinyl chloride are both examples of amorphous polymers. However, polyvinyl chloride (PVC) is rigid enough for use as drain pipes and window frames.

Explain how changing the monomer from ethene to chloroethene accounts for the increased strength and rigidity in polyvinyl chloride (PVC).

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5a3 marks

6.90 g of ethanol was reacted to form ethene, with a percentage yield of 80 %.

C2H5OH → C2H4 + H2O

i)
Name the type of reaction that is taking place.

ii)
Calculate the mass of ethene produced in the reaction.
5b2 marks

Plasticisers are chemicals that can be added to a polymer to increase its flexibility.

Explain how plasticisers affect the flexibility of a polymer like poly(vinyl chloride).

5c3 marks

Polymers are formed from the addition polymerisation of alkenes.

Explain why there is an increasing demand for polymers to be recycled rather than incinerated.

5d2 marks

Feedstock recycling is the process where polymers are heated to a temperature that breaks the polymer bonds and produces monomers. These monomers are then used to make new polymers.

Explain why this method of recycling can only be performed a limited number of times when recycling thermoplastic polymers such as polypropene.

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1a5 marks

Crude oil contains a mixture of hydrocarbons. These hydrocarbons are first separated in a process called fractional distillation. The longer hydrocarbon chains are then cracked to shorter saturated and unsaturated hydrocarbons.

Figure 1 shows two hydrocarbons which are obtained by cracking crude oil.

Figure 1

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i)
State which hydrocarbon is unsaturated and explain why.

ii)
State which hydrocarbon is more reactive and explain why.
1b2 marks

A student compares the physical properties of alkanes and alkenes.
Student A states that alkenes have similar boiling points to their equivalent alkanes.
Student B states that alkenes are soluble in water whereas alkanes are not.

State whether student A, B or both students are correct. Explain your answer.

1c4 marks

Draw the structure of ethane and ethene and include the bond angles. Name and explain the geometry of the two molecules.

1d2 marks

Both alkanes and alkenes can react with oxygen and combust. However, it is alkanes that are often used as fuels.

Explain why alkenes are not often used as fuels and state one other use of alkenes.

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2a6 marks

Alkenes are widely used as starting compounds for the synthesis of other organic compounds.

2-Methylbut-1-ene is an alkene commonly used as a solvent, as well as a starting material in the manufacture of chlorinated derivatives and other industrial substances.

2-Methylbut-1-ene reacts with a reagent to form a saturated halogenated compound.

i)
Suggest a reagent that will form a saturated monohalogenated compound when reacted with 2-methylbut-1-ene. 

ii)
Name and outline the mechanism for the reaction that occurs in part (i). You do not need to include any partial charges.
2b7 marks

The reaction described in part (a) produces a major and minor product.

i)
State the IUPAC name of the minor product and draw a mechanism for the formation of this product. You do not need to include any partial charges.

ii)
Explain why the structure named in part (i) is the minor product.
2c3 marks

A chemist wants to synthesise a mono-substituted chloroalkane from a different alkene. He uses a compound X which has the same molecular formula as 2-methylbut-1-ene, but is not branched.

Compound X produces fewer isomeric products than the reaction of 2-methylbut-1-ene with the reagent described in part (a).

State the IUPAC names of compound X and the isomeric products formed.

The double bond in compound X is in the same position as in 2-methylbut-1-ene.

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3a5 marks

Alcohols are valuable organic compounds which are often used as solvents, fuels and chemical feedstock. Alcohols can be prepared from the electrophilic addition reaction of alkenes.

The first step in this reaction involves the reaction of sulfuric acid with an alkene.

i)
State what the electrophile is in this electrophilic addition reaction.

ii)
Outline the mechanism for the reaction of but-1-ene with sulfuric acid to show the formation of the major product. Include the partial charges.
3b4 marks

The reaction described in part (a) also produces a product in which the sulfate group is bonded to a less substituted carbon atom.

State the name and draw the structure of this product. State whether this is the minor or  major product and explain why.

3c3 marks

The electrophilic addition reaction of sulfuric acid with but-1-ene results in the formation of three isomeric products.

Draw the structures of these isomeric products.

3d4 marks

The second step in the formation of an alcohol from an alkene is hydrolysis of the alkyl hydrogen sulfate formed in the first step.

i)
Write the equation for this step of the reaction using displayed formulae of the reactants and products.

ii)

Draw the skeletal formulae of all structural isomers of the compound formed in this reaction.

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4a5 marks

Ethene is an unsaturated hydrocarbon which can react with bromine in an electrophilic addition reaction.

i)
Explain why this is an example of an electrophilic addition reaction.

ii)
Bromine is a non-polar molecule, yet acts as an electrophile in this reaction. Explain why.

iii)
How can bromine be used as a test for unsaturation?
4b4 marks

Alkenes and substituted alkenes can also be used as monomers to form polymers in an addition reaction. The polymer which is formed from a propene monomer is widely used in the production of yoghurt containers and car bumpers.

i)
State the definition of monomers.

ii)
State the IUPAC name of the polymer which is formed from propene.

iii)
Addition polymers are unreactive. State one advantage and one disadvantage of this property.
4c3 marks

Another example of a polymer is polystyrene which is often used as packing material and electrical insulation.

Figure 1 shows 3 repeating units of the polymer.

Figure 1

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i)
Draw the structure and state the name of monomer X which is used to form the polymer shown in Figure 1.

ii)
Polystyrene is the common or trade name of the polymer. State the IUPAC name of the polymer shown in Figure 1.
4d2 marks

Monomer X can react with monomer Y to form a polymer. Two repeating units of this polymer are shown in Figure 2.

Figure 2

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Draw the structures of monomer Y which reacts with monomer X to form the polymer shown in Figure 2.

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5a4 marks

Polyvinyl chloride (PVC) is a polymer which is rigid enough for use as a drainpipe and flexible enough for plastic aprons.

i)
State the IUPAC name of PVC.

ii)
State how the properties of PVC can be modified. Explain your answer.
5b4 marks

Figure 1 below shows a section of the PVC polymer.

Figure 1

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i)
Draw the repeating unit of PVC.

ii)
Draw the structure and state the name of the monomer of the PVC polymer.

iii)
Write the equation to represent the formation of PVC from its monomer.
5c6 marks

The polymer poly(propene) is formed from the monomer propene.

i)
Draw the repeating unit of this polymer.

ii)

The properties of two different polymers, poly(methylpropene) and poly(ethene) are compared to each other. Complete Table 1 by placing an X in the correct box. You may place more than one X for each physical property.

Table 1

Physical property

Poly(methylpropene)

Poly(ethene)

Insoluble in water

 

 

Unreactive

 

 

High density

 

 



iii)

Explain why the melting point of poly(methylpropene) is much higher than that of methylpropene.

5d8 marks

Polymers are unreactive compounds due to their long non-polar chain of saturated carbon-carbon and carbon-hydrogen bonds. Though this means that they are safe to use due it also means that they are non-biodegradable as they are not attacked by biological agents such as enzymes.

Table 2 shows three different ways in which polymers can be disposed of.

Table 2

Method of disposal

Disadvantage

Recycling

 

Incerination

 

Landfill sites

 

i)
State the disadvantage of each method of disposal to complete Table 2.

ii)

Mechanical and feedstock recycling are two different methods to recycle polymers.

State the correct step for each process in Table 3 below.

Table 3

Mechanical recycling method

Step

Sort the clean plastics

 

Melt and remould the plastics into new plastics

 

Separate the different types of plastics

 

Ground the plastics up into smaller pellets

 

Wash the separated plastics

 

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