Poly Halogen Compounds

Q: Why is chloroform stored in dark coloured bottles?

In sunlight, air slowly oxidises chloroform to phosgene, COCl2, which is extremely poisonous. Chloroform is therefore kept in dark bottles filled to the brim to exclude light and air, and about 1 percent ethanol is added to convert any phosgene formed into harmless diethyl carbonate.

Q: What is the iodoform test?

The iodoform test detects compounds with a CH3CO- group or a CH3CH(OH)- group. When such a compound is warmed with iodine and sodium hydroxide, a yellow precipitate of iodoform, CHI3, forms. Ethanol, acetaldehyde, acetone and propan-2-ol give the test; methanol and propan-1-ol do not.

ChemistryHaloalkanes And HaloarenesFor NEET aspirants

Polyhalogen compounds are carbon compounds that contain more than one halogen atom, such as dichloromethane, chloroform, iodoform, carbon tetrachloride, freons and DDT. They are widely used as solvents, refrigerants, antiseptics and pesticides, but many of them harm health or the environment. For chloroform, exams focus on its reactions, especially the carbylamine and Reimer-Tiemann reactions, and on why it is stored in dark bottles. Polyhalogen compounds are a regular source of direct questions in JEE Main and NEET on preparation, uses and environmental effects.

Key Formulas - Quick Reference

$C H_{4} C l_{2}, h ν C H_{3} Cl \to C H_{2} C l_{2} \to CHC l_{3} \to CC l_{4}$
$2CHC l_{3} + O_{2} light 2COC l_{2} + 2HCl$ (phosgene)
Carbylamine: $R - N H_{2} + CHC l_{3} + 3KOH ⟶ R - NC + 3KCl + 3 H_{2} O$
$CHC l_{3} + 4KOH ⟶ HCOOK + 3KCl + 2 H_{2} O$
$2CH X_{3} + 6Ag Δ HC \equiv CH + 6AgX$
Iodoform test: $C H_{3} CO -$ or $C H_{3} CH (OH) -$ group + $I_{2}$ /NaOH gives yellow $CH I_{3}$
$CC l_{4} + 2 [H] Fe, H_{2} O CHC l_{3} + HCl$
Freon-12 (Swarts): $3CC l_{4} + 2Sb F_{3} ⟶ 3CC l_{2} F_{2} + 2SbC l_{3}$
BHC: $C_{6} H_{6} + 3C l_{2} UV C_{6} H_{6} C l_{6}$
DDT: $2 C_{6} H_{5} Cl + CC l_{3} CHO conc. H_{2} S O_{4} DDT + H_{2} O$

1. What Are Polyhalogen Compounds?

Carbon compounds containing more than one halogen atom are called polyhalogen compounds. Many are useful in industry and agriculture, but several are toxic or persist in the environment.

Compound	Formula	Main use	Main concern
Dichloromethane	$C H_{2} C l_{2}$	Solvent, paint remover	Harms the nervous system
Chloroform	$CHC l_{3}$	Solvent, making refrigerant R-22	Forms poisonous phosgene; liver damage
Iodoform	$CH I_{3}$	Antiseptic (earlier)	Unpleasant smell
Carbon tetrachloride	$CC l_{4}$	Solvent, making freons	Liver damage; ozone depletion
Freon-12	$CC l_{2} F_{2}$	Refrigerant, aerosol propellant	Ozone depletion
DDT	$(Cl C_{6} H_{4})_{2} CHCC l_{3}$	Insecticide	Persistent; builds up in food chains

2. Dichloromethane (Methylene Chloride)

Preparation: it is made industrially by direct chlorination of methane. The resulting mixture of chloromethanes is separated by fractional distillation.

Properties and uses: a colourless, sweet-smelling, volatile liquid (boiling point 313 K). Because of its low boiling point and low flammability, it is an effective extraction solvent in the pharmaceutical and food industries. It is also used as a paint remover and as a metal-cleaning solvent.

Health effects: dichloromethane harms the central nervous system. Low levels in air can slightly impair hearing and vision; higher levels cause dizziness, nausea and numbness in the fingers and toes. Direct contact burns the skin and can damage the eyes.

