Step 1 — Alcohol + PCl₅:
CH₃CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂Cl + POCl₃ (X) + HCl
Primary alcohol reacts with PCl₅ to give alkyl chloride + phosphoryl chloride (POCl₃) + HCl. So X = POCl₃ ✓
Step 2 — Dehydrohalogenation (alc. KOH, Δ):
CH₃CH₂CH₂Cl + alc. KOH →(Δ) CH₃CH=CH₂ (Y) + KCl + H₂O
Y = Propene (only one possible alkene from 1-chloropropane)
Step 3 — HBr addition with peroxide (anti-Markovnikov):
(C₆H₅CO)₂O₂ = benzoyl peroxide = free radical initiator
CH₃CH=CH₂ + HBr →(peroxide) CH₃CH₂CH₂Br (Z)
Peroxide → anti-Markovnikov (Kharasch effect) → Br goes to terminal (less substituted) carbon
Without peroxide: CH₃CH=CH₂ + HBr → CH₃CH(Br)CH₃ (Markovnikov)
Alcohols react with PCl₅, PCl₃, or SOCl₂ to give alkyl chlorides. With PCl₅: ROH + PCl₅ → RCl + POCl₃ + HCl. With PCl₃: 3ROH + PCl₃ → 3RCl + H₃PO₃. With SOCl₂ (thionyl chloride): ROH + SOCl₂ → RCl + SO₂ + HCl. SOCl₂ is the best reagent because SO₂ and HCl are gases — they escape and purification of RCl is easy. With PCl₅: X = POCl₃ (phosphoryl chloride, not H₃PO₃). Common mistake: confusing PCl₃ (gives H₃PO₃) with PCl₅ (gives POCl₃).
📌 Markovnikov's Rule (no peroxide): In HX addition to alkene, H goes to carbon with more H (less substituted), X goes to more substituted carbon → more stable carbocation intermediate
📌 Anti-Markovnikov (with peroxide): In HBr addition, Br• radical goes to less substituted carbon (more stable radical). H then adds to more substituted end
📌 ONLY HBr shows anti-Markovnikov with peroxide — NOT HCl or HI
📌 HCl: Cl• too reactive, doesn't show selectivity. HI: I• too stable/unreactive, can't add
📌 (C₆H₅CO)₂O₂ = benzoyl peroxide → generates free radicals → triggers anti-Markovnikov
Three conditions must be met: (1) The halogen radical must add to alkene (not too fast, not too slow). (2) The resulting radical must be stable enough to react with HX. (3) H-X bond must be weak enough for Cl•/Br•/I• to abstract H. For HBr: Br• reacts at a moderate rate — adds to alkene, giving alkyl radical, which then abstracts H from HBr. For HCl: Cl• is so reactive it's non-selective. For HI: I• is too stable, doesn't readily add to alkene. This makes HBr unique in showing Kharasch (anti-Markovnikov) addition.
Alkyl halides react with alcoholic KOH (or NaOH) at high temperature to give alkenes. This is β-elimination (E2 mechanism): base abstracts β-H while X leaves simultaneously. CH₃CH₂CH₂Cl + alc. KOH → CH₃CH=CH₂ + KCl + H₂O. For 1-chloropropane (primary): only one β-carbon (C2) → only propene possible. For 2-halopropane: could give propene from either β-carbon (same product here). Saytzeff's rule: major product is the more substituted alkene. Hofmann's rule: with bulky base, less substituted alkene predominates.
📌 SN2 (bimolecular): One step, backside attack, inversion of configuration (Walden inversion). Favoured by: primary alkyl halide, polar aprotic solvent, strong nucleophile. Rate = k[RX][Nu]
📌 SN1 (unimolecular): Two steps, carbocation intermediate, racemisation. Favoured by: tertiary alkyl halide, polar protic solvent, weak nucleophile. Rate = k[RX]
📌 Order: SN2 reactivity: CH₃X > 1° > 2° > 3°
📌 Order: SN1 reactivity: 3° > 2° > 1° > CH₃X
📌 Allylic/benzylic halides: both SN1 and SN2 work (resonance-stabilised carbocation/transition state)
Both nucleophilic substitution (SN) and elimination (E) compete when alkyl halides react with base/nucleophile. Substitution favoured: strong nucleophile, weak base, low temperature, primary substrate, polar aprotic solvent. Elimination favoured: strong base (not nucleophile), high temperature, tertiary substrate, bulky reagent, polar protic solvent. In practice: aqueous KOH at room temperature → SN. Alcoholic KOH at high temperature → E (elimination). This is the key to controlling which product forms.
📌 CHCl₃ (Chloroform): anaesthetic (now replaced), solvent, synthesis of DDT
📌 CCl₄ (Carbon tetrachloride): fire extinguisher (now banned — toxic), dry cleaning
📌 CH₂Cl₂ (Dichloromethane): paint remover, solvent in pharma industry
📌 CF₂Cl₂ (Freon-12): refrigerant (banned — ozone depletion)
📌 CH₃Br (Methyl bromide): fumigant for soil (restricted use)
📌 C₂H₅Br (Ethyl bromide): in synthesis, Grignard reagent preparation
Grignard reagent (RMgX) is prepared by reacting alkyl halide with magnesium in dry ether: RX + Mg →(dry ether) RMgX. The C–Mg bond is highly polar (C⁻–Mg⁺) — carbon acts as a carbanion (nucleophile). Reacts with: CO₂ → carboxylic acid (RCOOH). Aldehydes → secondary alcohol. Ketones → tertiary alcohol. HCHO → primary alcohol. H₂O → alkane (RH). Dry conditions essential — water destroys Grignard reagent. Grignard reactions are extremely important in organic synthesis for C–C bond formation.