ALKYL HALIDES — R–X

Classification · Preparation (All Routes) · Reactions · Mechanisms · Stereochemistry

§0 · Classification & General Formula
General formula: CnH2n+1X  |  X = F, Cl, Br, I  |  C–X bond is polar covalent (Cδ+–Xδ–)
TypeStructureExampleKey Character
1° (Primary)R–CH₂–XCH₃CH₂Br (bromoethane)SN₂ preferred; E2 rare
2° (Secondary)R₂CH–X(CH₃)₂CHBr (2-bromopropane)SN₂ & SN₁ compete; E2 with strong base
3° (Tertiary)R₃C–X(CH₃)₃CBr (t-butyl bromide)SN₁ & E1 preferred; SN₂ blocked
AllylicR–CH=CH–CH₂–XCH₂=CHCH₂Br (allyl bromide)Resonance-stabilised carbocation; SN₁ fast
BenzylicAr–CH₂–XC₆H₅CH₂Cl (benzyl chloride)Resonance-stabilised; very reactive SN₁
VinylicR–C=CH–XCH₂=CHCl (vinyl chloride)Extremely unreactive in SN; C–X has partial double-bond character
ArylAr–XC₆H₅Br (bromobenzene)No SN₁/SN₂; reacts only via EAS, Mg, or Pd coupling
Bond Properties & Halide Order:
Bond strength: C–F > C–Cl > C–Br > C–I  |  Bond length: C–F < C–Cl < C–Br < C–I
Reactivity in SN/E: R–I > R–Br > R–Cl > R–F (leaving group ability = bond polarisability)
Dipole moment: C–F highest individual bond, but CH₂Cl₂ highest molecule due to geometry
§1 · Preparation from Alkanes — Free Radical Halogenation
Mechanism: Free Radical Chain · Initiation → Propagation → Termination
Reaction:  R–H + X₂  ──hν or Δ──→  R–X + HX
Works for: Cl₂ and Br₂ only. F₂ is explosive and uncontrollable. I₂ does NOT halogenate (reaction endothermic, I• too unreactive).
CH₄ + Cl₂ ──hν──→ CH₃Cl + HCl (chloromethane) CH₄ + Br₂ ──hν──→ CH₃Br + HBr (bromomethane) CH₃CH₃ + Cl₂ ──hν──→ CH₃CH₂Cl + HCl (chloroethane)

🔴 Chlorination (Cl₂ / hν)

Selectivity: 3° : 2° : 1° = 5 : 3.8 : 1 (at 25°C)
Less selective; gives mixture of isomers
Propagation fast due to high reactivity of Cl•

🔵 Bromination (Br₂ / hν)

Selectivity: 3° : 2° : 1° = 1600 : 82 : 1
Highly selective; predominantly 3° or 2° product
Br• is less reactive → more selective (Hammond postulate)
Preferred when selectivity matters
Steps of Free Radical Chain:
Initiation: X₂ ──hν──→ 2 X•  (homolysis of X–X bond)
Propagation 1: X• + R–H → R• + H–X
Propagation 2: R• + X₂ → R–X + X•
Termination: R• + R• → R–R  |  R• + X• → R–X  |  X• + X• → X₂
Allylic and benzylic positions are brominated preferentially due to resonance stabilisation of the intermediate radical. NBS (N-bromosuccinimide) is used for selective allylic/benzylic bromination at low [Br₂] concentration.
Allylic bromination: CH₂=CH–CH₃ + NBS ──hν / CCl₄──→ CH₂=CH–CH₂Br + succinimide (propene) (allyl bromide)
§2 · Preparation from Alkenes
Key rule: Markovnikov's Rule — X adds to the C bearing more H atoms (H adds to C with more H)

🔴 Addition of HX (HCl, HBr, HI)

R–CH=CH₂ + HX → R–CHX–CH₃ (Markovnikov)
Mechanism: protonation of alkene → 2°/3° carbocation → X⁻ attack
Order of HX addition: HI > HBr > HCl

🔵 Anti-Markovnikov Addition (HBr + ROOR peroxide)

