Haloalkanes and Haloarenes: Mechanisms, Preparation and Reactivity

Organic Weightage: 3-4 Questions (12-16 Marks) JEE Unit 16

“This unit is where organic mechanism becomes the entire syllabus: almost every question here is really asking 'which pathway operates, and why' rather than 'what is the product.' GOC (Unit 14) and Hydrocarbons (Unit 15) are assumed knowledge throughout — carbocation/radical stability, resonance, hyperconjugation, and the EAS directing-effect rules are used here without re-explanation, just applied to a new functional group. The throughline worth holding onto: a haloalkane's C–X carbon is sp³ and genuinely reactive (SN1/SN2/E1/E2 all apply), while a haloarene's or vinyl halide's C–X carbon is sp² and resists all four under normal conditions — almost every trap in this chapter traces back to forgetting that one structural fact.”

— SCORECHEM ACADEMIC TEAM

1. Classification, Nomenclature and Optical Isomerism

Fig. 1: SN2/SN1 reactivity tracks hybridisation at the C–X carbon — sp3 allylic/benzylic halides are the MOST reactive of all (resonance-stabilised transition state/cation), while vinylic and aryl halides (sp2 C–X) resist both mechanisms entirely.

Classification by the C–X carbon's environment: alkyl halides (X on sp³ C: 1°, 2°, 3°), allylic halides (X on sp³ C adjacent to C=C), benzylic halides (X on sp³ C attached to an aromatic ring), vinylic halides (X directly on a C=C, sp²), and aryl halides (X directly on the aromatic ring, sp²). This classification is the single most load-bearing fact in the whole unit — it predicts SN1/SN2/E1/E2 reactivity before you even look at the specific reagent.
Allylic and benzylic are the fastest of all, for both SN1 (the resulting cation is resonance-stabilised by the adjacent π system) and SN2 (the developing p-orbital in the transition state is stabilised by conjugation with that same π system) — this is why C₆H₅CH₂Br reacts faster in SN2 than CH₃CH₂Br, a genuinely counter-intuitive JEE favourite.
Vinylic and aryl halides are inert to both SN1 and SN2 under ordinary conditions: SN1 fails because a vinyl/aryl cation (empty sp² orbital in the plane of, or part of, the π system) is far too unstable to form; SN2 fails because the C–X carbon's partial double-bond character and the ring/π-cloud physically block backside attack.
Optical isomerism in alkyl halides: a halide with four different groups on the C–X carbon is chiral. When counting how many structural isomers of a given molecular formula give an optically active substitution product, check each candidate's product (not the starting halide) for a stereocentre — JEE regularly asks this as a "count the isomers" numerical built on C₅H₁₁Br-type formulas.
Nomenclature: halogens are cited as substituent prefixes (chloro-, bromo-, iodo-, fluoro-) with the lowest locant set to all substituents together, not to the halogen alone; common names (chloroform, vinyl chloride, allyl chloride, benzyl chloride) remain expected alongside IUPAC names.

⚠️ JEE Trap: "C–Cl bond is weaker in vinyl chloride than in ethyl chloride" is FALSE — it's the other way round. The sp² carbon in CH₂=CHCl has more s-character than the sp³ carbon in CH₃CH₂Cl, AND resonance donation from Cl's lone pair into the π system adds partial double-bond character — both effects make the vinylic C–Cl bond shorter and stronger, not weaker.

2. Methods of Preparation

Fig. 6: Haloarenes can't be made by SN1/SN2 (inert sp2 C–X), so chemists use EAS for Cl/Br directly on benzene, and go through a diazonium salt intermediate for the two halogens EAS can't deliver cleanly — F (Balz-Schiemann) and I (KI).

