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

12 PYQs, none HARD — pure recall. 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 six are the war gases — which is tear gas (twice), 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

Intuition

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.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Halogen Derivatives of AlkanesEASY

Which is the most commonly used refrigerant Freon-12?

[Q91 · 9th May Shift 2 · 2023]

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

Intuition

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.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Halogen Derivatives of AlkanesEASY

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

[Q69 · 9th May Shift 1 · 2023]

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 5 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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