Class 12 Chemistry · Chapter 6
Short answer:
Ye Chemistry Part II ka pehla organic chemistry chapter hai. NCERT textbook me isme 33 exercise questions hain — IUPAC naming (dono directions me), preparation reactions (alcohol se halide, Finkelstein, Swarts), physical properties (boiling point trend, water me insoluble kyun), SN1 aur SN2 mechanism ka detailed comparison with stereochemistry, elimination vs substitution ki competition, haloarenes ki electrophilic substitution aur unki kam reactivity ka reason, aur kuch important compounds (DDT, freons, iodoform) ka brief coverage. Neeche is guide me 18 exercise-style questions solve kiye gaye hain jo poore chapter ko cover karte hain.
Haloalkanes aur haloarenes wo organic compounds hain jinme hydrocarbon ke H atom ko halogen (F, Cl, Br, I) replace karta hai. Ye chapter organic chemistry ki asli shuruat hai — yahan se reaction mechanism (SN1, SN2, elimination) padhna start hota hai jo aage alcohols, ethers, aur biomolecules tak use hoga. C-X bond polar hota hai (halogen zyada electronegative), isliye ye compounds substitution aur elimination dono reactions dete hain. Halogen ki electronegativity aur atomic size dono is chapter ki har trend (bond strength, reactivity, boiling point) explain karte hain.
Chapter 6 Summary — 5 Minute Revision
1. Classification
- Number of halogen atoms: Mono-, di-, tri-halo compounds (e.g. CH3Cl, CH2Cl2, CHCl3).
- Hybridisation of C-X carbon:
- sp3 C-X — alkyl halides (CH3CH2Cl)
- sp2 C-X, allylic — C=C-C-X (CH2=CH-CH2Cl)
- sp2 C-X, vinylic — C=C-X (CH2=CHCl) — bahut unreactive
- sp2 C-X, aryl — C6H5Cl — bahut unreactive
- Alkyl halides: primary (1°), secondary (2°), tertiary (3°) — depending on carbon jispe X attached hai, kitne alkyl groups se juda hai.
2. IUPAC Nomenclature
Halogen ko substituent (halo-) treat karte hain — parent chain longest continuous chain jisme functional group attached carbon include ho. Numbering aisi ki substituents (halogen + alkyl) ko collectively lowest locants milein. Multiple substituents alphabetical order me likhte hain (bromo pehle, chloro baad me, etc — 'b' before 'c' before 'e' before 'i').
Example: CH3-CHBr-CH2-CH3 → 2-bromobutane. C6H5Cl → chlorobenzene.
3. Nature of C-X Bond
Halogen carbon se zyada electronegative hai, isliye C-X bond polarised hota hai — carbon par δ+ aur halogen par δ- charge. Ye polarity hi nucleophilic substitution reactions ka basis hai (nucleophile δ+ carbon par attack karta hai). Bond length order: C-F < C-Cl < C-Br < C-I (atomic size badhne se bond length badhti hai), aur bond enthalpy order ulta hai: C-F > C-Cl > C-Br > C-I.
4. Methods of Preparation
- From alcohols: ROH + HX (ZnCl2 catalyst for HCl, Lucas reagent) → RX; ya SOCl2 (Darzens method, best — byproducts gaseous, halide pure); ya PX3/PX5.
- From hydrocarbons: Free radical halogenation of alkanes (mixture deta hai, less useful); electrophilic substitution on arenes with X2/Lewis acid (FeCl3) deta hai haloarenes.
- From alkenes: Addition of HX (Markovnikov) ya X2.
- Halogen exchange: Finkelstein reaction — RCl/RBr + NaI (dry acetone) → RI (NaCl/NaBr precipitate ho jaata hai, reaction aage badhti hai). Swarts reaction — RCl/RBr + AgF/Hg2F2/CoF2/SbF3 → RF.
5. Physical Properties
- Haloalkanes polar hote hain lekin water ke saath H-bond nahi bana sakte — isliye water me insoluble hain (thoda soluble hote hain kyunki weak dipole-induced dipole interaction bansakti hai, lekin water-water H-bonds todne jitni energy release nahi hoti).
- Boiling point order (same alkyl group): RI > RBr > RCl > RF — halogen ka size badhne se molecular mass aur van der Waals (London dispersion) forces badhte hain.
