Alkenes (Cambridge (CIE) A Level Chemistry): Exam Questions

This question is about alkenes with the molecular formula C₅H₁₀ .

Draw the skeletal formulae of three branched chain alkenes with the molecular formula C₅H₁₀ .

An alcohol can react with concentrated phosphoric acid, H₃PO_4, to form pen-2-ene. 

i) Give the formula of the alcohol that will form only pent-2-ene during this reaction. 

[1]

ii) Give a name for this type of reaction. 

[1]

Pent-2-ene reacts with hydrogen bromide, HBr, to form two bromoalkanes.

Complete the diagram to show the mechanism for the formation of 2-bromopentane in this reaction. Include curly arrows, and relevant lone pairs and dipoles.

A sample of pent-1-ene, with a mass of 1.33 g, is warmed to 60°C in a sealed container. The volume of the container is 500 cm³.

Calculate the pressure inside the container.

Include units and give your answer to an appropriate number of significant figures. [molar gas constant (R) = 8.31 J mol⁻¹ K⁻¹]

This question is about alkanes and alkenes. 

Propene and propane can be produced from the cracking of hexane, C₆H₁₄.

C₆H₁₄ → C₃H₈ + C₃H₆

Explain why propane is relatively unreactive.

Describe and give the result of a test that can be done to distinguish between the two hydrocarbons. 

Propene, C₃H₆ , is an alkene.

The reagents needed for three reactions of propene are shown in Fig. 2.1.

Complete Fig. 2.1 by drawing the structure of the organic product produced in each reaction. 

Fig. 2.1

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ii) Propene also reacts with hydrogen bromide, HBr.

Give the mechanism for this reaction to form the major product.

Include curly arrows, and relevant lone pairs and dipoles.

[3]

Propene reacts by addition polymerisation to form poly(propene).

Draw the structure of poly(propene), showing two repeat units.

This question is about alkenes. 

One method of preparing ethene in a school or college laboratory is from ethanol by using the apparatus shown below in Fig. 3.1.

Fig. 3.1

i) Write a balanced equation for this reaction.

[1]

ii) What type of reaction is this?

[1]

Ethene is bubbled into two separate test-tubes, one containing aqueous hydrogen bromide and the other containing cold, dilute acidified potassium manganate(VII).

In each case, complete Table 3.1 to describe any colour changes you would see and give the structural formula of the organic product.

Table 3.1

Cyclohexene has the following structural formula.

What is the molecular formula of cyclohexene?

Cyclohexene can react with bromine. 

i) Draw the structural formula of the compound formed when cyclohexene is reacted with bromine.

[1]

ii) State what type of mechanism is involved in forming this product.

[2]

Draw the structural formula of the compound formed when cyclohexene is reacted with hot concentrated acidified potassium manganate(VII).

Compound B contains carbon, hydrogen and oxygen only.

It has the following percentage composition by mass: C 54.5%, H 9.1%, O 36.4%.

Calculate the empirical formula of B.

Show your working.

Empirical formula =

Compound C is the secondary alcohol, butan-2-ol, CH₃CH(OH)CH₂CH₃.

i) Butan-2-ol can be converted into but-2-ene by dehydration.

State the reagent and conditions required for this reaction.

[2]

ii) Butan-2-ol is heated under reflux with acidified potassium dichromate(VI).

State the colour change observed.

[1]

iii) Name the organic product formed in (b)(ii).

[1]

But-2-ene shows geometrical (cis/trans) isomerism.

i) Explain the term geometrical isomerism.

In your answer, refer to the features of the double bond and the groups attached to it.

[2]

ii) Draw the displayed formula of trans-but-2-ene.

[1]

iii) Describe a chemical test to show the presence of the C=C double bond in but-2-ene.

Reagent: …………………………………………

Observation: …………………………………

[2]

Infrared spectroscopy is used to identify functional groups.

Use the Data Booklet to identify the bond responsible for the following absorption peaks found in organic spectra.

i) A strong absorption peak at 1700 cm⁻¹.

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ii) A broad absorption peak in the range 3200–3600 cm⁻¹.

[1]

iii) State how the infrared spectrum of the product formed in (b)(iii) would differ from the spectrum of butan-2-ol.

[2]

This question is about hydrocarbons.

10 cm³ of a gaseous hydrocarbon, C_xH_y, was reacted with 100 cm³ of oxygen gas, an excess. The final volume of the gaseous mixture was 95 cm³. 

This gaseous mixture was treated with concentrated, aqueous sodium hydroxide to absorb the carbon dioxide present. This reduced the gas volume to 75 cm³.

All gas volumes were measured at 298 K and 100 kPa.

i) Write an equation for the reaction between sodium hydroxide and carbon dioxide.

[1]

ii) Calculate the volume of carbon dioxide produced by the combustion of the hydrocarbon.

volume of CO₂ produced = ................. cm³ [1]

iii) Calculate the volume of oxygen used up in the reaction with the hydrocarbon.

volume of O₂ used = ................. cm³ [1]

iv) Use your answers to (b)(ii) and (b)(iii), together with the initial volume of hydrocarbon, to balance the equation below.

............C_xH_y + ............O₂ → ............CO₂ + zH₂O

[2]

v) Deduce the values of x, y and z in the equation in (iv).

x = .............................
y = .............................
z = .............................

[3]

A branched-chain hydrocarbon, W, with the formula C₄H₈, reacts with hydrogen bromide, HBr, to give two products X and Y.

X and Y are structural isomers of molecular formula C₄H₉Br. More X is produced than Y.

i) Give the structural formulae and names of the compounds W, X, and Y.

Name of W.................................................................................
Structural formula of W...............................................................
Name of X...................................................................................
Structural formula of X...............................................................
Name of Y...................................................................................
Structural formula of Y.................................................................

[3]

ii) When W reacts with hydrogen bromide, more X than Y is produced. Explain why.

[2]

Pent-2-ene will react with HBr.

Draw the mechanism for the reaction of pent-2-ene with hydrogen bromide. You do not need to include any partial charges.

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.

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 2.0 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.

The student’s titration results are shown in Table 2.1.

Table 2.1

The approximate M_r of the cooking oil is 1002 and its 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 this data and the titration results from Table 3.1 to deduce the number of C=C double bonds in a molecule of the oil. Show your working.

Fig. 2.1 shows the structure of a mono-unsaturated oil and a polyunsaturated oil.

Fig. 2.1

i) Suggest what is meant by the terms mono-unsaturated oil and polyunsaturated.

[2]

ii) Suggest the reagent and conditions necessary to produce the mono-unsaturated oil from the polyunsaturated oil.

[2]