Polyhalogen Compounds: CHCl₃, CCl₄, DDT, and CFCs

Master important polyhalogen compounds - chloroform, freons, DDT, and their environmental impact for JEE Chemistry

13 min read

The Hook: From Life-Saving Anesthetics to Environmental Villains

Connect: Real Life → Chemistry

Chloroform was once used to put patients to sleep during surgery - it revolutionized medicine in the 1800s! CFCs were miracle refrigerants that made air conditioning possible. DDT saved millions from malaria. But all three are now banned or restricted. Why?

Here’s the JEE question: Why does chloroform need to be stored in dark bottles? Why do CFCs destroy the ozone layer? And what makes DDT so persistent in the environment?

The Core Concept

What are Polyhalogen Compounds?

Organic compounds containing two or more halogen atoms.

Classification:

By position:

  • Geminal (gem): Both halogens on same carbon

    • Example: CHCl₃ (chloroform)

  • Vicinal (vic): Halogens on adjacent carbons

    • Example: CH₂Cl-CH₂Cl (ethylene dichloride)

By number:

  • Dihalogen: Two halogen atoms
  • Trihalogen: Three halogen atoms (CHCl₃)
  • Tetrahalogen: Four halogen atoms (CCl₄)
  • Polyhalogen: Multiple halogens (DDT has 5 Cl atoms)
JEE Weightage
Polyhalogen Compounds: 1-2 questions in JEE Main, 1-2 in JEE Advanced Focus areas: Preparation, properties, uses, environmental chemistry High relevance for current affairs and environmental chemistry questions!

Chloroform (Trichloromethane, CHCl₃)

Structure and Properties

Molecular formula: CHCl₃ IUPAC name: Trichloromethane Common name: Chloroform

Physical properties:

  • Colorless, volatile liquid
  • Sweet smell (characteristic odor)
  • Boiling point: 61°C
  • Denser than water (1.48 g/mL)
  • Slightly soluble in water, miscible with organic solvents

Preparation

Industrial Method: Chlorination of Methane

$$\boxed{\text{CH}_4 \xrightarrow{\text{Cl}_2/h\nu} \text{CH}_3\text{Cl} \xrightarrow{\text{Cl}_2/h\nu} \text{CH}_2\text{Cl}_2 \xrightarrow{\text{Cl}_2/h\nu} \text{CHCl}_3}$$

Free radical substitution - continues to CCl₄ if excess Cl₂

Laboratory Method: Haloform Reaction

From ethanol or acetone:

$$\boxed{\text{CH}_3\text{COCH}_3 + 3\text{Cl}_2 + 4\text{NaOH} \rightarrow \text{CHCl}_3 + 3\text{NaCl} + \text{CH}_3\text{COONa} + 3\text{H}_2\text{O}}$$

Mechanism:

Step 1: Halogenation of methyl group

$$\text{CH}_3\text{COCH}_3 \xrightarrow{3\text{Cl}_2/\text{OH}^-} \text{CCl}_3\text{COCH}_3$$

Step 2: Nucleophilic attack by OH⁻

$$\text{CCl}_3\text{COCH}_3 + \text{OH}^- \rightarrow \text{CCl}_3^- + \text{CH}_3\text{COOH}$$

Step 3: Protonation

$$\text{CCl}_3^- + \text{H}_2\text{O} \rightarrow \text{CHCl}_3 + \text{OH}^-$$

Works with:

  • Compounds with CH₃CO- group
  • Ethanol (oxidized to acetaldehyde first)
  • Acetone, methyl ketones
JEE Question: Haloform Reaction

Q: Which of the following will give haloform reaction?

(a) CH₃CH₂OH (b) CH₃COCH₃ (c) CH₃CHO (d) HCHO

Answer: (a), (b), and (c)

Explanation:

(a) CH₃CH₂OH: Yes

  • Oxidized to CH₃CHO (has CH₃CO- after oxidation)
  • Gives haloform

(b) CH₃COCH₃: Yes

  • Directly has CH₃CO- group
  • Classic haloform substrate

(c) CH₃CHO: Yes

  • Has CH₃CO- group (as aldehyde)
  • Gives haloform

(d) HCHO: No

  • No CH₃ group
  • Cannot give haloform

JEE Rule: Haloform test detects CH₃CO- or CH₃CHOH- groups!

Reactions of Chloroform

Reaction 1: Oxidation by Air and Light

$$\boxed{2\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} 2\text{COCl}_2 + 2\text{HCl}}$$

Product: Phosgene (COCl₂) - highly toxic gas

Why dangerous?

