Q.Some halogen containing compounds are useful in daily life. Some compounds of this class are responsible for exposure of flora and fauna to more and more of UV light which causes destruction to a great extent. Name the class of these halocompounds. In your opinion, what should be done to minimise harmful effects of these compounds.

Concept: Organic Compound Applications – Certain halogenated compounds deplete the ozone layer, increasing UV exposure.

Reasoning:

1. Chlorofluorocarbons (CFCs) and related halocarbons (e.g., halons, carbon tetrachloride) release chlorine/bromine radicals in the stratosphere.
2. These radicals catalytically destroy ozone ($O_3$), thinning the ozone shield and allowing more harmful UV-B radiation to reach Earth.
3. This UV increase damages DNA in flora and fauna, causing reduced crop yields, skin cancers, and ecosystem disruption.

To minimise harm:

• Phase out production and use of CFCs, halons, and other ozone-depleting substances under the Montreal Protocol. …

Why This Matters

The question connects two things: a specific class of halogen compounds that we use daily, and a global environmental problem — increased UV radiation reaching the Earth. The link is ozone layer depletion. Halogen compounds that are stable enough to reach the stratosphere can release chlorine or bromine atoms, which catalytically destroy ozone. With less ozone, more UV-B light penetrates, harming both plants and animals (including humans, through skin cancer and cataracts).

The compounds in question are chlorofluorocarbons (CFCs) — used in refrigerators, air conditioners, aerosol propellants, and foam-blowing agents. They were once considered miracle chemicals because they were non-toxic, non-flammable, and chemically inert near the ground. That very stability lets them drift up to the stratosphere unchanged, where UV light finally breaks them apart.

Step-by-Step Reasoning

1. Identify the class from the description

   The question says: "Some halogen containing compounds are useful in daily life" and "responsible for exposure of flora and fauna to more and more of UV light". The only class of halocompounds that causes increased UV exposure is the one that depletes the ozone layer. That class is chlorofluorocarbons (CFCs). Other halocompounds like halons (containing bromine) also deplete ozone, but CFCs are the most widely used and well-known.

2. Understand the mechanism (why they cause UV increase)

   CFCs (e.g., $\text{CCl}_3\text{F}$, $\text{CCl}_2\text{F}_2$) are photolysed in the stratosphere by UV light:

The chlorine atom then reacts with ozone:

The net reaction is $\text{O}_3 + \text{O} \rightarrow 2\text{O}_2$, with chlorine acting as a catalyst. One chlorine atom can destroy thousands of ozone molecules before being removed.

3. Name the class

   The class is chlorofluorocarbons (CFCs). In exam language, they are also called Freons (a trade name). The question expects the name "chlorofluorocarbons" or "CFCs".

4. What should be done to minimise harmful effects?

   This is an opinion-based part, but it must be scientifically sound. The core idea: stop releasing CFCs into the atmosphere. Practical steps include:

   • Ban production and use of CFCs under international agreements like the Montreal Protocol (1987). This has already been done successfully. …

Concept: Ozone Depletion by Halogen Compounds

Method: Cause-Effect-Solution Analysis

This method breaks down the problem into three logical steps: identifying the harmful class of compounds, understanding their mechanism of harm, and proposing practical solutions.

Step 1: Identify the Class of Halocompounds

The compounds described are chlorofluorocarbons (CFCs) — halogen-containing compounds (containing chlorine, fluorine, and carbon) that are stable, non-toxic, and widely used in refrigerants, aerosol propellants, and foam-blowing agents.

Why they cause UV exposure:

CFCs rise to the stratosphere, where UV radiation breaks them down, releasing chlorine atoms. These chlorine atoms catalytically destroy ozone ($O_3$) molecules, thinning the ozone layer. A thinner ozone layer allows more harmful UV-B radiation to reach Earth, damaging flora, fauna, and human health.

Step 2: Understand the Harmful Mechanism

The key reaction cycle (simplified):

1. $Cl + O_3 \rightarrow ClO + O_2$
2. $ClO + O \rightarrow Cl + O_2$

Net result: One chlorine atom can destroy thousands of ozone molecules before being removed.

Step 3: Propose Solutions to Minimise Harm

To reduce the harmful effects of CFCs, the following actions should be taken:

• Phase out CFC production and use — as mandated by the Montreal Protocol (1987), which has been highly successful. …

Here is a breakdown of the common mistakes students make on this specific question, along with the correct reasoning and exam-smart strategies to avoid them.

The Core Concept: Ozone Depletion by CFCs

The question is testing your understanding of Chlorofluorocarbons (CFCs) and their role in Ozone Layer Depletion.

• The "Why": CFCs are stable in the lower atmosphere but break down in the stratosphere under UV light, releasing chlorine atoms. These chlorine atoms act as catalysts, destroying ozone ($O_3$) molecules. This thinning of the ozone layer allows more harmful UV-B radiation to reach the Earth, harming flora and fauna.
• The "What": The class of compounds is Chlorofluorocarbons (CFCs) or Freons.

Common Mistake #1: Naming the Wrong Class of Compounds

The Mistake:

Students often write "Halogenated compounds," "Organohalogens," or "Alkyl halides." While technically true that CFCs are a subset of these, the question specifically asks for the class responsible for UV exposure. These answers are too broad and will lose marks.

How to Avoid:

• Be Specific: The question gives a clear clue: "responsible for exposure... to more UV light." This is the classic signature of CFCs (or Freons).
• Memorise the Link: In your mind, immediately connect "UV light + destruction + halocompounds" to CFCs and Ozone Depletion. Do not write "haloalkanes" or "haloarenes" unless the question is about their general properties.

Correct Answer: Chlorofluorocarbons (CFCs) or Freons.

Common Mistake #2: Giving a Vague or Impractical "Minimisation" Strategy

The Mistake:

Students write generic answers like:

• "Stop using all halogen compounds."
• "Ban all chemicals."
• "Use less of them."

These are impractical, unscientific, and show a lack of understanding of the real-world solution.

How to Avoid:

• Think Like a Chemist: The solution is not to ban all halocompounds (many are essential in medicine and industry). The solution is to replace the harmful ones with safer alternatives.
• Use Exam-Accurate Phrases: The textbook and examiners expect specific, actionable steps.

Correct Answer (Pick 2-3 points):

• Use substitutes: Replace CFCs with HCFCs (Hydrochlorofluorocarbons) or HFCs (Hydrofluorocarbons) which have lower ozone-depleting potential.
• Recycle and recover: Implement proper recycling of CFCs from old refrigerators and air conditioners instead of releasing them into the atmosphere.
• Follow international protocols: Adhere to the Montreal Protocol (1987), which successfully phased out the production of many CFCs.
• Develop non-CFC technologies: Use ammonia or hydrocarbons (like propane) as refrigerants in new appliances.

Common Mistake #3: Confusing the Mechanism (The "Why")

The Mistake:

Students write: "CFCs directly block UV light" or "CFCs absorb UV light and heat up the atmosphere." This is incorrect and shows a fundamental misunderstanding. …