Photo AI
Answer the questions below about methane (a saturated hydrocarbon), ethene (an unsaturated hydrocarbon) and benzene (an aromatic hydrocarbon) - Leaving Cert Chemistry - Question 8 - 2011
Question 8
Answer the questions below about methane (a saturated hydrocarbon), ethene (an unsaturated hydrocarbon) and benzene (an aromatic hydrocarbon). (a) Describe the mech... show full transcript
Worked Solution & Example Answer:Answer the questions below about methane (a saturated hydrocarbon), ethene (an unsaturated hydrocarbon) and benzene (an aromatic hydrocarbon) - Leaving Cert Chemistry - Question 8 - 2011
Step 1
Describe the mechanism of the monochlorination of methane.
Answer
The mechanism of the monochlorination of methane occurs in three stages: initiation, propagation, and termination.
-
Initiation: The process begins with the homolysis of the chlorine molecule (Cl₂) into two chlorine radicals (Cl·) under the influence of ultraviolet (UV) light, which provides the energy required to break the Cl-Cl bond:
-
Propagation: The chlorine radicals then react with methane (CH₄) in a sequence of steps. The first step involves a chlorine radical abstracting a hydrogen atom from methane, leading to the formation of hydrochloric acid (HCl) and a methyl radical (CH₃·):
Next, the newly formed methyl radical can react with another chlorine molecule:
-
Termination: The reaction can terminate when two radicals combine to form stable products, thus depleting the number of radicals and stopping the chain reaction:
This results in the formation of chloromethane and other byproducts.
Step 2
State three pieces of experimental evidence for the mechanism you have described.
Answer
-
Effect of uv light: The requirement of UV light suggests that the process involves the breaking of bonds via radical formation, supporting the initiation step as described.
-
Presence of products: The formation of chloromethane in the presence of excess chlorine confirms that the reaction is taking place and supports the propagation step.
-
Rate of reaction influenced by concentration: The rate of the reaction is observed to increase with the concentration of chlorine. This aligns with the mechanism where increasing radicals lead to more frequent reactions, supporting the propagation phase.
Step 3
Explain how each piece of evidence supports the mechanism you have described.
Answer
-
Effect of uv light: This indicates that the reaction requires energy to initiate the homolysis of Cl₂ into radicals, consistent with the initial step of the mechanism.
-
Presence of products: The appearance of chloromethane and HCl confirms that the proposed propagation steps occur, as they are the expected products from the interaction of chlorine radicals with methane.
-
Rate of reaction influenced by concentration: This evidence suggests that the reaction involves free radicals that react in a chain manner, fitting the defined mechanism where the number of radicals increases as the reaction progresses.
Step 4
Step 5
Step 6
Describe the bonding in benzene.
Answer
Benzene (C₆H₆) has a unique bonding structure characterized by:
- Hexagonal planar structure: Benzene has a planar structure with six carbon atoms arranged in a hexagonal ring.
- Sigma bonds: Each carbon atom forms single covalent (sigma) bonds with two adjacent carbon atoms as well as one hydrogen atom.
- Delocalized pi electrons: The p orbitals of the carbon atoms overlap and create a system of delocalized pi electrons above and below the plane of the ring, contributing to the stability of benzene through resonance.