Chapters

Everything NCERT Class 12 Unit 6 (Haloalkanes and Haloarenes) says that can be asked, with the haloarene part of the unit in the last numbered section, plus what JEE Main adds beyond the book (marked Beyond NCERT). The short note stays for the last week; this is for a proper revision pass.

Formula sheet

Every order and rule in the chapter on one screen, for the last pass. Details and examples are in the sections below.

Rule	Order or result	Watch
C–X bond length	C–F < C–Cl < C–Br < C–I	Longer bond, weaker bond
C–X bond enthalpy	C–F > C–Cl > C–Br > C–I
Dipole moment (CH₃X)	CH₃Cl > CH₃F > CH₃Br > CH₃I	Cl tops F: F's bond is too short
Boiling point, same R	RI > RBr > RCl > RF	Branching lowers it
Density	rises with C atoms, X atoms, X mass	RBr, RI, CHCl₃ sink in water
HX with alcohols	3° > 2° > 1°	1°, 2° with HCl need ZnCl₂
Leaving group	I⁻ > Br⁻ > Cl⁻ >> F⁻	Same in SN1 and SN2
SN2 rate	CH₃X > 1° > 2° > 3°	Steric; inversion
SN1 rate	3° > 2° > 1° > CH₃X	Carbocation; racemisation
SN1 rate law	rate = k[RX]	First order
SN2 rate law	rate = k[RX][Nu⁻]	Second order
Elimination	Saytzeff: more substituted alkene	Bulky base: less substituted · Beyond NCERT
Ambident	KCN → R–CN; AgCN → R–NC	KNO₂ → R–ONO; AgNO₂ → R–NO₂
Haloarene SN	needs –NO₂ at o/p	m-NO₂ does nothing
Haloarene EAS	deactivating, o/p-directing	–I sets the rate, +R the place

0 · What JEE Main asks from this chapter

102 questions in JEE Main 2019–2026, about 1 per paper, nearly all MCQ (4 numerical). Grouped by type (counts are approximate):

Type	Share	Typical question
SN1/SN2: rate order, mechanism, stereochemistry, solvent	~35%	Order of SN1 rate for four bromides (2023)
Elimination, and substitution vs elimination	~18%	1-Bromo-1-methylcyclopentane + OH⁻/C₂H₅OH → ? (2024)
Multi-step sequences (alc KOH, HBr, Br₂, NaNH₂, HgSO₄)	~15%	Optically active C₄H₉Br → … → test for D (2026)
Preparation and named reactions	~12%	Match Finkelstein, Swarts, Sandmeyer, Fittig to reagents (2026)
Ambident nucleophiles and the AgNO₃ test	~12%	1-Bromopropane + AgNO₂ and + AgCN (2025)
Classification, physical properties, polyhalogen uses	~10%	Match CCl₄, CH₂Cl₂, DDT, freons to uses (2024)

Most are statement or order questions. A numerical is usually a yield or an atom count: work in moles.

1 · Classification

By number of halogens: mono-, di-, tri-, tetra- (poly)halogen compounds.

By the carbon holding X:

Class	X is on	Example
Alkyl (haloalkane)	sp³ C of an alkyl group; CₙH₂ₙ₊₁X	CH₃CH₂Cl
→ 1°, 2°, 3°	a 1°, 2° or 3° carbon	(CH₃)₃CBr is 3°
Allylic	sp³ C next to C=C	CH₂=CH–CH₂Br
Benzylic	sp³ C attached to a ring	C₆H₅CH₂Cl
Vinylic	sp² C of C=C	CH₂=CHCl
Aryl	sp² C of an aromatic ring	C₆H₅Cl

Allylic and benzylic halides can also be 1°, 2° or 3° (C₆H₅CH(CH₃)Cl is 2° benzylic).

Example (2024) · "CH₂=CH–CH₂Cl is an allyl halide. Reason: in allyl halides X is on an sp² carbon." Assertion true, reason false: the allylic carbon is sp³; sp² is vinylic.