From alcohols (the main haloalkane route): R-OH + HX, or + PX₃/PX₅, or + SOCl₂ (thionyl chloride, the Darzens process). SOCl₂ is the preferred lab reagent because both by-products (SO₂ and HCl) are gases that simply escape, leaving pure alkyl chloride with no aqueous work-up needed.
From alkanes and alkenes (recap from Units 14–15): free-radical halogenation of alkanes (statistical + reactivity-weighted mixtures) and electrophilic/radical addition of HX or X₂ to alkenes (Markovnikov or anti-Markovnikov/Kharasch, as covered in Unit 15) both remain valid haloalkane routes here.
Finkelstein reaction: R-Cl/R-Br + NaI in dry acetone (SN2) → R-I. NaI is soluble in dry acetone while the NaCl/NaBr by-product precipitates out, pulling the equilibrium forward by Le Chatelier's principle; this works cleanly for 1° and 2° substrates.
Swarts reaction: R-Cl/R-Br heated with a metallic fluoride (AgF, Hg₂F₂, CoF₂, or SbF₃) → R-F. This remains the standard lab route to alkyl fluorides, since direct fluorination of alkanes with F₂ is far too violently exothermic to control.
Haloarenes, Route A — direct electrophilic halogenation: benzene + X₂ (X = Cl, Br) with a Lewis-acid catalyst (FeCl₃, FeBr₃) generates the electrophile X^⊕ for standard EAS. This only works cleanly for Cl₂ and Br₂ — F₂ reacts too violently and I₂ is reversible (needs an oxidiser like HIO₄ or HNO₃ to destroy the HI by-product and drive the reaction forward).
Haloarenes, Route B — via a diazonium salt: aniline → (NaNO₂/HCl, 273–278 K) → benzenediazonium chloride, which is then converted: CuCl/CuBr (Sandmeyer) or Cu powder/HX (Gattermann) → ArCl/ArBr; HBF₄ then heat (Balz–Schiemann) → ArF; KI(aq) directly → ArI. This diazonium route is the ONLY reliable lab path to aryl fluorides and aryl iodides.

⚠️ JEE Trap: counting "how many of these can be made by Sandmeyer's reaction" means counting only the Cl/Br targets. If the question lists a mix of aryl chlorides, bromides, fluorides and iodides among the "products," only the chlorides and bromides are genuine Sandmeyer products — fluorides need Balz–Schiemann and iodides need direct KI, neither of which is "Sandmeyer's reaction."

3. Physical Properties

Boiling point trend by halogen: R-I > R-Br > R-Cl > R-F for a given R group, tracking increasing polarizability and van der Waals attraction down the halogen group (not electronegativity, which runs the opposite way).
Boiling point trend by chain shape: straight-chain isomers boil higher than their branched isomers of the same formula, because branching reduces surface-area contact and therefore van der Waals forces — the identical logic used for alkane boiling points in Unit 15.
Dihalobenzene melting points: the para-isomer melts far higher than the ortho- or meta-isomer of the same dihalobenzene, because its symmetric shape packs into the crystal lattice much more efficiently — melting point here is a crystal-packing effect, not a bond-strength effect.
Density: chloro-, bromo-, and iodoalkanes are denser than water once there are enough halogen atoms per carbon (polyhalogenated or iodo-/bromo-substituted compounds especially); density increases with the number and atomic mass of halogens present.
Water solubility: haloalkanes are only sparingly soluble in water (the molecule can accept but not donate a hydrogen bond to water), despite having a net dipole moment — the same asymmetry seen with ethers in the GOC unit.

4. Nucleophilic Substitution: SN1 and SN2 Mechanisms

Fig. 2: Same starting halide, opposite outcomes — SN2's single concerted step forces clean backside attack (inversion, no rearrangement); SN1's free carbocation intermediate is attackable from both faces (racemisation) and is prone to hydride/alkyl shifts.