- Same halogen ke liye, isomers me: n-alkyl halide > sec > tert (branching se surface area/contact ghatta hai, dispersion forces kam hote hain).
- Density: haloalkanes generally water se dense hote hain; CH3I se CI4 tak density halogen number aur size ke saath badhti hai.
6. Chemical Reactions — Nucleophilic Substitution
SN2 (bimolecular): Ek hi step me nucleophile leaving group ke opposite side se backside attack karta hai. Rate depends on BOTH substrate and nucleophile concentration: Rate = k[RX][Nu⁻]. Transition state me carbon 5-coordinate (trigonal bipyramidal) hota hai. Steric hindrance zyada hone se rate ghat jaati hai, isliye reactivity order: methyl > 1° > 2° > 3° (3° halides SN2 practically nahi karte). Stereochemistry: complete inversion of configuration (Walden inversion) — jaise umbrella wind me palatti hai.
SN1 (unimolecular): Do steps — pehle slow step me C-X bond heterolytically break hokar carbocation banta hai, phir fast step me nucleophile attack karta hai. Rate sirf substrate concentration pe depend karti hai: Rate = k[RX]. Carbocation stability order 3° > 2° > 1° ke hisaab se reactivity: 3° > 2° > 1°. Stereochemistry: carbocation planar (sp2) hota hai, nucleophile dono taraf se attack kar sakta hai — isliye racemisation hoti hai (partial ya complete), pure inversion nahi.
7. Elimination Reactions
Alcoholic KOH ke saath haloalkane β-hydrogen elimination karke alkene banata hai (dehydrohalogenation). Saytzeff's rule: zyada substituted (more stable) alkene major product hota hai. Substitution (aqueous KOH/nucleophile) vs elimination (alcoholic KOH, strong base, heat) — dono competing reactions hain; bulky/strong base aur high temperature elimination favour karte hain.
8. Reactions of Haloarenes
- Nucleophilic substitution bahut mushkil hai kyunki: (i) resonance — lone pair on halogen delocalise hoke C-X bond ko partial double bond character deta hai, bond strong ho jaata hai; (ii) sp2 carbon zyada electronegative hai, C-X bond shorter/stronger; (iii) phenyl cation banna bahut unstable, SN1 bhi nahi hota; (iv) electron-rich benzene ring nucleophile ko repel karta hai.
- Electrophilic substitution: Halogen o,p-directing hai (resonance se ortho/para positions par electron density badhti hai) lekin net deactivating hai (inductive effect electron density kam karta hai overall) — isliye ring EAS me kam reactive hai halobenzene benzene se, but product mostly ortho/para hi milta hai.
- Wurtz-Fittig aur Fittig reaction: Aryl halide + alkyl halide + Na (dry ether) → alkylbenzene.
9. Some Important Compounds (brief)
| Compound | Use / note |
|---|---|
| DDT (dichlorodiphenyltrichloroethane) | Powerful insecticide — pesticide/environmental persistence ki wajah se ab restricted/banned kai countries me |
| Freons (CFCs) | Refrigerants, aerosol propellants — ozone layer depletion me role, ab phase-out ho rahe |
| Iodoform (CHI3) | Antiseptic (historical use) |
| Chloroform (CHCl3) | Solvent, historically anaesthetic |
↔ Table ko side me swipe karein
Poore Class 12 Chemistry ke handwritten colour notes
IITian & district toppers ke banaye short notes — revision-ready, diagram ke saath. Board se pehle poora syllabus 3 din me revise.
Exercise Questions — Solutions (Q1–Q18)
Q1. Write IUPAC names of: (i) CH3CH(Cl)CH2CH3 (ii) CH2=CHCH2Br
(i) Longest chain 4 carbon, Cl attached carbon-2 (dono end se number karke lowest locant 2 milta hai) → 2-chlorobutane. (ii) Double bond aur Br dono lowest locants ke liye number karo — double bond C1-C2, Br C3 par → 3-bromoprop-1-ene (allyl bromide).
CH3-CHCl-CH2-CH3 → 2-chlorobutane; CH2=CH-CH2Br → 3-bromoprop-1-ene
Q2. IUPAC naming me numbering rule kya hai jab multiple substituents ho? Ek example do jaha galat numbering se galat naam banta hai.