  • Phosgene was used as chemical weapon in WWI
  • Attacks respiratory system
  • Can be fatal

Prevention:

  • Store in dark brown bottles (prevent light)
  • Add 0.5-1% ethanol as stabilizer
  • Ethanol reacts with phosgene: COCl₂ + 2C₂H₅OH → (C₂H₅O)₂CO + 2HCl
JEE Fact: Why Dark Bottles?

Q: Why is chloroform stored in dark bottles with a little alcohol?

Answer:

Dark bottles: Prevent light-induced oxidation

$$\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} \text{COCl}_2 \text{ (phosgene - toxic!)}$$

Alcohol (ethanol): Neutralizes any phosgene formed

$$\text{COCl}_2 + 2\text{C}_2\text{H}_5\text{OH} \rightarrow \text{(C}_2\text{H}_5\text{O)}_2\text{CO} + 2\text{HCl}$$

This is a HIGH-YIELD JEE question - remember both reasons!

Interactive Demo: Visualize Polyhalogen Molecular Structures

Explore the 3D structures of chloroform, carbon tetrachloride, and DDT molecules.

Reaction 2: Reduction

$$\boxed{\text{CHCl}_3 + 6[\text{H}] \xrightarrow{\text{Zn/HCl}} \text{CH}_4 + 3\text{HCl}}$$

Product: Methane

Reaction 3: Carbylamine Reaction

With primary amine + KOH:

$$\boxed{\text{CHCl}_3 + \text{RNH}_2 + 3\text{KOH} \rightarrow \text{RNC} + 3\text{KCl} + 3\text{H}_2\text{O}}$$

Product: Isocyanide (RN≡C) - foul-smelling!

Uses:

  • Test for primary amines
  • Isocyanides have characteristic offensive odor

Reaction 4: Nitration

$$\boxed{\text{CHCl}_3 + \text{HNO}_3 \rightarrow \text{CCl}_3\text{NO}_2 + \text{H}_2\text{O}}$$

Product: Chloropicrin (trichloronitromethane)

  • Used as tear gas
  • Soil fumigant

Uses of Chloroform

Historical:

  • Anesthetic in surgery (no longer used - toxic to liver and heart)
  • First successful general anesthetic (1847)

Current uses:

  • Solvent in pharmaceutical industry
  • Intermediate in synthesis
  • Refrigerant (limited use)
  • Pesticide manufacturing

Why discontinued as anesthetic?

  • Damages liver (hepatotoxic)
  • Damages heart (cardiotoxic)
  • Carcinogenic
  • Replaced by safer alternatives (halothane, sevoflurane)

Carbon Tetrachloride (Tetrachloromethane, CCl₄)

Structure and Properties

Molecular formula: CCl₄ IUPAC name: Tetrachloromethane Common name: Carbon tetrachloride

Physical properties:

  • Colorless liquid
  • Characteristic sweet smell
  • Boiling point: 77°C
  • Denser than water (1.59 g/mL)
  • Insoluble in water, miscible with organic solvents
  • Non-flammable

Preparation

$$\boxed{\text{CHCl}_3 + \text{Cl}_2 \xrightarrow{h\nu} \text{CCl}_4 + \text{HCl}}$$

Or from methane:

$$\text{CH}_4 + 4\text{Cl}_2 \xrightarrow{h\nu} \text{CCl}_4 + 4\text{HCl}$$

Industrial: Chlorination of carbon disulfide

$$\text{CS}_2 + 3\text{Cl}_2 \xrightarrow{\text{catalyst}} \text{CCl}_4 + \text{S}_2\text{Cl}_2$$

Reactions

Reaction 1: Hydrolysis (Very Slow)

$$\boxed{\text{CCl}_4 + 4\text{H}_2\text{O} \xrightarrow{\text{very slow}} \text{CO}_2 + 4\text{HCl}}$$

Note: Extremely slow - CCl₄ is very stable

  • No H atom for nucleophilic attack
  • All positions blocked by Cl

Reaction 2: With HF (Swarts Reaction)

$$\boxed{\text{CCl}_4 + \text{HF} \xrightarrow{\text{SbF}_3} \text{CCl}_3\text{F, CCl}_2\text{F}_2, \text{CClF}_3}$$

Products: Freons (fluorochlorocarbons)

Uses of CCl₄

Historical uses (now discontinued):

  • Fire extinguisher (non-flammable, dense vapor)
  • Dry cleaning solvent
  • Degreasing agent

Why discontinued?