R–CH=CH₂ + HBr ──ROOR, hν──→ R–CH₂–CH₂Br
Mechanism: free radical; Br• adds to less substituted C
Only HBr shows anti-Markovnikov; not HCl or HI

🟢 Addition of X₂ (Cl₂, Br₂)

R–CH=CH₂ + X₂ ──CCl₄──→ R–CHX–CH₂X (vicinal dihalide)
Anti addition → trans product from cyclic halonium ion
Br₂/CCl₄ (orange → colourless) is alkene test

🟣 Addition of HOX (X₂ / H₂O)

R–CH=CH₂ + Cl₂/H₂O → R–CHCl–CH₂OH (halohydrin)
OH goes to more substituted C; X to less substituted C
Anti addition via halonium ion
Markovnikov: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane, major) Anti-Markovnikov (peroxide): CH₃CH=CH₂ + HBr ──ROOR──→ CH₃CH₂CH₂Br (1-bromopropane, major) Vicinal dihalide: CH₂=CH₂ + Br₂ ──CCl₄──→ BrCH₂CH₂Br (1,2-dibromoethane)
§3 · Preparation from Alcohols — Most Important Route
General: R–OH + Halogenating agent → R–X + byproduct  |  OH is a poor leaving group; must be activated

HX Acids (Lucas Test)

R–OH + HX → R–X + H₂O
Reactivity of HX: HI > HBr > HCl
Reactivity of R–OH: 3° > 2° > 1° > CH₃OH
3°: SN₁  |  1°: SN₂
ZnCl₂ catalyst needed for 1° and 2° with HCl

SOCl₂ (Thionyl Chloride)

R–OH + SOCl₂ ──pyridine──→ R–Cl + SO₂ + HCl
Retention of configuration (SNi mechanism) without pyridine
With pyridine: inversion (SN₂ via chlorosulfite)
Best reagent for 1° and 2° alcohols → chloride
SO₂ and HCl escape as gases (drives reaction)

PX₃ / PX₅ (Phosphorus Halides)

3 R–OH + PCl₃ → 3 R–Cl + P(OH)₃
3 R–OH + PBr₃ → 3 R–Br + P(OH)₃
R–OH + PCl₅ → R–Cl + POCl₃ + HCl
Inversion of configuration (SN₂)
PBr₃ excellent for 1° → bromides

Red P + Halogen (In Situ PX₃)

3 R–OH + P + Br₂ → 3 R–Br + P(OH)₃
Red P + Br₂ first generates PBr₃ in situ
Equivalent to using PBr₃ directly
Used in lab-scale preparation of alkyl bromides

Appel Reaction (PPh₃ / CCl₄ or CBr₄)

R–OH + PPh₃ + CCl₄ → R–Cl + Ph₃P=O + CHCl₃
Mild conditions; inversion of configuration
Excellent for sensitive substrates; no acid formed
CBr₄ gives R–Br; I₂ gives R–I
SOCl₂ example: (R)-butan-2-ol + SOCl₂ ──pyridine──→ (S)-2-chlorobutane [inversion] (R)-butan-2-ol + SOCl₂ ──no base──→ (R)-2-chlorobutane [retention, SNi] PBr₃ example: CH₃CH₂CH₂OH + PBr₃ → CH₃CH₂CH₂Br + H₃PO₃ (propan-1-ol → 1-bromopropane)
3° alcohols CANNOT be converted with SOCl₂ cleanly (elimination competes). Use HCl directly or Lucas reagent for 3°. Never use SOCl₂ on amino alcohols without protection.
§4 · Preparation via Halodecarboxylation (from Carboxylic Acids)

Hell–Volhard–Zelinsky (HVZ) Reaction

Acid + X₂ ──red P──→ α-Halo acid (α-halogenation)
R–CH₂–COOH + Br₂ ──P──→ R–CHBr–COOH
P converts acid → acid halide (more enolisable)
Only α-position halogenated
Works with Cl₂ and Br₂; not I₂

Hunsdiecker Reaction (Silver Salt)

