Alkenes (AQA 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 1 shows two hydrocarbons which are obtained by cracking crude oil.

Figure 1

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.

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

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.

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 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 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 hydrolysis of the alkyl hydrogen sulfate formed in the first step.

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.

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

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

Figure 2

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

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

Figure 1 below shows a section of the PVC polymer.

Figure 1

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

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

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

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

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

Draw and name the isomers.

Addition polymerisation is normally an exothermic reaction. In industry, a compromise temperature of 200 C 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.

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.

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.

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

Table 1

The approximate M_r 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.

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.)

Reaction 1 forms an alcohol when reacted with concentrated sulfuric acid, H₂SO₄ and steam.

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

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

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.

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

Figure 1

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.

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

Figure 2

Give two reasons why these polymers are unreactive.

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).

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

C₂H₅OH → C₂H₄ + H₂O

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).

Polymers are formed from the addition polymerisation of alkenes.

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

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.