3. Trichloromethane (Chloroform)

3.1 Preparation

(a) From methane: chlorination in the presence of light or a catalyst gives a mixture, from which $CHC l_{3}$ is separated by fractional distillation.

C H_{4} + C l_{2} h ν C H_{3} Cl + HCl

C H_{3} Cl + C l_{2} h ν C H_{2} C l_{2} + HCl

C H_{2} C l_{2} + C l_{2} h ν CHC l_{3} + HCl

(b) From chloral: chloral is warmed with sodium hydroxide.

CC l_{3} CHO + NaOH warm CHC l_{3} + HCOONa

(c) Laboratory method: ethanol or acetone is heated with a paste of bleaching powder and water. Bleaching powder supplies chlorine (an oxidising and chlorinating agent) and calcium hydroxide (a base).

CaOC l_{2} + H_{2} O Ca (OH)_{2} + C l_{2}

C H_{3} C H_{2} OH + C l_{2} oxidation C H_{3} CHO + 2HCl

C H_{3} CHO + 3C l_{2} chlorination CC l_{3} CHO + 3HCl

2CC l_{3} CHO + Ca (OH)_{2} hydrolysis 2CHC l_{3} + (HCOO)_{2} Ca

With acetone, chlorination gives trichloroacetone, which is then split by calcium hydroxide:

C H_{3} COC H_{3} + 3C l_{2} CC l_{3} COC H_{3} + 3HCl

2CC l_{3} COC H_{3} + Ca (OH)_{2} 2CHC l_{3} + (C H_{3} COO)_{2} Ca

(d) From carbon tetrachloride: partial reduction with iron filings and water.

CC l_{4} + 2 [H] Fe / H_{2} O CHC l_{3} + HCl

3.2 Physical properties

Chloroform is a colourless, oily liquid with a sweetish, sickly odour and taste. It is heavier than water (boiling point 334 K) and does not mix with it.

3.3 Chemical properties

Figure 1: Reactions of chloroform at a glance. The carbylamine and Reimer-Tiemann reactions are the ones most often asked in exams.

1. Oxidation by air in sunlight: chloroform is slowly oxidised to phosgene (carbonyl chloride), which is extremely poisonous.

2CHC l_{3} + O_{2} sunlight 2COC l_{2} + 2HCl

2. Hydrolysis: boiling with aqueous KOH gives potassium formate, through the unstable intermediate $HC (OH)_{3}$ .

CHC l_{3} + 3KOH [HC (OH)_{3}] + 3KCl

[HC (OH)_{3}] - H_{2} O HCOOH + H_{2} O

HCOOH + KOH HCOOK + H_{2} O

3. Reduction: zinc and HCl reduce chloroform to methylene chloride.

Zn + 2HCl ZnC l_{2} + 2 [H]

CHC l_{3} + 2 [H] C H_{2} C l_{2} + HCl

4. Reaction with acetone: in the presence of a base such as KOH, chloroform adds to acetone to form chloretone (1,1,1-trichloro-2-methylpropan-2-ol), which is used as a sleep-inducing (hypnotic) drug.

(C H_{3})_{2} C = O + CHC l_{3} KOH (C H_{3})_{2} C (OH) CC l_{3}

5. Reaction with nitric acid: on heating with concentrated $HN O_{3}$ , chloroform gives chloropicrin (trichloronitromethane), used as an insecticide and formerly as a war gas.

CHC l_{3} + HN O_{3} Δ CC l_{3} N O_{2} + H_{2} O

6. Carbylamine reaction: when chloroform is warmed with a primary amine (aliphatic or aromatic) and alcoholic KOH, an isocyanide (carbylamine) with a very unpleasant smell forms. Secondary and tertiary amines do not respond, so this is a test for primary amines.

C_{6} H_{5} N H_{2} + CHC l_{3} + 3KOH warm C_{6} H_{5} NC + 3KCl + 3 H_{2} O

7. Reaction with silver powder: on heating, chloroform gives acetylene.

2CHC l_{3} + 6Ag Δ HC \equiv CH + 6AgCl

8. Chlorination: further chlorination gives carbon tetrachloride.

CHC l_{3} + C l_{2} h ν CC l_{4} + HCl

9. Reimer-Tiemann reaction: chloroform reacts with phenol and aqueous NaOH at 333-343 K, and acidification gives salicylaldehyde (2-hydroxybenzaldehyde). The CHO group enters mainly at the ortho position.