Rule: parent chain ko is tarah number karo ki saare substituents (halogen + alkyl groups) collectively lowest set of locants paayein — individual sabse chhote substituent ko priority nahi, poore set ko dekhna hai. Example: CH3-CH2-CHCl-CH2-CH3 ko left se number karo to Cl C3 par (locant set {3}), right se bhi C3 — dono equal, to yahan koi confusion nahi. Lekin CH3-CHBr-CH2-CH2-CH3 ko agar right se number kiya (galat) to Br C4 milega — jo galat hai; sahi left se numbering se Br C2 par milta hai (locant set {2} vs {4}, 2 chhota hai) → sahi naam 2-bromopentane, galat naam '4-bromopentane' nahi likhna chahiye.
Q3. Alcohol se haloalkane banane ke teen methods likho — kaunsa sabse pure product deta hai aur kyun?
(1) ROH + HX (ZnCl2, Lucas reagent) → RX + H2O. (2) ROH + PCl5/PCl3 → RCl. (3) ROH + SOCl2 (pyridine ke saath, Darzens process) → RCl + SO2↑ + HCl↑.
SOCl2 wala method (3) sabse pure alkyl halide deta hai kyunki dono byproducts (SO2 aur HCl) gases hain jo apne aap nikal jaate hain — koi solid/liquid impurity mixture me nahi bachti, alag se purification ki zaroorat nahi.
ROH + SOCl2 --pyridine--> RCl + SO2 + HCl
Q4. Finkelstein reaction likho — ye reaction aage kyun badhti hai?
RCl/RBr + NaI (dry acetone me) → RI + NaCl↓/NaBr↓
R-Cl + NaI --dry acetone--> R-I + NaCl (ppt)
NaCl aur NaBr dry acetone me insoluble hain aur precipitate ban kar reaction mixture se bahar nikal jaate hain — Le Chatelier's principle ke hisaab se equilibrium product side (RI) ki taraf shift hota hai, isliye reaction complete ho jaati hai.
Q5. Swarts reaction kya hai?
Alkyl chloride/bromide ko AgF, Hg2F2, CoF2, ya SbF3 jaise metallic fluoride ke saath garam karne se alkyl fluoride milta hai — ye alkyl fluoride banane ka main method hai.
R-Br + AgF --heat--> R-F + AgBr
Q6. Haloalkanes water me kyun insoluble hote hain jabki wo polar molecules hain?
Haloalkane dissolve hone ke liye pehle water molecules ke beech ke H-bonds todne padte hain (energy chahiye), phir haloalkane aur water ke beech naye interactions (dipole-induced dipole, kyunki haloalkane khud H-bond donor/acceptor nahi hai proper tarah se) bante hain jo utni energy release nahi karte jitni H-bonds todne me lagi thi. Net energy balance unfavourable hone se haloalkanes water me practically insoluble rehte hain, halanki organic solvents me achhe se ghulte hain.
Q7. Boiling point order samjhao: CH3F, CH3Cl, CH3Br, CH3I
Order: CH3I > CH3Br > CH3Cl > CH3F
Halogen ka size (aur molecular mass) F<Cl<Br<I ke hisaab se badhta hai. Bade, zyada polarizable electron cloud wale halogen atoms me van der Waals (London dispersion) forces zyada strong hote hain, isliye boiling point bhi badhta jaata hai — inductive/dipole effect ka role secondary hai yahan, dispersion forces dominant hain.
Q8. n-propyl bromide, isopropyl bromide, tert-butyl bromide me boiling point order kya hoga aur kyun?
n-propyl bromide (n-C3H7Br) sabse zyada boiling point rakhta hai isomers me compare karein to: normal chain > secondary > tertiary. Straight chain isomer ki surface area zyada hoti hai jisse molecules ke beech contact aur van der Waals forces zyada strong hote hain. Branching (tertiary) se molecule compact/spherical ho jaata hai, surface contact kam hota hai, isliye boiling point ghat jaata hai.
Q9. SN1 aur SN2 mechanism me farak samjhao — rate law, steps, aur intermediate ke aadhar par.
SN2: single step, concerted — nucleophile aur leaving group dono transition state me involved. Rate = k[RX][Nu⁻] (second order). Koi stable intermediate nahi banta, sirf transition state.
SN1: do steps — (1) slow, rate-determining: C-X bond heterolysis se carbocation intermediate banta hai; (2) fast: nucleophile carbocation par attack karta hai. Rate = k[RX] (first order), nucleophile concentration rate ko affect nahi karti kyunki wo rate-determining step me involved nahi hai.
