Question

The gas 'A' is having very low reactivity reaches to stratosphere. It is non-toxic and non-flammable but dissociated by UV-radiations in stratosphere. The intermediates formed initially from the gas 'A' are:

• A. \(\dot{\text{C}}\text{H}_3 + \dot{\text{C}}\text{F}_2\text{Cl}\)
• B. \(\dot{\text{C}}\text{lO} + \dot{\text{C}}\text{H}_3\)
• C. \(\dot{\text{C}}\text{lO} + \dot{\text{C}}\text{F}_2\text{Cl}\)
• D. \(\dot{\text{C}}\text{l} + \dot{\text{C}}\text{F}_2\text{Cl}\)

Hint:

The story of ozone depletion is centered on CFCs. Remember that their stability in the lower atmosphere is why they reach the stratosphere, and the weaker C-Cl bond is the Achilles' heel that releases the ozone-destroying chlorine radical under UV radiation.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to identify the initial products formed when a specific type of gas, described by its properties, is broken down by UV radiation in the stratosphere.
Step 2: Key Formula or Approach:
The properties described for gas 'A' (low reactivity, non-toxic, non-flammable, stable until the stratosphere) are characteristic of chlorofluorocarbons (CFCs). A common example of a CFC is Freon-12, dichlorodifluoromethane (CF\(_2\)Cl\(_2\)). In the stratosphere, high-energy UV radiation causes photodissociation (breaking of chemical bonds). The bond with the lowest bond dissociation energy will break first. In CFCs, the C-Cl bond is weaker than the C-F bond. The bond breaks homolytically, forming free radicals.
Step 3: Detailed Explanation:
Let's assume gas 'A' is CF\(_2\)Cl\(_2\). The C-Cl bond has a bond energy of about 339 kJ/mol, while the C-F bond has a bond energy of about 485 kJ/mol. The UV radiation in the stratosphere has sufficient energy to break the weaker C-Cl bond. The photodissociation reaction proceeds via homolytic cleavage, where the two electrons in the covalent bond are split, one going to each atom, resulting in two free radicals. The reaction is: \[ \text{CF}_2\text{Cl}_2 (g) \xrightarrow{\text{UV light}} \dot{\text{C}}\text{l} (g) + \dot{\text{C}}\text{F}_2\text{Cl} (g) \] The initial intermediates formed are a chlorine free radical (\(\dot{\text{C}}\text{l}\)) and a chlorodifluoromethyl free radical (\(\dot{\text{C}}\text{F}_2\text{Cl}\)). This chlorine radical is highly reactive and initiates the catalytic cycle of ozone depletion.
Step 4: Final Answer:
The intermediates formed initially from the dissociation of gas 'A' are \(\dot{\text{C}}\text{l}\) and \(\dot{\text{C}}\text{F}_2\text{Cl}\). This corresponds to option (D).