Chemistry
Organic Compounds Containing Halogens
Organic Compounds Containing Halogens Formula Sheet
All key reactions, mechanisms, and reactivity orders for haloalkanes, haloarenes, SN1/SN2, E1/E2 & polyhalogens — JEE Main & Advanced quick revision.
Last-minute revision sheet for the entire Halogens Compounds chapter — every preparation reaction, reactivity order, mechanism, and high-yield environmental fact, grouped by sub-topic for fast scanning.
How to use this sheet
This is a mostly reaction- and fact-based chapter (organic chemistry), so the “formulas” here are named reactions, reactivity orders, and rate equations. Everything below is drawn only from the chapter pages — no outside additions.
General Formulas & Bond Nature
| Quantity | Formula | Notes |
|---|---|---|
| Alkyl halide | $\text{R-X}$ | X = F, Cl, Br, I |
| Aryl halide (haloarene) | $\text{Ar-X}$ | X on sp² ring carbon |
| C-X polarity | $\delta^+\text{C} - \delta^-\text{X}$ | Carbon is electrophilic |
Classification (1°/2°/3°): count carbons attached to the C bearing X — 1 neighbour = 1°, 2 = 2°, 3 = 3°.
Key Reactivity & Property Orders
These ordered relations are the single most-tested items in the chapter.
| Property | Order | Reason |
|---|---|---|
| Bond strength (C-X) | $\text{C-F} > \text{C-Cl} > \text{C-Br} > \text{C-I}$ | BDE: 485 > 339 > 285 > 213 kJ/mol |
| Reactivity in nucleophilic substitution | $\text{R-I} > \text{R-Br} > \text{R-Cl} > \text{R-F}$ | Leaving-group ability (opposite to bond strength) |
| Leaving group ability | $\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-$ | More stable anion = better LG |
| Boiling point | $\text{R-I} > \text{R-Br} > \text{R-Cl} > \text{R-F}$ | Molecular mass / van der Waals |
| Density | $\text{R-I} > \text{R-Br} > \text{R-Cl} > \text{R-F}$ | All denser than water |
| Dipole moment | $\text{CH}_3\text{F} > \text{CH}_3\text{Cl} > \text{CH}_3\text{Br} > \text{CH}_3\text{I}$ | EN decreases down the group |
| Alcohol reactivity with HX | $3° > 2° > 1°$ | Carbocation stability |
| HX reactivity | $\text{HI} > \text{HBr} > \text{HCl}$ | HF works poorly |
The reactivity paradox
C-I is the weakest bond yet R-I is the most reactive — because I⁻ is the most stable (best) leaving group. Bond strength and reactivity run opposite.
Branching → lower BP: 1-bromobutane (101 °C) vs 2-bromo-2-methylpropane (73 °C).
Preparation of Alkyl Halides
From Alcohols
$$\boxed{\text{R-OH} + \text{HX} \rightarrow \text{R-X} + \text{H}_2\text{O}}$$| Reagent | Reaction | Note |
|---|---|---|
| PCl₃ | $3\text{R-OH} + \text{PCl}_3 \rightarrow 3\text{R-Cl} + \text{H}_3\text{PO}_3$ | All alcohol types |
| PCl₅ | $\text{R-OH} + \text{PCl}_5 \rightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl}$ | |
| PBr₃ | $3\text{R-OH} + \text{PBr}_3 \rightarrow 3\text{R-Br} + \text{H}_3\text{PO}_3$ | |
| PI₃ | $3\text{R-OH} + \text{PI}_3 \rightarrow 3\text{R-I} + \text{H}_3\text{PO}_3$ | |
| SOCl₂ | $\text{R-OH} + \text{SOCl}_2 \rightarrow \text{R-Cl} + \text{SO}_2\uparrow + \text{HCl}\uparrow$ | Purest product (gaseous by-products escape) |
Halogenation of Alkanes (free radical)
$$\boxed{\text{R-H} + \text{X}_2 \xrightarrow{h\nu \text{ or heat}} \text{R-X} + \text{HX}}$$Gives a mixture of products — poor preparative method.
Addition of HX to Alkenes
$$\boxed{\text{R-CH=CH}_2 + \text{HX} \rightarrow \text{R-CHX-CH}_3 \quad (\text{Markovnikov})}$$$$\boxed{\text{R-CH=CH}_2 + \text{HBr} \xrightarrow{\text{peroxide}} \text{R-CH}_2\text{-CH}_2\text{Br} \quad (\text{anti-Markovnikov})}$$Peroxide effect works only with HBr (not HCl or HI).
