Industrial, Pharmaceutical & Environmental Applications

Application	Compound	Mechanism/Relevance	NEET Context
Refrigeration (CFCs)	$CCl_{2}F_{2}$ (Freon-12)	Chemically inert, low boiling point → ideal refrigerant; BUT cleaves C-Cl in stratosphere	Ozone depletion, Montreal Protocol
Pharmaceutical solvents	$CHCl_{3}$ (chloroform)	Polar, non-flammable → used as lab solvent; must store in dark (→ phosgene)	Phosgene formation hazard
Anesthesia (historical)	$CHCl_{3}$	Used as anesthetic before toxicity known	Safety chemistry
Pesticide (banned)	DDT	C-Cl bonds resistant to biodegradation → persistent in environment	Biomagnification, food chain
Phenol production	Dow process ( $C_{6}H_{5}Cl$ + NaOH → $C_{6}H_{5}OH$ )	Industrial nucleophilic aromatic substitution	623 K, 300 atm conditions
Teflon feedstock	$CF_{4}$ , PTFE polymers	C-F bonds extremely strong → non-stick, heat resistant	C-F highest bond energy
Refrigerant replacement	HFCs (no Cl)	Replace CFCs; no Cl→ no ozone depletion	Post-Montreal Protocol

Drug/Compound	Halogen Role	Mechanism
Chloroquine (antimalarial)	C-Cl bond in aromatic ring	Halogen improves lipophilicity for cell membrane penetration
Halothane (anesthetic)	$CF_{3}$ -CHBrCl	C-F bonds for stability; C-Br for metabolism
Many drugs	F substitution	C-F bonds resist metabolic oxidation → longer half-life

Problem	Compound	Chemistry	Solution
Ozone depletion	CFCs ( $CCl_{2}F_{2}$ , $CCl_{3}F$ )	UV cleaves C-Cl → Cl• → $O_{3}$ catalytic destruction	Montreal Protocol (1987): phase out CFCs
Persistent organic pollutants	DDT, PCBs	Highly chlorinated → non-biodegradable → bioaccumulation	Ban/restrict use; bioremediation research
Phosgene formation	$CHCl_{3}$ oxidation	$CHCl_{3}$ + $O_{2}$ → $COCl_{2}$ (toxic war gas)	Store in dark, add ethanol stabilizer
Groundwater contamination	Chlorinated solvents ( $CCl_{4}$ , $CHCl_{3}$ )	Dense non-aqueous phase liquids (DNAPLs) — sink in groundwater	Pump-and-treat, in situ bioremediation