R–COOAg + Br₂ ──CCl₄, Δ──→ R–Br + CO₂ + AgBr
Free radical mechanism
Net: RCOOH → R–Br (loss of one carbon as CO₂)
Works best with Br₂; Cl₂ gives poor yield
Chain shortens by 1C
HVZ: CH₃CH₂COOH + Br₂ ──red P──→ CH₃CHBrCOOH (2-bromopropanoic acid) Hunsdiecker: C₆H₅COOAg + Br₂ ──CCl₄──→ C₆H₅Br + CO₂ + AgBr (bromobenzene)
§5 · Halide Exchange Reactions

Finkelstein Reaction

R–Cl or R–Br + NaI ──dry acetone──→ R–I + NaCl or NaBr
NaCl and NaBr are insoluble in acetone → equilibrium pushed to product
SN₂ mechanism → inversion at chiral centre
Best for converting 1° and 2° chlorides/bromides to iodides

Swarts Reaction

R–Cl + AgF or SbF₃ or CoF₂ ──Δ──→ R–F + AgCl
Used to introduce fluorine (direct fluorination explosive)
AgF: mild conditions for reactive substrates
SbF₃ (Swarts reagent): industrial preparation of fluorides
Also: R–Cl + KF ──DMSO, Δ──→ R–F (polar aprotic)
Finkelstein: CH₃CH₂Br + NaI ──dry acetone──→ CH₃CH₂I + NaBr↓ Swarts: CCl₄ + SbF₃ ──SbCl₅ cat.──→ CCl₂F₂ (Freon-12) CH₃Cl + AgF → CH₃F + AgCl
In Finkelstein, use dry acetone strictly. Water dissolves NaBr/NaCl and ruins the driving force. Swarts reaction is the only practical route to alkyl fluorides from alkyl chlorides.
§6 · Miscellaneous Preparation Routes

From Alkynes (Addition of HX)

R–C≡CH + HX → R–CX=CH₂ (vinyl halide, Markovnikov)
R–C≡CH + 2HX → R–CX₂–CH₃ (gem-dihalide)
2 moles HX added in excess gives gem-dihalide

From Alkyl Sulfonates (Tosylates)

R–OTs + KBr ──DMF──→ R–Br + KOTs
R–OTs excellent leaving group (better than OH)
SN₂: clean inversion; retains carbon skeleton
R–OTs + NaI → R–I (Finkelstein variant)

From Diazonium Salts (Sandmeyer)

Ar–N₂⁺Cl⁻ + CuCl ──HCl──→ Ar–Cl + N₂
Ar–N₂⁺Br⁻ + CuBr ──HBr──→ Ar–Br + N₂
Ar–N₂⁺ + KI ──Δ──→ Ar–I + N₂ (no Cu needed for iodide)
Balz-Schiemann: Ar–N₂⁺BF₄⁻ ──Δ──→ Ar–F + BF₃ + N₂

Chloromethylation (Blanc)

ArH + HCHO + HCl ──ZnCl₂──→ Ar–CH₂Cl
Electrophilic aromatic substitution
Introduces –CH₂Cl on aromatic ring
Benzylic chloride formed directly

From Alcohols via Mitsunobu

R–OH + HX source + PPh₃ + DEAD →R–X
Complete inversion of configuration
Works under mild conditions; tolerates many groups
Excellent for chiral alcohol → inverted halide

Side-Chain Halogenation (Benzene)