SN2 (bimolecular): a single concerted step — the nucleophile attacks the carbon from the face directly opposite the leaving group as X departs, through a pentacoordinate transition state. Rate = k[RX][Nu^⊖] (second order). Reactivity: CH₃X > 1° > 2° >> 3° (essentially inert), governed purely by steric access to the back face.
SN1 (unimolecular): two steps — slow ionisation of R-X to a planar sp² carbocation (the rate-determining step), followed by fast attack of the nucleophile on either face of that cation. Rate = k[RX] (first order, independent of nucleophile concentration). Reactivity: 3° > 2° > 1° > CH₃X, governed by carbocation stability.
Stereochemical signature is the main way JEE distinguishes the two: SN2 gives 100% Walden inversion (complete, clean inversion of configuration at the reacting carbon, every time); SN1 gives racemisation (the flat carbocation is attacked from both faces, though inversion slightly exceeds retention in practice due to incomplete separation of the leaving ion pair — "mostly racemic" rather than perfectly 50:50).
Only SN1's carbocation intermediate allows rearrangement (1,2-hydride or 1,2-alkyl shifts to a more stable cation); SN2's single concerted step never passes through a free carbocation, so rearrangement is structurally impossible there.

Fig. 5: The three sp3 substituents flip through the carbon like an umbrella turning inside out in a gust of wind — the product's configuration is the mirror image of the substrate's at that carbon, every single time (100% inversion, never partial).

⚠️ JEE Trap: "SN1 reactions are stereospecific" is FALSE — that description belongs to SN2. SN2 is stereospecific (one defined stereochemical outcome: complete inversion). SN1 is stereoselective at best and typically gives a racemic (or near-racemic) mixture because the flat carbocation offers no preferred face for attack — a two-statement question swapping these two descriptions is one of the most repeated traps in this unit.

5. Factors Governing Substitution: Nucleophile, Solvent, Substrate

Fig. 4: The SAME halide ions reverse order depending on solvent — in water/alcohols small ions are solvation-caged and slow (I⊖ wins); in DMSO/acetone nothing cages them, so raw basicity decides (F⊖ wins). Always check the solvent before ranking nucleophiles.

Nucleophilicity trends depend entirely on the solvent — this is the single fact that resolves most "rank these nucleophiles" questions. In a polar PROTIC solvent (water, alcohols), small/basic ions get H-bond solvated and slowed, so nucleophilicity runs I^⊖ > Br^⊖ > Cl^⊖ > F^⊖ (tracks polarizability/size). In a polar APROTIC solvent (DMSO, DMF, acetone), nothing cages the anion, so nucleophilicity tracks raw basicity instead: F^⊖ > Cl^⊖ > Br^⊖ > I^⊖.
General nucleophilicity rules that hold regardless of solvent: anions beat their neutral conjugate acids (OH^⊖ > H₂O; OR^⊖ > ROH); nucleophilicity falls across a period with rising electronegativity (CH₃^⊖ > NH₂^⊖ > OH^⊖ > F^⊖); bulky nucleophiles are weaker than their less-hindered analogues (t-BuO^⊖ < EtO^⊖) because approach to the carbon is sterically slowed.
Leaving group ability tracks weak-base character: I^⊖ > Br^⊖ > Cl^⊖ > F^⊖, and more generally CF₃SO₃^⊖ > TsO^⊖ > RCOO^⊖ > PhO^⊖ > OH^⊖ — a weaker base is more stable on its own and therefore leaves more readily.
Ambident nucleophiles can attack through either of two atoms, and the counter-ion changes which atom attacks: NaCN (ionic, free CN^⊖) gives the cyanide R-CN (C-attack forms the stronger C–C bond); AgCN (covalent, Ag holds the softer C end) gives the isocyanide R-NC (only the N lone pair is free to attack). The same logic gives KNO₂ → alkyl nitrite (R-O-N=O) vs AgNO₂ → nitroalkane (R-NO₂).
The Darzens process is a mechanism-switching trap: R-OH + SOCl₂ ALONE proceeds with retention of configuration (an internal S_Ni mechanism via a chlorosulfite intermediate, no free chloride to attack from the back). R-OH + SOCl₂ + pyridine proceeds with inversion (pyridine mops up the HCl by-product into pyridinium chloride, releasing a free Cl^⊖ that then attacks via ordinary SN2).