SN2: Nu- + R-X -> [Nu...R...X]‡ -> Nu-R + X- | SN1: R-X -> R+ + X- (slow), R+ + Nu- -> R-Nu (fast)
Q10. SN1 aur SN2 me substrate preference kya hai (1°/2°/3°) aur kyun?
SN2: methyl > 1° > 2° > 3° halide — kyunki nucleophile ko backside se attack karne ke liye carbon ke paas jagah chahiye. Bulky alkyl groups (2°, 3°) steric hindrance create karte hain jo backside attack block karte hain, isliye 3° halides SN2 nahi karte.
SN1: 3° > 2° > 1° — kyunki rate-determining step me carbocation banta hai, aur carbocation stability order 3° > 2° > 1° (hyperconjugation + inductive electron donation se) hai. Zyada stable carbocation asani se banta hai, isliye 3° halides SN1 favour karte hain.
Q11. SN2 reaction me stereochemistry par kya effect hota hai? Example do.
SN2 me nucleophile leaving group ke exactly opposite side se backside attack karta hai — jaise umbrella tez hawa me ulti palat jaati hai. Isse carbon ka configuration completely invert ho jaata hai (Walden inversion). Agar starting material (R)-configuration me tha to product (S)-configuration me milega (agar priority order same rahe).
Nu- attacks from side opposite to leaving group X -> complete inversion of configuration (Walden inversion)
Q12. SN1 reaction me stereochemistry kaisi hoti hai aur kyun?
SN1 ke intermediate carbocation me central carbon sp2 hybridised aur planar hota hai. Nucleophile is planar carbocation par dono taraf se (front aur back, roughly equal probability) attack kar sakta hai. Isse product ka mixture banta hai — kuch inversion wala, kuch retention wala — jise racemisation kehte hain (agar optically active substrate use ho to product largely racemic mixture, poori tarah optically inactive, milta hai — chhoti si net inversion practically observe ho sakti hai kyunki leaving group thoda shield karta hai front attack).
Q13. Elimination aur substitution reaction me competition kaise decide hoti hai? Saytzeff rule kya hai?
Alcoholic KOH (strong base, high temp) elimination favour karta hai — β-hydrogen remove hoke alkene banta hai (dehydrohalogenation). Aqueous KOH (weaker base, nucleophile) substitution favour karta hai — alcohol banta hai. Bulky base (jaise potassium tert-butoxide) elimination ko aur favour karta hai kyunki wo substitution ke liye steric hindrance ki wajah se attack nahi kar paata.
Saytzeff's rule: Jab elimination se multiple alkenes ban sakte hain, to zyada substituted (zyada stable) alkene major product hota hai.
CH3-CH2-CHBr-CH3 + alc. KOH --> CH3-CH=CH-CH3 (major, more substituted) + CH3-CH2-CH=CH2 (minor)
Q14. Haloarenes me nucleophilic substitution reaction alkyl halides ke comparison me kyun difficult hai? Teen reasons do.
(1) Resonance: Halogen ka ek lone pair benzene ring ke π system ke saath delocalise hota hai, isse C-X bond ko partial double bond character milta hai — bond strong aur short ho jaata hai, todna mushkil.
(2) Carbon hybridisation: Aryl carbon sp2 hai jo sp3 se zyada electronegative hota hai — electrons ko zyada tightly hold karta hai, C-X bond ko strong banata hai.
(3) Instability of phenyl cation: Agar SN1 path try kare to phenyl cation banega jo empty sp2 orbital me hota hai (ring ke plane me, π system se conjugated nahi) — bahut unstable, isliye SN1 bhi possible nahi.
(4) Electron-rich aromatic ring negatively charged nucleophile ko repel karta hai.
Q15. Chlorobenzene me electrophilic substitution reaction ortho/para directing hoti hai lekin ring deactivate bhi karti hai — ye contradictory kaise nahi hai?
Halogen ke do opposing effects hote hain: resonance effect (+R) — lone pair donation se ortho/para positions par electron density badhti hai, isliye wahi positions substitution ke liye activate hoti hain (directing effect). Inductive effect (-I) — halogen bahut electronegative hai, poore ring se electron density kheenchta hai, jisse overall ring benzene se kam reactive ho jaati hai (deactivating).