  • Toxic to liver and kidneys
  • Ozone depleting substance
  • Environmental hazard
  • Banned under Montreal Protocol

Current uses (limited):

  • Laboratory solvent (with precautions)
  • Intermediate in chemical synthesis
  • CFC production (being phased out)
Environmental Impact

Why CCl₄ was banned as fire extinguisher:

  1. Toxic fumes: Decomposes to phosgene (COCl₂) at high temperature

  2. Ozone depletion: Cl radicals destroy ozone

    $$\text{CCl}_4 \xrightarrow{\text{UV}} \text{CCl}_3^\bullet + \text{Cl}^\bullet$$ $$\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$$ $$\text{ClO}^\bullet + \text{O}^\bullet \rightarrow \text{Cl}^\bullet + \text{O}_2$$

    Net: O₃ + O → 2O₂ (Cl• acts as catalyst!)

  3. Persistent: Does not decompose easily in environment

JEE Tip: Ozone depletion mechanism is frequently asked!

Freons (Chlorofluorocarbons - CFCs)

What are Freons?

Freons are chlorofluorocarbons (CFCs) - compounds containing C, F, and Cl.

Common examples:

NameFormulaFreon Number
Freon-11CCl₃FF-11
Freon-12CCl₂F₂F-12
Freon-22CHClF₂F-22
Freon-113CCl₂F-CClF₂F-113

Preparation

From CCl₄ (Swarts Reaction):

$$\boxed{\text{CCl}_4 + \text{HF} \xrightarrow{\text{SbF}_3} \text{CCl}_3\text{F} + \text{CCl}_2\text{F}_2 + \text{CClF}_3 + \text{CF}_4}$$

Example:

$$\text{CCl}_4 + 2\text{HF} \rightarrow \text{CCl}_2\text{F}_2 + 2\text{HCl}$$

Properties

Why freons were popular:

  1. Chemically inert - stable, non-reactive
  2. Non-toxic - safe to handle
  3. Non-flammable - safe in homes
  4. Low boiling points - good refrigerants
  5. Non-corrosive - compatible with metals

Seemed like perfect compounds!

Uses (Historical - Now Banned)

Refrigerants:

  • Air conditioners
  • Refrigerators
  • Freezers

Propellants:

  • Aerosol sprays
  • Deodorants
  • Insecticides

Blowing agents:

  • Foam production
  • Insulation materials

Environmental Impact: Ozone Depletion

The problem: CFCs destroy stratospheric ozone layer

Mechanism of ozone destruction:

Step 1: UV radiation breaks C-Cl bond

$$\text{CCl}_2\text{F}_2 \xrightarrow{\text{UV}} \text{CClF}_2^\bullet + \text{Cl}^\bullet$$

Step 2: Cl• attacks ozone

$$\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$$

Step 3: Regeneration of Cl•

$$\text{ClO}^\bullet + \text{O}^\bullet \rightarrow \text{Cl}^\bullet + \text{O}_2$$

Net reaction:

$$\text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2$$

Cl• acts as catalyst - one Cl atom can destroy 100,000 O₃ molecules!

JEE Concept: Ozone Hole

Why CFCs are so dangerous:

  1. Stability: Don’t decompose in troposphere

    • Reach stratosphere intact (15-50 km altitude)

  2. UV absorption: Only break down in stratosphere

    • Release Cl• radicals at ozone layer

  3. Catalytic destruction: Cl• regenerates

    • One CFC molecule destroys thousands of O₃

  4. Long lifetime: Persist 50-100 years

    • Continue destroying ozone for decades

Consequences of ozone depletion:

  • More UV-B radiation reaches Earth
  • Increased skin cancer rates
  • Damage to marine phytoplankton
  • Crop damage

Solution: Montreal Protocol (1987)

  • Phased out CFC production
  • Replaced by HFCs (hydrofluorocarbons)

JEE Tip: Know the catalytic cycle and why Cl• regenerates!