⚠️ JEE Trap: the same halide ion can be ranked in opposite orders depending on whether the solvent is protic or aprotic. A question giving "F^⊖, Cl^⊖, Br^⊖, I^⊖ in methanol" wants the protic order (I^⊖ wins); the same four ions "in DMSO" want the aprotic order (F^⊖ wins). Always read which solvent is named before ranking.

6. Elimination Reactions: E1, E2, E1cb and the Zaitsev/Hofmann Split

Fig. 3: The two axes that decide everything — does a carbocation form (rearrangement risk, racemisation) and is the base/nucleophile bulky or unhindered (Zaitsev vs Hofmann, SN2 vs E2)? JEE loves testing both at once.

E1 (unimolecular elimination): two steps via the same carbocation intermediate as SN1 — ionisation, then loss of a β-hydrogen to form the alkene. Competes directly with SN1 at the same substrate; higher temperature favours elimination (E1) over substitution (SN1).
E2 (bimolecular elimination): a single concerted step requiring an anti-periplanar arrangement between the β-hydrogen and the leaving group X, with a strong base removing that H as X departs. Rate = k[RX][Base], competing directly with SN2 at the same substrate; a strong, bulky base favours elimination (E2) over substitution (SN2).
E1cb (elimination, unimolecular conjugate base): the rare third mechanism, seen when the substrate carries a strong electron-withdrawing group that makes the β-hydrogen unusually acidic, combined with a poor leaving group (such as –F or –OH). The base removes the acidic proton FIRST, forming a carbanion intermediate that only then expels the leaving group.

Fig. 8: Same substrate, same reaction type (E2) — swap a small base for a bulky one and the product flips from the more-substituted (Zaitsev) to the less-substituted (Hofmann) alkene, because steric bulk decides which β-H the base can actually reach.

Zaitsev (Saytzeff) product — the more-substituted, more stable alkene: favoured by small, unhindered bases (OH^⊖, EtO^⊖, MeO^⊖) that can reach any β-hydrogen and simply pick the pathway giving the thermodynamically more stable (more hyperconjugated) alkene.
Hofmann product — the less-substituted, terminal alkene: forced by either (1) a sterically bulky base (t-BuOK, LDA, Et₃N) that can physically only reach the least-hindered β-hydrogen, or (2) a poor leaving group (such as –F) that induces E1cb-like carbanion character, favouring removal of the most acidic (least substituted, least stabilised-by-hyperconjugation) proton instead.

⚠️ JEE Trap: substitution and elimination are always in competition on the SAME substrate. A strong, small, unhindered nucleophile/base pushes toward SN2/E2 Zaitsev product mixtures; a weak, bulky base pushes toward E2 Hofmann; heat generally favours elimination over substitution at a given substrate (entropy favours the extra molecule produced). A question is rarely testing "substitution OR elimination" in isolation — it's testing which one dominates under the stated conditions.

7. Why Haloarenes Resist Substitution (and When They Don't)

Fig. 7: In chlorobenzene, one of chlorine's lone pairs conjugates into the aromatic π system — shortening and strengthening the C–Cl bond (giving it partial double-bond character) and spreading the negative charge into the ring, which is exactly why haloarenes resist both SN1 and SN2.

