Organic chemistry - Alkenes (3)
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1.
Ethylene reacts with steam in the presence of an acid catalyst to produce ethanol. Describe the reaction, including the conditions required, the observations made, and the chemical equation. Draw the structural formulae of the reactants and products.
Description of Reaction: Ethene reacts with steam (H2O) in the presence of an acid catalyst (e.g., sulfuric acid, H2SO4) to produce ethanol. The acid catalyst protonates the alkene, forming a carbocation intermediate. This carbocation then reacts with water to form a protonated alcohol, which deprotonates to give ethanol. The reaction is typically carried out at a high temperature (e.g., 350-400 °C) and moderate pressure.
Chemical Equation:
C2H4(g) + H2O(g) ⇌ C2H5OH(g)
Observations: The reaction is endothermic, meaning heat is required. The product, ethanol, will be collected as a gas. The reaction is carried out at a high temperature, and the presence of an acid catalyst is essential for the reaction to proceed at a reasonable rate.
Structural Formulae:
- Ethene (Reactant): CH2=CH2
- Steam (Reactant): H2O
- Ethanol (Product): CH3-CH2OH
2.
A student carried out a test to distinguish between two hydrocarbons, X and Y. They added each hydrocarbon to aqueous bromine and recorded their observations. The results are shown below:
| Hydrocarbon | Observation after adding aqueous bromine |
| X | Solution decolourises |
| Y | Solution remains brown |
State one possible identity for hydrocarbon X and one possible identity for hydrocarbon Y. Explain your answers in terms of the chemical test performed.
Hydrocarbon X: A possible identity for hydrocarbon X is ethene (C2H4). This is because the decolourisation of the bromine solution indicates that X is an unsaturated hydrocarbon containing a double bond. Ethene is a common alkene.
Hydrocarbon Y: A possible identity for hydrocarbon Y is ethane (C2H6). This is because the bromine solution remaining brown indicates that Y is a saturated hydrocarbon containing only single bonds. Ethane is a common alkane.
Explanation: The chemical test relies on the reactivity of bromine with unsaturated hydrocarbons. The addition reaction of bromine to a double bond results in the loss of the characteristic brown colour of bromine. Saturated hydrocarbons, lacking double or triple bonds, are unreactive and therefore do not cause a colour change.
3.
Describe the difference between a single bond, a double bond and a triple bond in terms of the number of electrons shared and the strength of the bond. Relate your answer to the bonding found in alkenes.
| Type of Bond | Number of Electrons Shared | Strength |
|---|
| Single Bond | 1 pair (2 electrons) | Weakest |
| Double Bond | 2 pairs (4 electrons) | Stronger than a single bond, weaker than a triple bond |
| Triple Bond | 3 pairs (6 electrons) | Strongest |
Alkenes contain a double bond, which consists of two shared pairs of electrons. This double bond is stronger than a single bond but weaker than a triple bond. The presence of this double bond is what defines alkenes as unsaturated hydrocarbons. Because the double bond is present, the alkene has fewer hydrogen atoms than a saturated hydrocarbon with the same number of carbon atoms. This makes it reactive and able to undergo addition reactions.