Replacement: HFCs and HCFCs

HFCs (Hydrofluorocarbons):

  • Example: CH₂FCF₃ (HFC-134a)
  • No chlorine - don’t deplete ozone
  • Still greenhouse gases (being phased out)

HCFCs (Hydrochlorofluorocarbons):

  • Example: CHClF₂ (HCFC-22)
  • Less harmful than CFCs
  • Temporary transition compounds
  • Being replaced by HFCs

DDT (Dichlorodiphenyltrichloroethane)

Structure and Properties

Full name: Dichlorodiphenyltrichloroethane

Structure:

        CCl₃
         |
    H—C—H
       /   \
      /     \
   Cl-⚬   ⚬-Cl

Formula: (ClC₆H₄)₂CH(CCl₃)

Properties:

  • White crystalline solid
  • Insoluble in water
  • Soluble in organic solvents
  • Fat-soluble (lipophilic)
  • Very stable (persistent)

Preparation

Condensation of chlorobenzene with chloral:

$$\boxed{2\text{C}_6\text{H}_5\text{Cl} + \text{CCl}_3\text{CHO} \xrightarrow{\text{H}_2\text{SO}_4} (\text{ClC}_6\text{H}_4)_2\text{CH(CCl}_3) + \text{H}_2\text{O}}$$

Reaction: Electrophilic substitution on chlorobenzene

Uses and Historical Significance

Insecticide:

  • Most effective insecticide ever discovered (1940s)
  • Saved millions of lives from malaria
  • Nobel Prize awarded to Paul Müller (1948)

Applications:

  • Mosquito control (malaria, dengue)
  • Agricultural pest control
  • Lice and typhus control

Success story:

  • Near eradication of malaria in many countries
  • Credited with saving 500 million lives

Why DDT was Banned

Environmental problems:

1. Persistence (Non-biodegradable)

  • Half-life: 15-30 years in soil
  • Does not decompose easily
  • Accumulates in environment

2. Bioaccumulation

  • Fat-soluble → stored in adipose tissue
  • Not excreted easily
  • Builds up in organisms over time

3. Biomagnification

Concentration increases up the food chain!

Water (0.001 ppm)
    ↓
Plankton (0.01 ppm) - 10× increase
    ↓
Small fish (0.1 ppm) - 10× increase
    ↓
Large fish (1 ppm) - 10× increase
    ↓
Birds/Humans (10+ ppm) - 10× increase

Top predators get toxic doses!

4. Ecological Impact

Effects on birds:

  • Interferes with calcium metabolism
  • Thin eggshells (eggs break during incubation)
  • Population decline in eagles, pelicans, falcons

Effects on humans:

  • Potential carcinogen
  • Endocrine disruptor
  • Affects nervous system
  • Accumulates in breast milk

5. Insect Resistance

  • Mosquitoes developed resistance
  • Required higher doses
  • Vicious cycle
JEE Concept: Biomagnification

Biomagnification = Increase in concentration of persistent pollutants up the food chain

Why DDT biomagnifies:

  1. Fat-soluble: Stored in fatty tissues
  2. Non-biodegradable: Not broken down
  3. Not excreted: Remains in body
  4. Food chain transfer: Passed to predators
  5. Accumulation: Each level concentrates it more

Mathematical example:

If efficiency of transfer = 10%, but DDT transfer = 100%:

Trophic Level          Energy    DDT
Water                  -         1 unit
Plankton (1° producer) 100%      10 units
Small fish (1° consumer) 10%     100 units
Large fish (2° consumer) 1%      1000 units
Eagle (3° consumer)    0.1%      10000 units

Result: Eagle has 10,000× more DDT than water!

JEE Strategy: Understand why persistent, fat-soluble pollutants are most dangerous!

Current Status

Banned: In most countries (since 1970s)

  • USA: Banned 1972
  • India: Banned 1989 for agriculture, allowed for malaria control

Still used: Limited use in some malaria-endemic countries

  • Indoor residual spraying only
  • Strict WHO guidelines
  • When benefits outweigh risks

Alternatives:

  • Synthetic pyrethroids
  • Biological control
  • Mosquito nets (treated with safer insecticides)

Iodoform (CHI₃)

Structure and Properties

Molecular formula: CHI₃ IUPAC name: Triiodomethane

Properties:

  • Yellow crystalline solid
  • Characteristic antiseptic smell
  • Melting point: 119°C
  • Insoluble in water
  • Soluble in organic solvents

Preparation: Iodoform Test

Haloform reaction with iodine:

$$\boxed{\text{CH}_3\text{COCH}_3 + 3\text{I}_2 + 4\text{NaOH} \rightarrow \text{CHI}_3 \downarrow + 3\text{NaI} + \text{CH}_3\text{COONa} + 3\text{H}_2\text{O}}$$

Yellow precipitate of iodoform forms!