The structural reason haloarenes resist SN1/SN2: one of chlorine's lone pairs conjugates into the aromatic π system, giving the C–Cl bond partial double-bond character. This shortens and strengthens the bond (1.69 Å in chlorobenzene vs ~1.78–1.80 Å in a typical haloalkane) and spreads the negative charge into the ring rather than leaving it localised on chlorine — both effects work against ionisation (no SN1) and the ring's electron density blocks backside approach (no SN2).
The same resonance makes the C–Cl bond in chlorobenzene LESS polar than in a haloalkane, even though it is shorter and stronger — resonance donation partially cancels the inductive electron-withdrawal, reducing net charge separation across the bond. "Shorter/stronger" and "less polar" are not contradictory here; they share the same cause.
EAS on haloarenes keeps the same deactivating-but-o/p-directing behaviour established in Unit 15: a halogen withdraws electron density inductively overall (net deactivation, slower than benzene) but donates it back by resonance specifically at the ortho and para positions (hence o/p-directing). Nitration, sulphonation, and halogenation of chlorobenzene all give predominantly ortho/para products.
Friedel–Crafts reactions still work on haloarenes (unlike on the strongly deactivated rings from Unit 15, such as nitrobenzene) — a halogen is only weakly deactivating, so alkylation/acylation proceeds, just more slowly than on benzene itself.
Haloarenes are NOT permanently inert to nucleophilic substitution — under forcing conditions it does happen, via an addition–elimination (benzyne or Meisenheimer-intermediate) mechanism rather than SN1/SN2: chlorobenzene converts to phenol with NaOH at 623 K and 300 atm (Dow's process), while 2,4-dinitrochlorobenzene reacts with aqueous NaOH readily even at room temperature because the ortho/para –NO₂ groups resonance-stabilise the anionic (Meisenheimer) intermediate that forms before chloride leaves.

⚠️ JEE Trap: "chlorobenzene never undergoes nucleophilic substitution" overstates the real rule. It resists SN1/SN2 under ORDINARY conditions, but strong electron-withdrawing groups ortho/para to the halogen (as in 2,4-dinitrochlorobenzene) or forcing industrial conditions (Dow's process) switch it on via a completely different addition–elimination mechanism — a two-statement question can legitimately call this reaction correct while still calling chlorobenzene "resistant to nucleophilic substitution" in general.

8. Reactions with Metals: Wurtz, Wurtz-Fittig, Fittig and Grignard

Wurtz reaction (recap from Unit 15): 2 R-X + 2 Na (dry ether) → R-R + 2 NaX. Fails for 3° halides (elimination dominates over coupling due to steric hindrance at the 3° carbon) and gives a statistical mixture of three products when two different alkyl halides are coupled together, so it is only a clean synthesis when used with a single, non-3° halide.
Wurtz–Fittig reaction: an aryl halide + an alkyl halide + Na in dry ether couples unsymmetrically to give an alkylbenzene (Ar–R). This is the standard route to introduce an alkyl chain onto a benzene ring via a pre-formed haloarene.
Fittig reaction: two molecules of an aryl halide + Na in dry ether couple symmetrically to give a biaryl (Ar–Ar), such as biphenyl from bromobenzene.
Grignard reagent formation: R-X + Mg turnings in dry ether → R-MgX (an organomagnesium halide). The reaction and the reagent are both extremely moisture- and protic-group sensitive — any trace of water, or any –OH/–NH/–COOH/–SH group elsewhere in the same molecule, destroys the Grignard reagent by simple acid-base protonation before it can be used.
Core Grignard reactivity (full carbonyl chemistry is Unit 17, but the general pattern belongs here): R-MgX + H₂O → R-H + Mg(OH)X (this is why "dry" conditions are non-negotiable); R-MgX + CO₂ (dry ice), then H₃O^⊕ → R-COOH (a one-carbon chain extension to a carboxylic acid) — a favourite numerical set-up (identify the gas from a known molar volume, then compute the mass of the final acid).

⚠️ JEE Trap: Wurtz coupling of two DIFFERENT alkyl halides is a messy mixture, not a clean synthesis. 2 R-X + 2 R'-X + 4 Na gives R-R, R'-R', AND R-R' all together (a statistical mixture), so Wurtz is only reliable for making a symmetrical R-R product from a single halide — a reaction scheme proposing Wurtz coupling of two different halides as a clean route to one specific cross-product has already gone wrong.