C_{6} H_{5} OH + CHC l_{3} + 3NaOH 333-343 K o - HO C_{6} H_{4} CHO + 3NaCl + 2 H_{2} O

Figure 2: The three Cl atoms make the H of chloroform acidic. Base removes it, and the

CC l_{3}^{-}

ion loses

C l^{-}

to give dichlorocarbene,

\ddot{C} C l_{2}

JEE Advanced

The carbylamine and Reimer-Tiemann reactions both go through dichlorocarbene, $\ddot{C} C l_{2}$ . The base removes the acidic H of chloroform to give $CC l_{3}^{-}$ , which loses $C l^{-}$ . The electron-deficient carbene is then attacked by the amine nitrogen or by the ortho carbon of the phenoxide ion.

3.4 Uses, storage and health effects

Uses: solvent for fats, oils, varnishes and resins; in medicine; as a preservative; as a laboratory reagent; and in making the refrigerant R-22 ( $CHCl F_{2}$ ).
Anaesthesia: it was once used as a general anaesthetic but has been replaced by safer anaesthetics such as ether.
Health: it depresses the central nervous system; long exposure can damage the liver and kidneys.
Storage: kept in closed, dark-coloured bottles filled to the brim, so that air and light cannot convert it to phosgene. About 1% ethanol is added, which converts any phosgene formed into harmless diethyl carbonate.

COC l_{2} + 2 C_{2} H_{5} OH (C_{2} H_{5} O)_{2} CO + 2HCl

Exam Trick

Dark, full, with a dash of alcohol. These three storage steps (dark bottle, filled to the brim, 1% ethanol) all exist to stop or remove phosgene.

4. Triiodomethane (Iodoform)

4.1 Preparation

Iodoform is prepared by heating ethanol or acetone with iodine and sodium hydroxide (or sodium carbonate) in water.

C H_{3} C H_{2} OH + 4 I_{2} + 6NaOH Δ CH I_{3} + HCOONa + 5NaI + 5 H_{2} O

C H_{3} COC H_{3} + 3 I_{2} + 4NaOH Δ CH I_{3} + C H_{3} COONa + 3NaI + 3 H_{2} O

4.2 Properties

Iodoform is a yellow crystalline solid (melting point about 392 K) with a characteristic smell. It is insoluble in water but dissolves readily in organic solvents.

Figure 3: Reactions of iodoform. Its antiseptic action comes from the iodine it releases.

CH I_{3} + HI red P C H_{2} I_{2} + I_{2}

C H_{3} N H_{2} + CH I_{3} + 3KOH C H_{3} NC + 3KI + 3 H_{2} O

2CH I_{3} + 6Ag Δ HC \equiv CH + 6AgI

4.3 The iodoform test

Compounds containing a $C H_{3} CO -$ group, or a $C H_{3} CH (OH) -$ group that can be oxidised to it, give a yellow precipitate of iodoform when warmed with iodine and NaOH.

Gives the iodoform test	Does not give the test
Ethanol, $C H_{3} C H_{2} OH$	Methanol, $C H_{3} OH$
Acetaldehyde, $C H_{3} CHO$	Propan-1-ol, $C H_{3} C H_{2} C H_{2} OH$
Acetone, $C H_{3} COC H_{3}$	Pentan-3-one, $C H_{3} C H_{2} COC H_{2} C H_{3}$
Propan-2-ol, $C H_{3} CH (OH) C H_{3}$	Formaldehyde, $HCHO$
Acetophenone, $C_{6} H_{5} COC H_{3}$	Benzaldehyde, $C_{6} H_{5} CHO$

Uses: iodoform was used as an antiseptic for dressing wounds, an action due to the free iodine it releases, not to iodoform itself. It has been replaced by other antiseptics because of its unpleasant smell. It is still used in making some pharmaceuticals.

5. Tetrachloromethane (Carbon Tetrachloride)

5.1 Preparation

(a) From methane: by chlorination in sunlight with excess chlorine.

C H_{4} + 4C l_{2} h ν CC l_{4} + 4HCl

(b) From carbon disulphide: by the action of chlorine in the presence of $AlC l_{3}$ .