This is a TEST for:

  • CH₃CO- group (methyl ketones)
  • CH₃CHOH- group (secondary alcohols with CH₃)
  • Ethanol (oxidized to acetaldehyde)

Uses

Antiseptic:

  • Wound dressing
  • Minor cuts and abrasions
  • Characteristic smell identifies it

Common Mistakes to Avoid

Mistake #1: Confusing Haloform Products

Wrong: “All halogenation reactions give haloform”

Correct: Haloform reaction is specific!

Requirements:

  • CH₃CO- or CH₃CHOH- group
  • Excess halogen (Cl₂, Br₂, or I₂)
  • Base (NaOH)

Product: CHX₃ (haloform) + carboxylate salt

JEE Tip: Haloform test distinguishes methyl ketones from other ketones!

Mistake #2: Forgetting Stabilizers

Wrong: “Chloroform can be stored in any bottle”

Correct: Must use dark bottles + 1% ethanol

Reason:

  • Light + O₂ → COCl₂ (phosgene - toxic!)
  • Ethanol neutralizes phosgene

JEE Question type: “Why dark bottle?” - Always give BOTH reasons!

Mistake #3: Wrong Ozone Depletion Mechanism

Wrong: “CFCs directly react with ozone”

Correct: CFCs release Cl• which catalytically destroys ozone

Key points:

  • UV breaks C-Cl bond → Cl•
  • Cl• + O₃ → ClO• + O₂
  • ClO• + O• → Cl• + O₂
  • Cl• regenerates (catalytic cycle)

JEE Tip: Emphasize “catalytic” - one Cl• destroys many O₃!

Practice Problems

Level 1: Foundation (NCERT)

Problem 1: Haloform Reaction

Q: Write the reaction of acetone with Cl₂ in presence of NaOH.

Solution:

$$\text{CH}_3\text{COCH}_3 + 3\text{Cl}_2 + 4\text{NaOH} \rightarrow \text{CHCl}_3 + 3\text{NaCl} + \text{CH}_3\text{COONa} + 3\text{H}_2\text{O}$$

Products:

  • Chloroform (CHCl₃)
  • Sodium acetate (CH₃COONa)
  • Sodium chloride
  • Water

This is haloform reaction!

Problem 2: Storage of Chloroform

Q: Why is chloroform stored in dark-colored bottles?

Solution:

Reason: Prevent oxidation by air and light

Reaction:

$$2\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} 2\text{COCl}_2 + 2\text{HCl}$$

Product: Phosgene (COCl₂) - highly toxic

Prevention methods:

  1. Dark bottles - block light
  2. Add 1% ethanol - neutralizes phosgene $$\text{COCl}_2 + 2\text{C}_2\text{H}_5\text{OH} \rightarrow (\text{C}_2\text{H}_5\text{O})_2\text{CO} + 2\text{HCl}$$

Level 2: JEE Main

Problem 3: Iodoform Test

Q: Which of the following will give positive iodoform test?

(a) CH₃CH₂OH (b) (CH₃)₂CHOH (c) CH₃COCH₃ (d) CH₃CHO

Solution:

Positive test: (a), (b), (c), and (d)

(a) CH₃CH₂OH: Yes

  • Ethanol is oxidized to CH₃CHO
  • CH₃CHO has CH₃CO- group
  • Gives CHI₃

(b) (CH₃)₂CHOH: Yes

  • Has CH₃CHOH- group directly
  • Gives CHI₃

(c) CH₃COCH₃: Yes

  • Has CH₃CO- group
  • Classic iodoform substrate

(d) CH₃CHO: Yes

  • Has CH₃CO- group (as aldehyde)
  • Gives CHI₃

Only primary alcohols with CH₃CH₂OH structure and methyl ketones give positive test!

Problem 4: Environmental Chemistry

Q: Explain how CFCs cause ozone depletion.

Solution:

Mechanism:

Step 1: UV breaks C-Cl bond in stratosphere

$$\text{CCl}_2\text{F}_2 \xrightarrow{\text{UV}} \text{CClF}_2^\bullet + \text{Cl}^\bullet$$

Step 2: Cl• attacks ozone

$$\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$$

Step 3: ClO• reacts with atomic oxygen

$$\text{ClO}^\bullet + \text{O}^\bullet \rightarrow \text{Cl}^\bullet + \text{O}_2$$

Step 4: Cl• regenerates (returns to Step 2)

Net reaction:

$$\text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2$$

Key point: Cl• acts as catalyst

  • Not consumed in net reaction
  • One Cl• destroys ~100,000 O₃ molecules
  • This is why CFCs are so dangerous!

Level 3: JEE Advanced

Problem 5: Biomagnification

Q: Explain why DDT concentration is highest in birds at the top of food chain.