Halogen Exchange & Special Reactions
| Reaction | Equation | Purpose |
|---|---|---|
| Finkelstein | $\text{R-X} + \text{NaI} \xrightarrow{\text{dry acetone}} \text{R-I} + \text{NaX}\downarrow$ | R-Cl/R-Br → R-I |
| Swarts | $\text{R-X} + \text{AgF (or Hg}_2\text{F}_2) \rightarrow \text{R-F} + \text{AgX}$ | Make R-F |
| Hunsdiecker | $\text{R-COOAg} + \text{Br}_2 \xrightarrow{\text{CCl}_4} \text{R-Br} + \text{CO}_2 + \text{AgBr}$ | Product has one C less |
Finkelstein driving force
NaCl/NaBr are insoluble in acetone and precipitate out, pulling the equilibrium toward R-I.
Nucleophilic Substitution: SN1 vs SN2
General reaction:
$$\boxed{\text{R-X} + \text{Nu}^- \rightarrow \text{R-Nu} + \text{X}^-}$$| Feature | SN2 | SN1 |
|---|---|---|
| Steps | 1 (concerted, backside attack) | 2 (via carbocation) |
| Rate law | $\text{Rate} = k[\text{R-X}][\text{Nu}^-]$ | $\text{Rate} = k[\text{R-X}]$ |
| Substrate order | $\text{CH}_3 > 1° > 2° > 3°$ | $3° > 2° > 1° > \text{CH}_3$ |
| Stereochemistry | 100% inversion (Walden) | Racemization (50:50) |
| Nucleophile | Strong Nu⁻ needed | Strength irrelevant to rate |
| Solvent | Polar aprotic (acetone, DMSO, DMF, CH₃CN) | Polar protic (H₂O, ROH, HCOOH, CH₃COOH) |
| Rearrangement | None | Possible (1,2-shifts) |
SN1 two steps:
$$\text{R-X} \xrightarrow{\text{slow}} \text{R}^+ + \text{X}^- \qquad \text{R}^+ + \text{Nu}^- \xrightarrow{\text{fast}} \text{R-Nu}$$Carbocation stability:
$$\boxed{3° > 2° > 1° > \text{CH}_3^+}$$Stabilised by hyperconjugation + inductive (+I) effect.
Nucleophilicity (protic solvent):
$$\boxed{\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-} \qquad \text{RS}^- > \text{RO}^- > \text{OH}^- > \text{NH}_2^-$$Negative charge > neutral species.
Mnemonics
SN2 = 2 sides → inversion. SN1 = 1 planar carbocation → attack from both faces → racemization.
SPRINT (SN2): Small substrate, Polar aprotic, Robust Nu, Inversion, No rearrangement, Transition state.
CRISP (SN1): Carbocation, Rearrangement, Ionizing (protic) solvent, Stable substrate, Planar → racemic.
Common traps
SN2 gives inversion, not racemization. SN1 gives racemization, not complete inversion. Nucleophilicity ≠ basicity — for SN2 think nucleophilicity. Always check SN1 carbocations for rearrangement (e.g. neopentyl bromide + H₂O → tert-butanol).
Special substrates: allylic / benzylic halides do both SN1 & SN2; SN1 favoured by resonance-stabilised carbocation.
Elimination: E1 vs E2
General (β-elimination):
$$\boxed{\text{R-CH}_2\text{-CHX-R'} + \text{Base} \rightarrow \text{R-CH=CH-R'} + \text{HX}}$$| Feature | E2 | E1 |
|---|---|---|
| Steps | 1 (concerted) | 2 (via carbocation) |
| Rate law | $\text{Rate} = k[\text{R-X}][\text{Base}]$ | $\text{Rate} = k[\text{R-X}]$ |
| Substrate | $3° \geq 2° > 1°$ | $3° > 2° \gg 1°$ |
| Base | Strong base required | Weak base OK |
| Stereochemistry | Anti-periplanar (H, X at 180°) | No restriction |
| Rearrangement | None | Possible |
| Competes with | SN2 | SN1 |
E1 two steps:
$$\text{R-X} \xrightarrow{\text{slow}} \text{R}^+ + \text{X}^- \qquad \text{R}^+ \xrightarrow{-\text{H}^+,\ \text{fast}} \text{Alkene}$$Saytzeff vs Hofmann
Alkene stability:
$$\boxed{\text{Tetra} > \text{Tri} > \text{Di} > \text{Mono-substituted}}$$| Rule | Major product | Conditions |
|---|---|---|
| Saytzeff (Zaitsev) | More substituted alkene | Normal (e.g. alcoholic KOH) |
| Hofmann | Less substituted alkene | Bulky base or quaternary R₄N⁺ |
Example: 2-bromobutane → ~80% 2-butene : 20% 1-butene (Saytzeff).
Substitution vs Elimination cheat-grid
1° → SN2 loves it. 3° + strong base → E2. 2° → depends on everything.