Ar–CH₃ + X₂ ──hν──→ Ar–CH₂X (benzylic)
No Lewis acid; UV light required (free radical)
Ar–CH₃ + X₂ ──FeBr₃──→ Ar–X (ring halogenation via EAS)
Lewis acid directs to ring; light directs to side chain
Sandmeyer: C₆H₅NH₂ ──NaNO₂/HCl, 0–5°C──→ C₆H₅N₂⁺Cl⁻ ──CuCl/HCl──→ C₆H₅Cl + N₂ Balz-Schiemann: C₆H₅N₂⁺Cl⁻ + HBF₄ → C₆H₅N₂⁺BF₄⁻ ──Δ──→ C₆H₅F + N₂ + BF₃ Alkyne → gem-dihalide: CH≡CH + 2HCl → CH₃CHCl₂ (1,1-dichloroethane)
§7 · All Reactions of Alkyl Halides
7.1 · SN₂ Substitution — Bimolecular
R–X + Nu⁻ → R–Nu + X⁻  |  Rate = k[R–X][Nu]
One-step, concerted; inversion of configuration (Walden inversion)
Favoured by: 1° > 2° > 3° (in that order) · Strong nucleophile · Polar aprotic solvent (DMSO, DMF, acetone)
Bulky groups at β-carbon slow SN₂ dramatically
CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr (R)-2-bromobutane + NaOH → (S)-butan-2-ol [inversion] CH₃Br + NaCN ──DMSO──→ CH₃CN + NaBr (nitrile, +1C)
7.2 · SN₁ Substitution — Unimolecular
R–X → R⁺ + X⁻ (slow) → R⁺ + Nu → R–Nu (fast)  |  Rate = k[R–X] only
Two steps; carbocation intermediate → racemisation (planar carbocation attacked from both faces)
Favoured by: 3° > 2° > allyl/benzyl · Polar protic solvent (H₂O, ROH) · Weak nucleophile
Rearrangements (hydride/methyl shift) possible via carbocation
(CH₃)₃CBr + H₂O → (CH₃)₃C⁺ + Br⁻ → (CH₃)₃COH + HBr Allyl / benzyl bromide: very fast SN₁ due to resonance-stabilised cation
7.3 · E2 Elimination — Bimolecular
Base abstracts β–H while X leaves → alkene formed  |  Rate = k[R–X][Base]
Anti-periplanar arrangement required (H and X must be 180° apart)
Zaitsev product (more substituted alkene) with strong, small base (KOH/EtOH)
Hofmann product (less substituted) with bulky base (t-BuOK / t-BuOH)
Favoured by: 2° and 3° · Strong, concentrated base · High temperature
CH₃CH₂Br + KOH ──EtOH, Δ──→ CH₂=CH₂ + KBr + H₂O (CH₃)₂CHBr + KOH ──EtOH──→ CH₃CH=CH₂ (propene, Zaitsev) (CH₃)₂CHBr + (CH₃)₃COK ──t-BuOH──→ CH₂=CHCH₃ (propene, but less sub., if applicable)
7.4 · E1 Elimination — Unimolecular
R–X → R⁺ + X⁻ (slow) → loss of β–H → alkene  |  Rate = k[R–X]
Occurs alongside SN₁; weak base / weak nucleophile; polar protic solvent; high temperature
Always gives Zaitsev product (more stable alkene, more substituted)
(CH₃)₃CBr + H₂O ──Δ──→ (CH₃)₂C=CH₂ (isobutylene) + (CH₃)₃COH
7.5 · With NaOH / KOH
Aqueous NaOH / KOH → Alcohol (SN₂ for 1°, SN₁ for 3°)
Alcoholic NaOH / KOH + heat → Alkene (E2 for 1° and 2°; E1 for 3°)