C S_{2} + 3C l_{2} AlC l_{3} CC l_{4} + S_{2} C l_{2}

Sulphur monochloride, $S_{2} C l_{2}$ , is the by-product.

5.2 Properties

A colourless, oily liquid with a characteristic sickly smell (boiling point 350 K). It is heavier than water and insoluble in it, but soluble in organic solvents such as ether. It does not burn.

(i) Reduction: moist iron filings reduce it to chloroform.

CC l_{4} + 2 [H] Fe / H_{2} O CHC l_{3} + HCl

(ii) Hydrolysis: heating with alcoholic KOH gives the unstable $C (OH)_{4}$ , which loses water to form $C O_{2}$ ; this dissolves in the excess KOH.

CC l_{4} + 4KOH [C (OH)_{4}] + 4KCl

[C (OH)_{4}] C O_{2} + 2 H_{2} O

C O_{2} + 2KOH K_{2} C O_{3} + H_{2} O

(iii) Stability: it is stable at red heat, but its hot vapour reacts with water (steam) to give phosgene.

CC l_{4} + H_{2} O high temp. COC l_{2} + 2HCl

5.3 Uses and effects

Uses: industrial and laboratory solvent, manufacture of chloroform and of freon refrigerants and aerosol propellants, dry cleaning and spot removal, and formerly as a fire extinguisher under the trade name Pyrene.
Health: exposure can cause dizziness, nausea and liver damage, and is linked to liver cancer.
Environment: released $CC l_{4}$ rises to the upper atmosphere and depletes the ozone layer.

6. Freons (Chlorofluorocarbons)

The chlorofluorocarbons of methane and ethane are called freons. They are extremely stable, non-toxic, non-corrosive and easily liquefied, which made them ideal refrigerants. Freon-12, $CC l_{2} F_{2}$ , is one of the most common. It is made from tetrachloromethane by the Swarts reaction:

3CC l_{4} + 2Sb F_{3} SbC l_{5} (catalyst) 3CC l_{2} F_{2} + 2SbC l_{3}

Uses: refrigeration and air conditioning, and aerosol propellants.

Ozone depletion: because freons are so stable, they are not broken down in the lower atmosphere. They slowly reach the stratosphere, where ultraviolet light breaks the C-Cl bond and releases chlorine atoms. Each chlorine atom starts a chain reaction that destroys many ozone molecules:

CC l_{2} F_{2} UV \cdot CCl F_{2} + Cl \cdot

Cl \cdot + O_{3} ClO \cdot + O_{2}

ClO \cdot + O Cl \cdot + O_{2}

7. Other Polyhalogen Compounds

7.1 p-Dichlorobenzene

It is prepared by chlorination of benzene. It is a white, volatile solid (melting point 325 K) that sublimes readily. It is used as an insecticide, germicide and soil fumigant, and as a deodorant and moth repellent.

7.2 Perfluorocarbons (PFCs)

They are prepared by controlled fluorination of alkanes in the vapour phase, with the mixture diluted by nitrogen to keep the violent reaction in check.

C_{7} H_{16} + 16 F_{2} vapour phase, N_{2} 573 K, Co F_{3} C_{7} F_{16} + 16HF

PFCs are non-toxic, non-flammable, non-corrosive and extremely stable. They are used as lubricants, for surface coatings and as electrical insulators. Teflon, the non-stick polymer $(- C F_{2} - C F_{2} -)_{n}$ , is a well-known perfluorinated material.

7.3 Benzene hexachloride (BHC)

BHC (1,2,3,4,5,6-hexachlorocyclohexane) is made commercially by adding chlorine to benzene in ultraviolet light. Its $γ$ -isomer, known as lindane or gammaxene, is the active insecticide, used as a pesticide in agriculture.

C_{6} H_{6} + 3C l_{2} UV light C_{6} H_{6} C l_{6}

7.4 DDT

DDT (p,p'-dichlorodiphenyltrichloroethane) is manufactured by condensing chlorobenzene with chloral (trichloroacetaldehyde) in the presence of concentrated sulphuric acid.

2 C_{6} H_{5} Cl + CC l_{3} CHO conc. H_{2} S O_{4} (Cl C_{6} H_{4})_{2} CH - CC l_{3} + H_{2} O

Figure 4: DDT is made by condensing chlorobenzene with chloral. The central carbon joins the para positions of two chlorobenzene rings.