9. Polyhalogen Compounds and Environmental Chemistry

Chloroform (CHCl₃): slowly oxidised by air and light to the highly toxic gas phosgene (COCl₂), so chloroform is stored in dark, amber bottles completely filled (to exclude air) and containing about 1% ethanol, which converts any phosgene formed into harmless diethyl carbonate. Being covalent, pure chloroform gives no precipitate with AgNO₃.
Carbylamine reaction: a primary amine (R-NH₂) + CHCl₃ + alcoholic KOH → a foul-smelling isocyanide (R-NC), via a dichlorocarbene (:CCl₂) intermediate — this reaction is specific to 1° amines and is often used as a qualitative test for them.
Reimer–Tiemann reaction: phenol + CHCl₃ + KOH → salicylaldehyde, again via the same dichlorocarbene intermediate attacking the phenoxide ring (this reaction properly belongs with phenol chemistry in Unit 17, but the CHCl₃-derived carbene mechanism is identical to the carbylamine reaction above, so JEE often tests both together).
Carbon tetrachloride (CCl₄): once a widely used solvent and dry-cleaning agent, now restricted because it is a potent ozone-depleting substance and forms toxic phosgene when it contacts flame, ending its historical use in fire extinguishers (pyrene).
Iodoform (CHI₃): a pale-yellow solid formed by the haloform reaction (I₂/NaOH acting on ethanol, acetaldehyde, or any compound containing a CH₃-CO- or CH₃-CH(OH)- group); historically used as an antiseptic, and its formation (the iodoform test) is a standard diagnostic for that specific structural fragment.
Freons (chlorofluorocarbons): named Freon-cba where c = (number of C atoms − 1), b = (number of H atoms + 1), a = (number of F atoms) — e.g. Freon-12 is CCl₂F₂. Chemically inert and non-flammable (ideal as refrigerants/aerosol propellants historically), but in the upper atmosphere UV light photolyses the C–Cl bond to release chlorine radicals that catalytically destroy stratospheric ozone, which is why CFC production is now heavily restricted.
DDT (dichlorodiphenyltrichloroethane): a historically important insecticide; non-biodegradable and fat-soluble, so it accumulates in fatty tissue and biomagnifies up the food chain, which is why its agricultural use is now banned or heavily restricted in most countries (though it is still used in limited, controlled settings for malaria-vector control).

10. Laboratory Identification Tests

Beilstein test (general halogen presence): heat a small sample on a clean copper wire in a Bunsen flame — a green flash confirms the presence of a halogen (via volatile copper halide formation), but does not distinguish which halogen or which class of halide is present.
Alcoholic AgNO₃ test (distinguishes alkyl from vinylic/aryl halides): warming with alcoholic AgNO₃ gives a precipitate (AgCl white, AgBr pale yellow, AgI yellow) from an alkyl halide, via Ag^⊕-assisted ionisation; a vinylic or aryl halide gives NO precipitate even on prolonged heating, since there is no accessible ionisation pathway. The RATE of precipitate formation among alkyl halides also tracks SN1 character: 3° > 2° > 1°.
Iodoform test: warming with I₂/NaOH; a yellow precipitate (CHI₃) confirms a CH₃-CO- or CH₃-CH(OH)- fragment is present somewhere in the molecule (see Section 9).

11. How JEE Frames These Questions

"Count the optically active products" numericals: given several structural isomers of a haloalkane treated with aqueous KOH (SN2, no rearrangement), check EACH resulting alcohol individually for a stereocentre (four different groups on one carbon) — the starting halide's own chirality is not what's being asked about.
Rate-ordering questions across a mixed set of substrates: first identify which mechanism is specified (SN1 or SN2) — the correct ranking reverses completely depending on which one the question names, so never apply a memorised order without checking the mechanism label first.
"How many can be made by Sandmeyer's reaction" counting questions: filter the list down to only the aryl chlorides and bromides before counting (see Section 2's trap box).
Assertion-reason on benzylic/allylic SN2 rate enhancement: the correct reasoning is resonance/conjugation stabilisation of the developing transition state's partial p-character, not simple inductive or steric effects — a reason choice invoking the wrong mechanism (e.g. "steric hindrance is reduced") is a common distractor even when the assertion itself is true.
C–X bond-character comparison questions (haloalkane vs haloarene, or vinylic vs alkylic): work through bond length, bond strength, hybridization, and bond polarity as four SEPARATE properties — a statement can get three right and the fourth (usually polarity) backwards, since polarity does not simply track bond strength here.
Multi-step "identify A, B, C" sequences mixing alkene, haloalkane, and haloarene chemistry: name the specific mechanism operating at each individual step (EAS, SN1, SN2, Sandmeyer, Grignard) before assigning a structure, exactly as emphasised for Unit 15's sequences.