CH₃CH₂Br + NaOH (aq) → CH₃CH₂OH (substitution) CH₃CH₂Br + NaOH (alc) ──Δ──→ CH₂=CH₂ (elimination)
7.6 · With AgOH / Ag₂O → Alcohol
R–X + AgOH → R–OH + AgX↓ (silver halide precipitate, confirmatory test)
Ag⁺ assists ionisation → even unreactive halides react
7.7 · With NaCN / KCN → Nitrile (+1C)
R–X + NaCN ──DMSO──→ R–CN + NaX  (nitrile, chain +1C)
R–CN ──H₂/Ni──→ R–CH₂–NH₂ (amine) · R–CN ──H₃O⁺/Δ──→ R–COOH (acid)
Ambident nucleophile: CN⁻ attacks via C (major, nitrile) or N (minor, isonitrile)
CH₃Br + KCN ──DMSO──→ CH₃CN + KBr (acetonitrile) CH₃Br + AgCN → CH₃NC + AgBr (methyl isocyanide, N-attack with Ag⁺)
7.8 · With NaNO₂ / AgNO₂ — Ambident NO₂⁻
R–X + NaNO₂ ──DMSO──→ R–O–N=O (nitrite ester, O-attack) — soft conditions
R–X + AgNO₂ → R–NO₂ (nitroalkane, N-attack) — Ag⁺ assists ionisation, hard ion pairs
NaNO₂ → alkyl nitrite (ester)  |  AgNO₂ → nitroalkane
C₂H₅Br + AgNO₂ → C₂H₅NO₂ + AgBr (nitroethane) C₂H₅Br + NaNO₂ ──DMSO──→ C₂H₅–O–NO (ethyl nitrite)
7.9 · With Ammonia / Amines → Amines (Hofmann Method)
R–X + NH₃ → R–NH₂ · HX (primary ammonium salt) ──base──→ R–NH₂
Excess R–X gives: R–NH₂ → R₂NH → R₃N → R₄N⁺X⁻ (quaternary ammonium salt)
This successive alkylation is called Hofmann exhaustive alkylation
Gabriel synthesis avoids over-alkylation (phthalimide → 1° amine only)
CH₃Br + NH₃(excess) → (CH₃)₄N⁺Br⁻ (tetramethylammonium bromide) CH₃Br + NH₃(1:1) → CH₃NH₃⁺Br⁻ ──NaOH──→ CH₃NH₂ (methylamine)
7.10 · With Sodium Alkoxide / Phenoxide → Ether (Williamson Synthesis)
R–X + R'ONa → R–O–R' + NaX  (unsymmetrical ether)
SN₂ mechanism; use 1° R–X only (3° gives elimination with alkoxide base)
R'O⁻ must come from less hindered alkyl; X on more hindered → use different strategy
CH₃Br + NaOC₂H₅ → CH₃–O–C₂H₅ + NaBr (methyl ethyl ether) (CH₃)₃CBr + NaOCH₃ → (CH₃)₂C=CH₂ (mainly elimination, not ether)
7.11 · With Metals
Mg / Et₂O: R–X + Mg → R–MgX (Grignard reagent)
Zn / Et₂O: R–X + Zn → R–ZnX (Reformatsky reagent)
Li / Et₂O: R–X + 2Li → R–Li + LiX (organolithium, most reactive)
2Na (Wurtz): 2 R–X + 2Na → R–R + 2 NaX (symmetrical coupling)
Wurtz-Fittig: ArX + RX + 2Na → Ar–R + 2 NaX (mixed, aryl-alkyl coupling)
Wurtz reaction: 2 CH₃Br + 2Na → CH₃–CH₃ + 2 NaBr (ethane) 2 C₂H₅Br + 2Na → C₄H₁₀ + 2 NaBr (butane) Grignard: C₂H₅Br + Mg ──dry Et₂O──→ C₂H₅MgBr
7.12 · Wurtz Reaction — Limitations
Mixed Wurtz (R–X + R'–X + 2Na) gives R–R, R'–R' AND R–R' (3 products) → not useful synthetically
Best results: both halides same, or one is methyl/primary
Cannot be used for odd-carbon-number target from even halide pairs