Properties: a white powder, insoluble in water but soluble in oils and fats.
History: DDT was the first chlorinated organic insecticide. Paul Müller discovered its insecticidal power in 1939 and received the 1948 Nobel Prize in Physiology or Medicine. It was widely used against the mosquitoes that spread malaria and the lice that carry typhus.
Problems: many insects became resistant; it is highly toxic to fish; and it is chemically very stable, so animals cannot break it down quickly. Being fat-soluble, it is stored in fatty tissue and becomes more concentrated at each step of the food chain.
Status: banned in the United States in 1973 and restricted in many countries, though still used in some parts of the world.

Exam Trick

DDT = "Don't Degrade, Travel up". It does not break down, and it travels up the food chain (biomagnification). That is the reason it was banned.

8. Solved Examples

Solved Example 1

Which compound, when heated with KOH and a primary amine, gives the carbylamine test?
(A)

CHC l_{3}

(B)

C H_{3} Cl

(C)

C H_{3} OH

(D)

C H_{3} CN

Solution:

Answer: (A). Only chloroform can lose HCl to give dichlorocarbene, which converts the primary amine into a foul-smelling isocyanide.

Solved Example 2

When chloroform reacts with acetone, the product is:
(A) ethylidene dichloride
(B) mesitylene
(C) chloretone
(D) chloral

Solution:

Answer: (C). In the presence of KOH, chloroform adds across the C=O of acetone:

(C H_{3})_{2} C = O + CHC l_{3} KOH (C H_{3})_{2} C (OH) CC l_{3}

The product, chloretone, is a hypnotic drug.

Solved Example 3

The H atom of chloroform is acidic. Why?

Solution:

The three chlorine atoms pull electron density away from carbon by their $- I$ effect. Carbon in turn pulls the C-H bonding electrons towards itself, leaving the hydrogen partly positive and easy to remove as $H^{+}$ . The anion formed, $CC l_{3}^{-}$ , is also stabilised by the same $- I$ effect of the three chlorine atoms (Figure 2).

Solved Example 4

Chloroform is stored in dark-coloured bottles. Why?

Solution:

In sunlight, air oxidises chloroform to phosgene, $COC l_{2}$ , which is highly poisonous. Dark bottles keep out light, and filling them to the brim keeps out air. A little ethanol is also added to destroy any phosgene that forms.

Solved Example 5

What does DDT stand for? What is its IUPAC name?

Solution:

DDT stands for p,p'-dichlorodiphenyltrichloroethane. Its IUPAC name is 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane.

Solved Example 6

Which of these give the iodoform test: ethanol, methanol, propan-2-ol, propan-1-ol, acetone, pentan-3-one?

Solution:

Ethanol, propan-2-ol and acetone. Ethanol and propan-2-ol have a $C H_{3} CH (OH) -$ group, which iodine oxidises to $C H_{3} CO -$ ; acetone already has $C H_{3} CO -$ . Methanol, propan-1-ol and pentan-3-one have neither group.

Solved Example 7

How can chloroform be distinguished from iodoform?

Solution:

Warm each with aqueous $AgN O_{3}$ . Iodoform gives a yellow precipitate of AgI, because its C-I bonds are weak. Chloroform gives no precipitate. Physically, iodoform is a yellow solid, while chloroform is a colourless liquid.

Solved Example 8

Complete:

CC l_{4}

Sb F_{3}

gives A. Why is A harmful to the ozone layer?

Solution:

3CC l_{4} + 2Sb F_{3} 3CC l_{2} F_{2} + 2SbC l_{3}

A is Freon-12, $CC l_{2} F_{2}$ . It is so stable that it reaches the stratosphere unchanged. There, UV light breaks the C-Cl bond, and each chlorine atom destroys many ozone molecules in a chain reaction.

Solved Example 9

Write the equation for the carbylamine reaction of ethylamine. Why can this reaction distinguish primary amines from secondary amines?