⚠️ JEE Trap: a Grignard-based numerical is almost always a two-stage identification, not a direct calculation. Identify the gas/product from its given molar mass or molar volume FIRST (often via the ideal gas law or vapour density), name the specific compound that must be, and only then compute whatever the question actually asks — skipping straight to arithmetic without first pinning down the structure is where marks are lost.

12. Quick Sheet and Checklist

Concept	Key Fact
SN2 reactivity	CH₃X > 1° > 2° >> 3° (steric); 100% Walden inversion; rate = k[RX][Nu]
SN1 reactivity	3° > 2° > 1° > CH₃X (cation stability); racemisation; rate = k[RX]
Allylic/benzylic	Fastest of all for BOTH SN1 (stable cation) and SN2 (stabilised TS via resonance)
Vinylic/aryl halides	Inert to SN1 (no stable cation) and SN2 (blocked backside, partial double bond)
Nucleophilicity (protic solvent)	I^⊖ > Br^⊖ > Cl^⊖ > F^⊖ (polarizability, less solvated)
Nucleophilicity (aprotic solvent)	F^⊖ > Cl^⊖ > Br^⊖ > I^⊖ (basicity, unsolvated)
Leaving group ability	I^⊖ > Br^⊖ > Cl^⊖ > F^⊖ (weaker base leaves more easily)
Darzens process	SOCl₂ alone → retention (SNi); SOCl₂ + pyridine → inversion (SN2)
Zaitsev vs Hofmann	Small base → Zaitsev (more substituted); bulky base or poor LG → Hofmann (less substituted)
Haloarene prep	Cl₂/Br₂+FeX₃ direct on benzene; F via Balz–Schiemann; I via KI on diazonium
Sandmeyer/Gattermann	CuCl/CuBr or Cu+HX on Ar-N₂^⊕ → ArCl/ArBr ONLY
Chlorobenzene C–Cl	Shorter (1.69 Å), stronger, LESS polar than a haloalkane C–Cl (resonance)
Freon naming	Freon-cba: c = C−1, b = H+1, a = F

Before the exam, check you can:

Classify any halide (1°/2°/3°/allylic/benzylic/vinylic/aryl) on sight and predict its SN1/SN2/E1/E2 reactivity from that classification alone.
State the SN1 and SN2 reactivity orders separately and explain why they run in opposite directions.
Explain Walden inversion mechanistically (the umbrella-flip trigonal bipyramidal transition state) and contrast it with SN1 racemisation.
Rank a given set of halide-ion nucleophiles correctly in BOTH a protic and an aprotic solvent.
Distinguish the four preparation routes to haloarenes (direct EAS, Sandmeyer, Gattermann, Balz–Schiemann, KI) and know exactly which halogen each one delivers.
Explain, from resonance, why chlorobenzene's C–Cl bond is shorter, stronger, AND less polar than an alkyl chloride's.
State the one condition (strong o/p electron-withdrawing groups, or forcing industrial conditions) under which haloarenes DO undergo nucleophilic substitution.
Distinguish Wurtz, Wurtz–Fittig, and Fittig reactions by which two fragments they couple.
Name the storage protocol for chloroform and explain the chemistry behind it (phosgene formation and its prevention).
Explain how Freons deplete stratospheric ozone and why DDT's persistence makes it an environmental hazard distinct from its toxicity.