7.13 · With Silver Salts (Ag⁺ promotes SN₁)
R–X + AgNO₃ ──EtOH──→ R–OEt or R–OH + AgX↓ (qualitative test)
Rate: 3° > 2° > 1° (SN₁; Ag⁺ abstracts X⁻ to form AgX↓, driving ionisation)
Aryl and vinyl halides give NO precipitate with AgNO₃/EtOH (use for distinction)
AgNO₃ / EtOH test: (CH₃)₃CBr → immediate white/yellow ppt (3°, fast SN₁) CH₃CH₂Br → ppt on warming (1°, slow) C₆H₅Br → no ppt even on heating (aryl, unreactive)
7.14 · With Na/K (Corey-House) / Organocuprate
R–X + R'Li → R'Li (then + CuI → R'₂CuLi) + R–X → R–R' (Corey-House)
Organocuprate (Gilman) + R–X → new C–C bond · Best for coupling 2° and 3° halides
Tolerates ester, ketone, nitrile on the molecule (unlike Grignard)
Corey-House synthesis: CH₃Li + CuI → (CH₃)₂CuLi ──+ (CH₃)₃CBr──→ (CH₃)₄C (neopentane) (CH₃)₂CuLi + CH₂=CHCHO → CH₃CH₂CHO (1,4-addition, not 1,2)
7.15 · Reduction → Alkane
R–X + H₂ ──Pd/C──→ R–H + HX (catalytic hydrogenolysis)
R–X + LiAlH₄ ──dry Et₂O──→ R–H + LiX + AlX (delivers H⁻ as nucleophile, SN₂)
R–X + Zn + HCl → R–H (dissolving metal reduction)
R–X + Bu₃SnH ──AIBN, hν──→ R–H (radical reduction; Barton-McCombie variant)
CH₃CH₂Br + LiAlH₄ ──Et₂O──→ CH₃CH₃ + LiBr + AlH₃ C₆H₅Br + H₂ ──Pd/C──→ C₆H₆ + HBr (benzene)
7.16 · Dehalogenation of Vicinal Dihalides → Alkene
–CHX–CHX– + Zn ──EtOH──→ –CH=CH– + ZnX₂
Used to protect a double bond (add X₂ then remove with Zn) or to form trans-alkene
Also: –CHX–CHX– + KOH/EtOH (2 equiv) → alkyne (double E2)
CH₂BrCH₂Br + Zn ──EtOH──→ CH₂=CH₂ + ZnBr₂ CHBr₂CHBr₂ + 2 KOH ──alc──→ HC≡CH + 2 KBr + 2 H₂O (acetylene)
§8 · SN₁ vs SN₂ vs E1 vs E2 — Decision Table
FactorSN₂SN₁E2E1
Substrate1° > 2° (never 3°)3° > 2° > allyl/benzyl2°, 3°3° > 2°
Nucleophile/BaseStrong Nu, high conc.Weak Nu or solventStrong, conc. baseWeak base
SolventPolar aprotic (DMSO, DMF)Polar protic (H₂O, ROH)Polar aprotic or protic + basePolar protic
TemperatureLow–moderateModerateHigh (favours elimination)High
StereochemistryInversionRacemisationAnti-periplanar (H and X 180°)Racemic alkene
Rate law2nd order (bimolecular)1st order (unimolecular)2nd order1st order
Rearrangement?NoYes (hydride/methyl shift)NoYes (via carbocation)
Alkene regiochem.——Zaitsev (KOH/EtOH) or Hofmann (bulky base)Zaitsev (more subst.)
Allylic and benzylic halides can do BOTH SN₁ AND SN₂ rapidly due to resonance stabilisation of carbocation AND good orbital overlap for backside attack. With strong Nu in polar aprotic → SN₂; with weak Nu in protic → SN₁.
§9 · Stereochemistry Summary