High temperature → favours elimination (ΔS positive). Bulky strong base (t-BuO⁻) → E2. Strong small nucleophile (CN⁻, I⁻) + polar aprotic → SN2.
| Substrate | Small base | Bulky base |
|---|---|---|
| 1° | SN2 major | E2 |
| 2° | SN2 or E2 | E2 major |
| 3° | SN1/E1 | E2 only |
Haloarenes (Ar-X)
Why unreactive (RIP): Resonance (partial C=X double-bond character, C-Cl ≈ 169 pm), Inert sp² carbon (higher s-character), unstable Phenyl cation. ⇒ ~1000× slower than alkyl halides.
Preparation
| Method | Equation |
|---|---|
| Direct halogenation | $\text{C}_6\text{H}_6 + \text{X}_2 \xrightarrow{\text{Lewis acid}} \text{C}_6\text{H}_5\text{X} + \text{HX}$ |
| Sandmeyer | $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{CuX} \rightarrow \text{C}_6\text{H}_5\text{X} + \text{N}_2$ (X = Cl, Br) |
| Gattermann | $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{HX/Cu} \rightarrow \text{C}_6\text{H}_5\text{X} + \text{N}_2$ |
| Iodobenzene | $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{KI} \rightarrow \text{C}_6\text{H}_5\text{I} + \text{N}_2 + \text{KCl}$; or C₆H₆ + I₂ with HNO₃ |
| Balz-Schiemann (F) | $\text{C}_6\text{H}_5\text{N}_2^+\text{BF}_4^- \xrightarrow{\Delta} \text{C}_6\text{H}_5\text{F} + \text{N}_2 + \text{BF}_3$ |
Lewis acid catalysts: Cl₂ → FeCl₃/AlCl₃; Br₂ → FeBr₃/AlBr₃; I₂ → needs HNO₃ (oxidant).
Reactions
| Reaction | Equation / Note |
|---|---|
| Replacement by OH (industrial) | $\text{C}_6\text{H}_5\text{Cl} + \text{NaOH} \xrightarrow{623\text{ K, 300 atm}} \text{C}_6\text{H}_5\text{OH} + \text{NaCl}$ |
| Reduction | $\text{C}_6\text{H}_5\text{Cl} + \text{H}_2 \xrightarrow{\text{Ni},\ \Delta} \text{C}_6\text{H}_6 + \text{HCl}$ |
| Wurtz-Fittig | $\text{C}_6\text{H}_5\text{Br} + \text{CH}_3\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{-CH}_3 + 2\text{NaBr}$ |
| Fittig | $2\text{C}_6\text{H}_5\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{-C}_6\text{H}_5 + 2\text{NaBr}$ (biphenyl) |
| Grignard | $\text{C}_6\text{H}_5\text{Br} + \text{Mg} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_5\text{MgBr}$ |
SNAr (Activated Nucleophilic Aromatic Substitution)
- Requires electron-withdrawing groups at o/p positions → addition-elimination via Meisenheimer complex.
- Activation order: $\text{-NO}_2 > \text{-CN} > \text{-COCH}_3 > \text{-CHO} > \text{-COOH}$.
- Reactivity: chlorobenzene ≪ 2,4-dinitrochlorobenzene < 2,4,6-trinitrochlorobenzene (picryl chloride).
Directing effect trap
Halogens are ortho/para directing BUT deactivating (“Odd: O/p but Deactivating”). Resonance donation governs orientation; inductive withdrawal slows the ring. Synthesis tip: for meta products use a meta-director (NO₂) first, then introduce halogen.
Boiling Points of Haloarenes
| Compound | BP (°C) |
|---|---|
| Fluorobenzene | 85 |
| Chlorobenzene | 132 |
| Bromobenzene | 156 |
| Iodobenzene | 188 |
Polyhalogen Compounds
Chloroform (CHCl₃, trichloromethane)
| Reaction | Equation | Product/Note |
|---|---|---|
| Industrial prep | $\text{CH}_4 \xrightarrow{\text{Cl}_2/h\nu} \text{CH}_3\text{Cl} \rightarrow \text{CH}_2\text{Cl}_2 \rightarrow \text{CHCl}_3$ | Free-radical; goes to CCl₄ with excess Cl₂ |
| Haloform (lab) | $\text{CH}_3\text{COCH}_3 + 3\text{Cl}_2 + 4\text{NaOH} \rightarrow \text{CHCl}_3 + 3\text{NaCl} + \text{CH}_3\text{COONa} + 3\text{H}_2\text{O}$ | |
| Air + light oxidation | $2\text{CHCl}_3 + \text{O}_2 \xrightarrow{h\nu} 2\text{COCl}_2 + 2\text{HCl}$ | Phosgene (toxic) |
| Reduction | $\text{CHCl}_3 + 6[\text{H}] \xrightarrow{\text{Zn/HCl}} \text{CH}_4 + 3\text{HCl}$ | |
| Carbylamine | $\text{CHCl}_3 + \text{RNH}_2 + 3\text{KOH} \rightarrow \text{RNC} + 3\text{KCl} + 3\text{H}_2\text{O}$ | Test for 1° amine (foul smell) |
| Nitration | $\text{CHCl}_3 + \text{HNO}_3 \rightarrow \text{CCl}_3\text{NO}_2 + \text{H}_2\text{O}$ | Chloropicrin (tear gas) |
Why dark bottles + 1% ethanol (HIGH-YIELD)
Dark bottle blocks light-induced oxidation to phosgene; ethanol neutralises any phosgene formed:
$$\text{COCl}_2 + 2\text{C}_2\text{H}_5\text{OH} \rightarrow (\text{C}_2\text{H}_5\text{O})_2\text{CO} + 2\text{HCl}$$Always state both reasons.