Solution:

Process: Biomagnification

Properties of DDT:

  1. Fat-soluble (lipophilic) - stored in adipose tissue
  2. Non-biodegradable - not broken down by organisms
  3. Persistent - remains in environment for decades
  4. Not excreted - accumulates in body

Biomagnification mechanism:

Food Chain Level          DDT Concentration
Water                     0.001 ppm
Phytoplankton            0.01 ppm (10× increase)
Zooplankton              0.1 ppm (10× increase)
Small fish               1 ppm (10× increase)
Large fish               10 ppm (10× increase)
Fish-eating birds        25 ppm (2.5× increase)
Eagle (top predator)     100+ ppm (4× increase)

Why concentration increases:

  • Each organism eats many from level below
  • DDT transferred but not eliminated
  • Accumulates in fatty tissues
  • Higher trophic level = more accumulated DDT

Consequence for birds:

  • Interferes with calcium metabolism
  • Thin, fragile eggshells
  • Eggs break during incubation
  • Population decline

This is why persistent, fat-soluble pollutants are most dangerous!

Problem 6: Multi-concept Question

Q: Arrange in order of increasing ozone-depleting potential: (a) HFC-134a (CH₂FCF₃) (b) CFC-12 (CCl₂F₂) (c) HCFC-22 (CHClF₂) (d) CCl₄

Solution:

Order: (a) < (c) < (b) ≈ (d)

Analysis:

(a) HFC-134a: Zero ozone depletion

  • No chlorine atoms
  • Cannot produce Cl• radicals
  • Safe for ozone layer (but greenhouse gas)

(c) HCFC-22: Low ozone depletion

  • One Cl atom
  • Has C-H bond → breaks down faster in troposphere
  • Less reaches stratosphere
  • Transitional replacement

(b) CFC-12: High ozone depletion

  • Two Cl atoms
  • No C-H bonds → very stable
  • Reaches stratosphere intact
  • Major ozone depleter

(d) CCl₄: Very high ozone depletion

  • Four Cl atoms
  • Extremely stable
  • All four can produce Cl•
  • Highest ozone-depleting potential

Key factors:

  1. Number of Cl atoms
  2. Presence of C-H bonds (faster breakdown)
  3. Stability (reaches stratosphere or not)

JEE Strategy: No Cl = no ozone depletion!

Quick Revision Box

CompoundFormulaKey PropertyUseEnvironmental Issue
ChloroformCHCl₃Sweet smell, oxidizes to phosgeneSolvent, former anestheticToxic to liver
CCl₄CCl₄Non-flammableFormer fire extinguisherOzone depleter, toxic
FreonsCCl₂F₂Inert, volatileRefrigerantsOzone depletion
DDT(ClC₆H₄)₂CH(CCl₃)Fat-soluble, persistentInsecticideBiomagnification
IodoformCHI₃Yellow solid, antiseptic smellAntiseptic, test reagentNone (limited use)

Connection to Other Topics

Prerequisites:

Related Topics:

Applications:

Teacher’s Summary

Key Takeaways

1. Important Polyhalogen Compounds:

  • CHCl₃: Former anesthetic, oxidizes to phosgene (store in dark + ethanol)
  • CCl₄: Former fire extinguisher, ozone depleter (banned)
  • CFCs: Refrigerants, destroy ozone (Montreal Protocol)
  • DDT: Effective insecticide, biomagnifies (banned in most countries)

2. Haloform Reaction (HIGH-YIELD):

  • Test for CH₃CO- or CH₃CHOH- groups
  • Products: CHX₃ + carboxylate
  • Iodoform test: Yellow precipitate of CHI₃

3. Environmental Chemistry (VERY IMPORTANT):

Ozone Depletion:

  • CFCs release Cl• in stratosphere
  • Cl• catalytically destroys O₃
  • One Cl• destroys ~100,000 O₃ molecules
  • Solution: HFCs (no chlorine)

Biomagnification:

  • DDT is fat-soluble and persistent
  • Concentration increases up food chain
  • Top predators most affected
  • Causes thin eggshells in birds

4. JEE Focus Areas:

  • Haloform test mechanism
  • Why chloroform needs dark bottles
  • Ozone depletion catalytic cycle
  • Biomagnification concept

“These compounds changed the world - both for better (medicine, refrigeration, pest control) and worse (environmental damage). Understanding their chemistry explains why!”

This completes the Halogens Compounds chapter. Next, study Biomolecules to learn about carbohydrates, proteins, and life’s chemistry!