Solution:

C_{2} H_{5} N H_{2} + CHC l_{3} + 3KOH warm C_{2} H_{5} NC + 3KCl + 3 H_{2} O

The isocyanide carbon bonds to a nitrogen that has lost both of its hydrogen atoms, so the amine must be primary ( $R - N H_{2}$ ). Secondary and tertiary amines do not form isocyanides and give no foul smell.

Practice Questions

Why is the hydrogen in $CHC l_{3}$ slightly acidic?Answer: the $- I$ effect of three Cl atoms makes the C-H bond polar and stabilises the $CC l_{3}^{-}$ ion formed when H leaves.
Why can Pyrene ( $CC l_{4}$ ) be used as a fire extinguisher?Answer: $CC l_{4}$ does not burn, and its dense vapour forms a blanket over the fire that cuts off oxygen. It is no longer recommended because hot $CC l_{4}$ can form poisonous phosgene.

Common Mistakes to Avoid

Watch out

Applying the carbylamine test to secondary or tertiary amines. Only primary amines give isocyanides.
Writing methanol as the hydrolysis product of chloroform. Aqueous KOH gives potassium formate.
Thinking the ethanol added to chloroform is a diluent. It converts phosgene into harmless diethyl carbonate.
Expecting methanol or propan-1-ol to give the iodoform test. They lack the $C H_{3} CH (OH) -$ group.
Calling the whole of BHC "lindane". Lindane is only its $γ$ -isomer, and BHC is a cyclohexane, not an aromatic compound.
Giving para-hydroxybenzaldehyde as the main Reimer-Tiemann product. The main product is the ortho isomer, salicylaldehyde.
Attributing iodoform's antiseptic action to iodoform itself. It is due to the iodine it releases.

Frequently Asked Questions

What are polyhalogen compounds?

Polyhalogen compounds are carbon compounds that contain more than one halogen atom. Important examples are dichloromethane, chloroform, iodoform, carbon tetrachloride, freons such as $CC l_{2} F_{2}$ , and DDT. They are used as solvents, refrigerants, antiseptics and insecticides, but many are toxic or harm the environment.

Why is chloroform stored in dark coloured bottles?

In sunlight, air slowly oxidises chloroform to phosgene, $COC l_{2}$ , which is extremely poisonous. Chloroform is therefore kept in dark bottles filled to the brim to exclude light and air, and about 1 percent ethanol is added to convert any phosgene formed into harmless diethyl carbonate.

What is the carbylamine reaction?

In the carbylamine reaction, a primary amine is warmed with chloroform and alcoholic KOH to form an isocyanide, which has a very offensive smell. For example, aniline gives phenyl isocyanide. Secondary and tertiary amines do not react this way, so it is used as a test for primary amines.

What is the iodoform test?

The iodoform test detects compounds with a $C H_{3} CO -$ group or a $C H_{3} CH (OH) -$ group. When such a compound is warmed with iodine and sodium hydroxide, a yellow precipitate of iodoform, $CH I_{3}$ , forms. Ethanol, acetaldehyde, acetone and propan-2-ol give the test; methanol and propan-1-ol do not.

Why is the hydrogen of chloroform acidic?

The three chlorine atoms withdraw electrons by their inductive effect, making the C-H bond polar and leaving hydrogen slightly positive. The trichloromethyl anion formed when the proton leaves is also stabilised by the same effect. This acidity lets bases convert chloroform into dichlorocarbene.

How do freons damage the ozone layer?

Freons are very stable, so they drift unchanged into the stratosphere. There, ultraviolet light breaks their C-Cl bonds and releases chlorine atoms. Each chlorine atom reacts with ozone to form ClO and oxygen, and is then regenerated, so a single chlorine atom can destroy thousands of ozone molecules.

Which polyhalogen compounds are important for NEET?

For NEET, learn the uses and harmful effects of dichloromethane, chloroform, iodoform, carbon tetrachloride, freons and DDT, as given in NCERT. Questions often ask why chloroform is stored in dark bottles, how freons deplete ozone, why DDT was banned, and which compounds give the iodoform test.

What reactions of chloroform are important for JEE Main?

JEE Main commonly tests the carbylamine reaction, the Reimer-Tiemann reaction, oxidation to phosgene, hydrolysis to formate, and reaction with silver powder to give acetylene. Knowing that the carbylamine and Reimer-Tiemann reactions both go through dichlorocarbene helps with mechanism-based questions.

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