Carbon Tetrachloride (CCl₄)
| Reaction | Equation |
|---|---|
| Hydrolysis (very slow) | $\text{CCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{CO}_2 + 4\text{HCl}$ |
| Swarts (→ Freons) | $\text{CCl}_4 + \text{HF} \xrightarrow{\text{SbF}_3} \text{CCl}_3\text{F}, \text{CCl}_2\text{F}_2, \text{CClF}_3$ |
| From CS₂ (industrial) | $\text{CS}_2 + 3\text{Cl}_2 \xrightarrow{\text{catalyst}} \text{CCl}_4 + \text{S}_2\text{Cl}_2$ |
Freons (CFCs) & Ozone Depletion
| Freon | Formula |
|---|---|
| Freon-11 | CCl₃F |
| Freon-12 | CCl₂F₂ |
| Freon-22 | CHClF₂ |
| Freon-113 | CCl₂F-CClF₂ |
Prep: $\text{CCl}_4 + 2\text{HF} \rightarrow \text{CCl}_2\text{F}_2 + 2\text{HCl}$ (SbF₃).
Ozone-destruction catalytic cycle:
$$\text{CCl}_2\text{F}_2 \xrightarrow{\text{UV}} \text{CClF}_2^\bullet + \text{Cl}^\bullet$$$$\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2$$$$\text{ClO}^\bullet + \text{O}^\bullet \rightarrow \text{Cl}^\bullet + \text{O}_2$$$$\boxed{\text{Net: } \text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2 \quad (\text{Cl}^\bullet = \text{catalyst})}$$One Cl• destroys ~100,000 O₃ molecules. Solution: Montreal Protocol (1987) → HFCs (no Cl, no ozone depletion).
Ozone-depleting potential
No Cl = no depletion. HFC-134a (CH₂FCF₃, 0 Cl) < HCFC-22 (CHClF₂, has C-H → breaks down sooner) < CFC-12 (CCl₂F₂) ≈ CCl₄ (4 Cl, most stable). More Cl + no C-H + higher stability = worse.
DDT & Iodoform
- DDT = (ClC₆H₄)₂CH(CCl₃); prep: $2\text{C}_6\text{H}_5\text{Cl} + \text{CCl}_3\text{CHO} \xrightarrow{\text{H}_2\text{SO}_4} (\text{ClC}_6\text{H}_4)_2\text{CH(CCl}_3) + \text{H}_2\text{O}$.
- Fat-soluble + non-biodegradable → biomagnification (concentration rises up the food chain; thins bird eggshells).
- Iodoform $\text{CH}_3\text{COCH}_3 + 3\text{I}_2 + 4\text{NaOH} \rightarrow \text{CHI}_3\downarrow + 3\text{NaI} + \text{CH}_3\text{COONa} + 3\text{H}_2\text{O}$ — yellow ppt; tests for CH₃CO- / CH₃CH(OH)- groups.
Haloform / iodoform test scope
Positive for compounds with CH₃CO- or CH₃CH(OH)- groups: methyl ketones, acetaldehyde, ethanol (→ CH₃CHO), 2° alcohols of type CH₃CH(OH)-. HCHO does not respond (no CH₃ group).
Compound Summary Table
| Compound | Formula | Key fact | Environmental issue |
|---|---|---|---|
| Chloroform | CHCl₃ | Oxidises to phosgene; store dark + 1% EtOH | Hepatotoxic |
| Carbon tetrachloride | CCl₄ | Non-flammable, very stable | Ozone depleter, toxic |
| Freons | CCl₂F₂ etc. | Inert, volatile refrigerants | Ozone depletion |
| DDT | (ClC₆H₄)₂CH(CCl₃) | Fat-soluble, persistent insecticide | Biomagnification |
| Iodoform | CHI₃ | Yellow solid, antiseptic | Limited use |