❗ SN₂ — Walden Inversion

Nucleophile attacks rear face of C–X bond
(R) substrate → (S) product and vice versa
Transition state: trigonal bipyramidal (Nu—C—X linear)
Configuration fully inverted at chiral centre
Retention possible only via double-inversion (two SN₂ steps)

🔵 SN₁ — Racemisation

Planar carbocation intermediate attacked from both faces equally
Gives 50:50 R and S products (racemic mixture)
Slight excess of inversion product in practice (ion pair effect)
Chiral information destroyed at reaction centre

✅ E2 — Anti Periplanar

H and X must be anti (dihedral 180°) for concerted E2
Cyclohexyl halides: axial H and axial X must both be present
trans-2-halocyclohexane reacts faster than cis (in chair conformation)
Syn-elimination possible in special cases (cyclic, no anti H)

🟣 SOCl₂ Mechanism (SNi vs SN₂)

Without pyridine: chlorosulfite ester → frontside attack (SNi) → retention
With pyridine: pyridinium salt formed → intermolecular SN₂ → inversion
Pyridine deprotonates HCl to give chloride that attacks backside
§10 · Qualitative Tests & Reactivity Order
TestReagentObservationInference
AgNO₃ / EtOH testAgNO₃ in ethanol3°: immediate ppt; 2°: on warming; 1°: slow; Ar/vinyl: no pptType of R–X (SN₁ reactivity)
NaI / Acetone (Finkelstein)NaI in dry acetoneR–Cl and R–Br: precipitate (NaCl or NaBr insoluble); R–I: no pptIdentifies chloride or bromide; SN₂ reactivity
Beilstein TestCu wire in flameGreen flamePresence of any halogen (Cl, Br, I)
Na / Lassaigne (fusion)Na fusion + AgNO₃White ppt AgCl; pale yellow AgBr; yellow AgIIdentifies specific halide
Alcoholic AgNO₃ rateAgNO₃ / EtOHAllylic/benzylic: very fast; 3° > 2° > 1°; vinyl/aryl: no reactionConfirms substrate class
Overall SN₁ Reactivity: Allyl ≈ Benzyl > 3° > 2° > 1° > CH₃ > Vinyl ≈ Aryl (no reaction)
Overall SN₂ Reactivity: CH₃ > 1° > 2° >> 3° (essentially zero for 3°)
Leaving Group Ability (both SN₁ & SN₂): I⁻ > Br⁻ > Cl⁻ > F⁻  |  TsO⁻ > I⁻ > Br⁻ > Cl⁻
Correlates with conjugate acid pKa: weaker conjugate acid = better leaving group
Nucleophilicity in Polar Aprotic (DMSO, DMF, acetone): I⁻ > Br⁻ > Cl⁻ > F⁻ (polarisability order)
Nucleophilicity in Polar Protic (H₂O, ROH): F⁻ > Cl⁻ > Br⁻ > I⁻ (charge density order; protic solvent solvates small ions more)
§11 · Quick Reference — All Reactions of R–X
ReagentProductType / Notes
NaOH (aq)R–OH (alcohol)SN₂ (1°) / SN₁ (3°)
KOH (alc, Δ)AlkeneE2 (Zaitsev)
NaCN / KCNR–CN (nitrile, +1C)SN₂; C-attack
AgCNR–NC (isonitrile)N-attack via Ag⁺
NaNO₂ (DMSO)R–ONO (nitrite ester)O-attack
AgNO₂R–NO₂ (nitroalkane)N-attack via Ag⁺
NaOR' (Williamson)R–O–R' (ether)SN₂; use 1° R–X only
NH₃ (excess)R₄N⁺X⁻ (quat. ammonium)Over-alkylation
NH₃ (limited)R–NH₂ (1° amine)Hofmann method
Mg / Et₂OR–MgX (Grignard)Organometallic
2Na (Wurtz)R–R (alkane, +2C)Symmetric only useful
NaI / dry acetoneR–I (iodide)Finkelstein SN₂
AgF or SbF₃R–F (fluoride)Swarts reaction
LiAlH₄R–H (alkane)Reduction (H⁻ as Nu)
H₂ / Pd–CR–H (alkane)Hydrogenolysis
Zn + HClR–H (alkane)Dissolving metal reduction
Na + R'X (Wurtz-Fittig)Ar–R' (aryl-alkyl)Mixed coupling
R'₂CuLi (Gilman)R–R' (new C–C bond)Corey-House; no over-addition
AgNO₃ / EtOHAgX↓ + alcoholQualitative test; SN₁
R–X + Zn (vicinal diX)AlkeneDehalogenation
R–X + 2KOH/alc (gem/vic diX)AlkyneDouble E2
§12 · Must-Remember Summary Rules
🔴 Reaction Conditions Decide Everything:
Aq. NaOH → alcohol  |  Alc. KOH + Δ → alkene  |  NaCN → nitrile  |  Mg/Et₂O → Grignard  |  2Na → Wurtz alkane
🟢 Preparation Memory Map:
Alkane + X₂/hν → R–X (free radical)  |  Alkene + HX → R–X (Markovnikov)  |  Alkene + HBr/ROOR → anti-Markovnikov  |  Alcohol + SOCl₂/PBr₃/HX → R–X
🔵 Key Selectivity Facts:
Bromination more selective than chlorination (NBS for allylic/benzylic)  |  SN₂ needs polar aprotic + 1° substrate  |  SN₁ needs polar protic + 3° or stabilised cation  |  AgCN → isocyanide; NaCN → nitrile  |  AgNO₂ → nitroalkane; NaNO₂ → nitrite ester  |  SOCl₂ with pyridine → inversion; without pyridine → retention (SNi)