-I effect > +R effect in magnitude (net), isliye ring overall deactivate hoti hai, lekin jahan bhi substitution ho, wo mostly ortho/para positions par hi hoti hai kyunki wahan resonance se relatively zyada electron density bachti hai.
Q16. Wurtz-Fittig aur Fittig reaction me farak batao.
Wurtz-Fittig reaction: Aryl halide + alkyl halide + Na (dry ether) → alkylbenzene (mixed coupling).
C6H5Br + CH3CH2Br + 2Na --dry ether--> C6H5-CH2CH3 + 2NaBr
Fittig reaction: Sirf do aryl halide molecules + Na (dry ether) → biphenyl derivative (symmetric aryl-aryl coupling), koi alkyl halide involved nahi.
2 C6H5Br + 2Na --dry ether--> C6H5-C6H5 + 2NaBr
Q17. Distinguish test: ethyl chloride aur chlorobenzene me kaise farak karoge?
AgNO3 test: Ethyl chloride (alkyl halide) ko AgNO3 solution ke saath garam karne se turant white precipitate (AgCl) ban jaata hai kyunki C-Cl bond weak/reactive hai aur asani se hydrolyse ho sakta hai. Chlorobenzene me C-Cl bond resonance ki wajah se strong hai, isliye same conditions me precipitate nahi banta (ya bahut slowly banta hai) — halogen ionise nahi hota easily.
Q18. DDT aur Freons ka ek-ek use aur environmental concern likho.
DDT (dichlorodiphenyltrichloroethane): powerful insecticide/pesticide ke roop me use hota tha; concern — environment me bahut persistent hai, food chain me bio-accumulate hota hai, isliye kai countries me restricted/banned hai.
Freons (CFCs): refrigerants aur aerosol propellants me use hote the; concern — stratosphere me pahunch kar ozone layer ko deplete karte hain (UV radiation se free radicals release karke), isliye phase-out kiye ja rahe hain.
Important Equations — Ek Nazar Me
| Feature | SN2 | SN1 |
|---|---|---|
| Kinetics | Rate = k[RX][Nu⁻] (2nd order) | Rate = k[RX] (1st order) |
| Mechanism | Single step, concerted, backside attack | Two steps — carbocation intermediate |
| Substrate order | methyl > 1° > 2° > 3° | 3° > 2° > 1° |
| Stereochemistry | Complete inversion (Walden inversion) | Racemisation (planar carbocation) |
| Solvent preference | Polar aprotic (DMSO, acetone) | Polar protic (water, alcohol) — stabilises carbocation/ions |
| Rearrangement | Nahi hota | Ho sakta hai (carbocation rearrange kar sakta hai for more stability) |
↔ Table ko side me swipe karein
| Named reaction | Equation / use |
|---|---|
| Finkelstein | RCl/RBr + NaI (dry acetone) → RI + NaCl/NaBr↓ (best method for alkyl iodide) |
| Swarts | RCl/RBr + AgF/Hg2F2/CoF2/SbF3 → RF (best method for alkyl fluoride) |
| Darzens process | ROH + SOCl2 (pyridine) → RCl + SO2 + HCl (purest alkyl chloride) |
| Wurtz-Fittig | ArX + RX + 2Na (dry ether) → Ar-R (aryl-alkyl coupling) |
| Fittig | 2 ArX + 2Na (dry ether) → Ar-Ar (aryl-aryl coupling) |
↔ Table ko side me swipe karein
| Reactivity trend | Order |
|---|---|
| Bond enthalpy (C-X) | C-F > C-Cl > C-Br > C-I |
| Bond length (C-X) | C-F < C-Cl < C-Br < C-I |
| Boiling point (same R, diff X) | RI > RBr > RCl > RF |
| SN2 reactivity (alkyl halides, same X) | methyl > 1° > 2° > 3° |
| SN1 reactivity (alkyl halides, same X) | 3° > 2° > 1° |
| Reactivity toward nucleophilic substitution (same R) | RI > RBr > RCl > RF (better leaving group) |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- SN1/SN2 substrate preference ulta kar dena. Students yaad rakhte waqt confuse ho jaate hain — SN2 me steric hindrance ki wajah se 1° fast hai, SN1 me carbocation stability ki wajah se 3° fast hai. Dono opposite orders hain, ek doosre se mix mat karo.
