Alkenes (OCR A Level Chemistry)

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.

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.

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.

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.

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.

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.

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.

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.

Figure below shows a section of the PVC polymer.

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).

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. 

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

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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.

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 

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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. 

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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. 

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. 

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Polymers can be disposed of by incineration. 

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