Alkenes (CIE A Level Chemistry)

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

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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. 

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

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. 

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