- SN1 me pure inversion likh dena. SN1 racemisation deta hai (planar carbocation, dono side se attack), inversion sirf SN2 ki property hai. Bahut students SN1 answer me bhi 'inversion of configuration' likh dete hain jo galat hai.
- Haloarenes ko haloalkanes jaisa reactive maan lena nucleophilic substitution me. Chlorobenzene aur ethyl chloride dono ko AgNO3 ke saath same treat karna galat hai — resonance ki wajah se aryl C-X bond bahut strong hai, easily substitute nahi hota.
- Halogen ko sirf deactivating ya sirf directing bol dena, dono effects na explain karna. Sahi answer: halogen -I se deactivating hai (poori ring ke liye) lekin +R se ortho/para directing hai (jahan substitution ho wahi positions ke liye) — dono baat sath likhni chahiye, marks poore effect samjhaane se milte hain.
- IUPAC numbering me sirf ek substituent ka lowest locant dekh lena. Rule 'collectively lowest set of locants' hai — sabhi substituents (halogen + alkyl) ka combined set compare karo, sirf pehle mile substituent ka locant nahi.
- Boiling point trend ko inductive effect se explain karne ki koshish karna. RI > RBr > RCl > RF ka reason van der Waals/dispersion forces hai (size aur polarizability badhne se), inductive/dipole effect nahi — ye common confusion hai kyunki halogen electronegativity F sabse zyada hai but boiling point sabse kam.
Board-Style Important Questions
- 1 mark: Chlorobenzene me C-Cl bond, ethyl chloride ke C-Cl bond se strong kyun hota hai?
- 2 marks: Finkelstein reaction likho aur samjhao ye reaction complete kaise hoti hai.
- 2 marks: SN1 aur SN2 reaction ki stereochemistry me kya farak hai? Ek-ek example ke saath explain karo.
- 3 marks: Haloalkanes water me insoluble kyun hote hain jabki polar molecules hain? Iske saath boiling point trend RI > RBr > RCl > RF bhi samjhao.
- 3 marks: Haloarenes nucleophilic substitution ke liye resistant kyun hote hain? Teen reasons likho.
- 5 marks: SN1 aur SN2 mechanism ko step-by-step likho (rate law, intermediate, transition state, stereochemistry, substrate preference sabka comparison do).
Aksar Poochhe Jaane Wale Sawaal
Haloalkane aur haloarene me basic farak kya hai?
Haloalkane me halogen sp3 carbon se juda hota hai (alkyl halide, jaise CH3CH2Cl), haloarene me halogen directly benzene ring ke sp2 carbon se juda hota hai (aryl halide, jaise C6H5Cl). Isse reactivity, bond strength, aur mechanism sab alag ho jaate hain.
SN1 aur SN2 me kaunsa fast hai?
Depend karta hai substrate pe. Methyl aur 1° halides SN2 fast karte hain (kam steric hindrance), 3° halides SN1 fast karte hain (stable carbocation banta hai). 2° halides dono mechanism se react kar sakte hain, conditions (solvent, nucleophile strength) pe depend karta hai.
Haloarenes electrophilic substitution me deactivating hote hue bhi ortho/para directing kaise hain?
Halogen ka -I effect (inductive) poori ring ko deactivate karta hai kyunki halogen electronegative hai aur electron density kheenchta hai. Lekin +R effect (resonance, lone pair donation) ortho/para positions par relatively zyada electron density chhod deta hai — isliye substitution wahi hoti hai jahan ho, bas overall rate benzene se kam hoti hai.
Saytzeff's rule kya kehta hai?
Jab elimination reaction se ek se zyada alkene ban sakte hain, to zyada substituted (zyada stable) alkene hi major product banta hai — kam substituted alkene minor product hota hai.
Alcohol se alkyl halide banane ka sabse accha method kaunsa hai?
SOCl2 (thionyl chloride, pyridine ke saath) — Darzens process. Ye sabse pure product deta hai kyunki dono byproducts (SO2 aur HCl) gases hote hain jo apne aap escape ho jaate hain, koi extra purification nahi chahiye.
DDT abhi bhi use hota hai kya?
Bahut countries me DDT ka agricultural use ban ya heavily restricted hai kyunki ye environment me persist karta hai aur food chain me bio-accumulate hota hai. Kuch jagah malaria control jaise limited public health uses ke liye WHO guidelines ke under still allowed hai.
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