MHT-CET Chemistry · Halogen Derivatives of Alkanes

Polyhalogen Compounds: Freons, DDT, BHC and War Gases

A handful of named polyhalogen compounds recur: Freon-12 (CCl₂F₂) the refrigerant, DDT and BHC the insecticides, and the war gases — phosgene COCl₂, mustard gas (ClCH₂CH₂)₂S, tear gas CCl₃NO₂ — each asked by name, formula or atom count.

All Halogen Derivatives of Alkanes notes MHT-CET Chemistry strategy

Why this matters

19 PYQs, none HARD — pure recall. Of the eleven set from 2021 to 2025, two ask Freon-12 by name, two ask DDT's structure from four drawings, one asks its replacement (BHC), one asks the gas chloroform gives on oxidation (phosgene), and five are the war gases — which is tear gas, the formula of mustard gas, how many Cl or N or S atoms in n moles, which has the most chlorines. Two tables.

Concept 1 of 2: Freons, DDT and BHC

Freons are chlorofluoromethanes and -ethanes: stable, non-toxic, easily liquefied — ideal refrigerants until their ozone damage phased them out. Freon-12 is dichlorodifluoromethane. DDT is 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane: a CHCCl₃ carbon carrying two p-chlorophenyl rings. BHC (benzene hexachloride, lindane/gammexane) replaced it as an insecticide.

Definition

Freon-12: $\text{CCl}_2\text{F}_2$ , dichlorodifluoromethane, made from $\text{CCl}_4$ with SbF₃/SbCl₅ (Swarts). Refrigerant-22 $\text{CHClF}_2$ from chloroform.
DDT: two p-chlorophenyl rings on one carbon that also carries a $\text{CCl}_3$ — $(p\text{-ClC}_6\text{H}_4)_2\text{CH-CCl}_3$ ; made from chlorobenzene and chloral with H₂SO₄. Non-biodegradable, banned.
BHC: $\text{C}_6\text{H}_6\text{Cl}_6$ , benzene + Cl₂ in sunlight; used instead of DDT.
Chloroform (anaesthetic, forms phosgene in light; stored in dark bottles), iodoform (antiseptic), carbon tetrachloride (fire extinguisher, solvent).

Compound	Formula	Use
Freon-12	$\text{CCl}_2\text{F}_2$	Refrigerant, aerosol propellant Dichlorodifluoromethane — two Cl, two F.
Refrigerant-22	$\text{CHClF}_2$	Refrigerant; made from chloroform
DDT	$(p\text{-ClC}_6\text{H}_4)_2\text{CH-CCl}_3$	Insecticide (banned) Both rings carry a PARA chlorine; the side carbon carries CCl₃, not CCl₂.
BHC (lindane)	$\text{C}_6\text{H}_6\text{Cl}_6$	Insecticide used in place of DDT
Chloroform	$\text{CHCl}_3$	Solvent; once an anaesthetic
Iodoform	$\text{CHI}_3$	Antiseptic

Freon-12 is the two-and-two halomethane; DDT is two para-chlorophenyls plus a trichloromethyl on one carbon.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 2 · Q100Easy

Example 1 · Halogen Derivatives of Alkanes · Polyhalogen Compounds — Freon, DDT, War Gases

Which is the most commonly used refrigerant Freon-12?

Picking the DDT drawing without the ring chlorines

1,1,1-Trichloro-2,2-diphenylethane is offered every time. DDT's rings each carry a chlorine at the para position; the trichloromethyl alone is not enough.

Drill 5 more on freons, ddt and bhc

Concept 2 of 2: Phosgene, Mustard Gas and Tear Gas

Three named agents with three formulas. Phosgene COCl₂ (a choking gas, formed from chloroform in light). Mustard gas (ClCH₂CH₂)₂S, a blister agent with one sulphur and two chlorines. Tear gas chloropicrin CCl₃NO₂, three chlorines and one nitrogen — the most chlorine-rich of the three. The paper asks atom counts per n moles.

Definition

Phosgene $\text{COCl}_2$ : 2 Cl. Carbonyl chloride, choking agent.
Mustard gas $\text{Cl-CH}_2\text{CH}_2\text{-S-CH}_2\text{CH}_2\text{-Cl}$ : 2 Cl, 1 S — n mol carries n mol S. Blister agent.
Tear gas (chloropicrin) $\text{CCl}_3\text{NO}_2$ : 3 Cl, 1 N — n mol carries 3n mol Cl and n mol N. Lachrymator. The compound with the MOST chlorine atoms per molecule among the three.
Phosphine PH₃ is not a halogen compound at all — a planted option.

Agent	Formula	Cl per molecule	Type
Phosgene	$\text{COCl}_2$	2	Choking
Mustard gas	$(\text{ClCH}_2\text{CH}_2)_2\text{S}$	2	Blister (vesicant) One S per molecule.
Tear gas (chloropicrin)	$\text{CCl}_3\text{NO}_2$	3	Lachrymator Highest chlorine count of the three — the official key on the 2022 paper.

Tear gas has three chlorines; the other two have two each.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 1 · Q69Easy

Example 2 · Halogen Derivatives of Alkanes · Polyhalogen Compounds — Freon, DDT, War Gases

What is the number of moles of Cl atoms and N atoms respectively present in n moles of tear gas?

Giving mustard gas the most chlorine

Mustard gas has two chlorines, one at each end. Chloropicrin has three on one carbon. The 2022 paper keyed tear gas.

Drill 4 more on phosgene, mustard gas and tear gas

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Freons, DDT and BHC6 rows

Compound	Formula	Use
Freon-12	$\text{CCl}_2\text{F}_2$	Refrigerant, aerosol propellant Dichlorodifluoromethane — two Cl, two F.
Refrigerant-22	$\text{CHClF}_2$	Refrigerant; made from chloroform
DDT	$(p\text{-ClC}_6\text{H}_4)_2\text{CH-CCl}_3$	Insecticide (banned) Both rings carry a PARA chlorine; the side carbon carries CCl₃, not CCl₂.
BHC (lindane)	$\text{C}_6\text{H}_6\text{Cl}_6$	Insecticide used in place of DDT
Chloroform	$\text{CHCl}_3$	Solvent; once an anaesthetic
Iodoform	$\text{CHI}_3$	Antiseptic

Freon-12 is the two-and-two halomethane; DDT is two para-chlorophenyls plus a trichloromethyl on one carbon.

Phosgene, Mustard Gas and Tear Gas3 rows

Agent	Formula	Cl per molecule	Type
Phosgene	$\text{COCl}_2$	2	Choking
Mustard gas	$(\text{ClCH}_2\text{CH}_2)_2\text{S}$	2	Blister (vesicant) One S per molecule.
Tear gas (chloropicrin)	$\text{CCl}_3\text{NO}_2$	3	Lachrymator Highest chlorine count of the three — the official key on the 2022 paper.

Tear gas has three chlorines; the other two have two each.

Watch out for (2)

Picking the DDT drawing without the ring chlorines
→ Freons, DDT and BHC
Giving mustard gas the most chlorine
→ Phosgene, Mustard Gas and Tear Gas

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