Alkenes (OCR A Level Chemistry A): Exam Questions

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 below shows two hydrocarbons that are obtained by cracking crude oil.

i) State which hydrocarbon is unsaturated and explain why.

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

Draw the structure of ethane and ethene and include the bond angles.

Name and explain the geometry around the carbon atoms of each molecule.

Alkenes B, C, D, E and F are shown below

You will have to refer to these alkenes throughout this part of the question.

Describe, using the orbital overlap model, how the π-bond in alkene D is formed.

Many alkenes show E/Z isomerism.

i) Explain why E/Z isomerism is shown in some alkenes.

ii) Which two alkenes are a pair of E/Z isomers?    

Choose from B, C, D, E and F.

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) to show the major product. You do not need to include any partial charges.

The reaction in part (a) results in the formation of three isomeric products. The properties of these isomeric products are shown in Table below.

Draw and label the structures of each isomeric product.

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.

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.

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. 

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.

The following molecule shows E/Z isomerism.

Use the Cahn–Ingold–Prelog priority rules to identify whether this alkene is an E or Z stereoisomer. Explain how you came to your decision.

Which of the bond(s) shown in the following structure is/are in the Z configuration? 

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?

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.

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

Figure below shows three repeating units of the polymer.

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

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

Monomer X can react with monomer Y to form a polymer.

Two repeating units of this polymer are shown in Figure below.

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

One way of disposing of poly(chloroethene) is incineration.

This process can cause environmental damage. Incineration produces a mixture of carbon dioxide, carbon monoxide and hydrogen chloride.

Carbon dioxide can cause climate change because it is a greenhouse gas.

i) Describe examples of environmental damage that may result from carbon monoxide and from hydrogen chloride.

ii) Outline the methods developed by chemists to reduce the environmental damage caused by incineration.

Figure below shows a section of the PVC polymer.

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.

Polymers are unreactive compounds due to their long non-polar chain of saturated carbon-carbon and carbon-hydrogen bonds.

Although this means that they are safe to use, it also means that they are non-biodegradable, as they are not attacked by biological agents such as enzymes. 

Table below shows three different ways in which polymers can be disposed of:

State the disadvantage of each method of disposal to complete Table.

This part of the question is about 2-chloropropene, C₃H₅Cl. Three reactions of 2-chloropropene are shown in the flowchart below.
Complete the flowchart to show the organic products formed in the reactions

2-chloropropene can be polymerised to form poly(2-chloropropene).

i) Write a balanced equation for the formation of this polymer. The equation should include the structure of the repeating unit of the polymer.

ii) After their useful life, waste polymers can be disposed of by combustion. State one particular problem with disposal of poly(2-chloropropene) by combustion.

This question is about the preparation of alcohols.

Alcohols can be prepared via the electrophilic addition reaction of alkenes. 

The first step involves the alkene reacting with sulfuric acid. 

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

[1]

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. 

[4]

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

i) State the name and draw the structure of this product.

[2]

ii) State whether this is the minor or major product and explain why.

[2]

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.

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

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

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

This question is about isomers. 

The structure of isomer X shown below. 

Classify the isomer as either an E or Z isomer. 

Explain your answer. 

Hex-3-ene also exhibits E/Z isomerism 

i) Recall what is meant by a stereoisomer.

[2]

ii) Draw the structures for the E and Z stereoisomers of hex-3-ene. 

[2]

iii) Explain why hex-3-ene can also be described as having cis-trans isomerism. 

You should correctly identify the isomers in part (ii) as cis or trans. 

[2]

Outline the mechanism for the electrophilic addition of bromine to cis-hex-3-ene.

Give the name of the product formed. 

Explain why trans hex-3-ene react with bromine to produce the same structural isomer. 

This question is about polymers. 

Poly(methyl methacrylate) is used as a glass substitute in Perspex. 

A section of the polymer chain is shown below. 

Draw the skeletal formula of the monomer. 

Explain why poly(methylmethacrylate) is fairly unreactive. 

Polymer waste has been an environmental concern for many years now. 

The availability, cost and convenience of polymers for use in many everyday household items mean the the demand for them has increased bringing with it huge environmental effects.

Evaluate the three methods below for reducing the environmental impact of plastics. 

• Recycling

  • Combustion

  • Feedstock recycling 

Bioplastics, such as biodegradable plastics offer a sustainable alternative to oil based products.

Where the use of plant based polymers is not possible, photodegradable polymers are being developed. 

i) Explain how photodegradable polymers are broken down. 

[1]

ii) Suggest two disadvantages of using photodegradable polymers.

[2]

This question is about alkenes. 

But-1-ene is an organic compound widely used in the petrochemical industry. 

Explain why but-1-ene is more reactive than butane. 

But-1-ene has the molecular formula C₄H₈. 

For the reactions of but-1-ene with bromine, hydrogen bromide and steam: 

• Predict the organic products made

• Write the equation for the reactions 

• Give the conditions for the reactions. 

During the reaction between but-1-ene and HBr, the HBr breaks by heterolytic fission. 

i) Explain what is meant by heterolytic fission. 

[1]

ii) Draw the mechanism for the electrophilic addition of HBr to but-1-ene to show the formation of the major product. 

[4]

The structure of but-2-en-1-ol is shown below. 

Explain the bond angle and shape around each atom A, B and C. 

You should refer to electron repulsion theory in your answer. 

This question is about alkenes. 

A chloroalkene, compound Y has the following percentage composition by mass of carbon and chlorine: 

   C= 24.7 %    Cl: 73.2% 

The relative molecular mass of compound Y is 145.5

Show that the relative molecular formula of compound Y is C₃H₃Cl₃. 

Explain how p-orbitals are involved in the formation of the C=C bond in chloroalkenes.

You should include a sketch in your answer. 

Compound Y has six possible structural isomers of C₃H₃Cl₃. 

Two of these isomers are shown below. 

i) Name isomer 1.

[1]

ii) Draw three other structural isomers of C₃H₃Cl₃ that are chloroalkenes. 

[3]

iii) Show two repeat units for the polymer formed by isomer 2. 

[1]

Polymers can be disposed of by incineration. 

i) Write a balanced symbol equation for the incineration of poly(chloroethene). 
Carbon dioxide, water and HCl are formed.

[1]

ii) Describe the environmental impact of this reaction and explain how it could be reduced. 

[3]