2 · Nomenclature

Common names: alkyl group + halide. IUPAC: halo-substituted hydrocarbon. Dihalides with the same halogen: gem (both on one carbon; common name alkylidene halide) and vic (adjacent carbons; alkylene dihalide). IUPAC calls both dihaloalkanes: CH₃CHCl₂ is 1,1-dichloroethane (ethylidene chloride), ClCH₂CH₂Cl is 1,2-dichloroethane (ethylene dichloride).

Structure	Common	IUPAC
CH₃CH₂CH(Cl)CH₃	sec-butyl chloride	2-chlorobutane
(CH₃)₃CCH₂Br	neopentyl bromide	1-bromo-2,2-dimethylpropane
(CH₃)₃CBr	tert-butyl bromide	2-bromo-2-methylpropane
CH₂=CHCl	vinyl chloride	chloroethene
CH₂=CHCH₂Br	allyl bromide	3-bromopropene
CH₂Cl₂	methylene chloride	dichloromethane
CHCl₃, CHBr₃	chloroform, bromoform	trichloro-, tribromomethane
CCl₄	carbon tetrachloride	tetrachloromethane
C₆H₅CH₂Cl	benzyl chloride	chlorophenylmethane
o-ClC₆H₄CH₃	o-chlorotoluene	1-chloro-2-methylbenzene

For benzene derivatives, common names use o-, m-, p-; IUPAC uses 1,2-, 1,3-, 1,4-.

Example (NCERT Example 6.1) · C₅H₁₁Br has 8 structural isomers. 1°: 1-bromopentane, 1-bromo-3-methylbutane, 1-bromo-2-methylbutane, 1-bromo-2,2-dimethylpropane. 2°: 2-bromopentane, 3-bromopentane, 2-bromo-3-methylbutane. 3°: 2-bromo-2-methylbutane. (C₄H₉Br, exercise 6.6, has 4.)

3 · Nature of the C–X bond

X is more electronegative than C, so the bond is polar: Cδ+–Xδ−. Down the group the halogen grows, so the bond gets longer and weaker.

Bond	Length (pm)	Enthalpy (kJ mol⁻¹)
CH₃–F	139	452
CH₃–Cl	178	351
CH₃–Br	193	293
CH₃–I	214	234

Dipole moments (D): CH₃F 1.847, CH₃Cl 1.860, CH₃Br 1.830, CH₃I 1.636. CH₃Cl is highest: F has the larger charge but the shorter bond.

Example (NCERT exercise 6.4) · Highest dipole moment among CH₂Cl₂, CHCl₃, CCl₄: CH₂Cl₂. CCl₄ is symmetrical (0); in CHCl₃ the three C–Cl dipoles partly cancel.

4 · Preparation of haloalkanes

From alcohols (best route: alcohols are easy to get). The –OH is replaced by X:

• SOCl₂ is preferred: both by-products are gases, so the alkyl chloride comes out pure.
• 1° and 2° alcohols need ZnCl₂ with HCl (or dry HCl gas through the alcohol); a 3° alcohol only needs shaking with conc. HCl at room temperature.
• Reactivity with a given HX: 3° > 2° > 1°.
• Bromides: constant-boiling HBr (48%). Iodides: NaI or KI in 95% H₃PO₄, not H₂SO₄, which oxidises HI to I₂ (intext 6.2).
• PBr₃ and PI₃ are made in situ from red P and Br₂ or I₂.
• None of these work on phenol: its C–O bond has partial double-bond character.

From alkanes (free radical, Cl₂ or Br₂ with light or heat): gives a mixture of isomeric mono- and polyhalides, so the yield of any one is low.

Example (NCERT Example 6.3) · (CH₃)₂CHCH₂CH₃ has 4 kinds of H, so 4 monochlorides: (CH₃)₂CHCH₂CH₂Cl, (CH₃)₂CHCHClCH₃, (CH₃)₂CClCH₂CH₃, ClCH₂CH(CH₃)CH₂CH₃.

Example (intext 6.4, exercise 6.5) · Of the C₅H₁₂ isomers, neopentane gives 1 monochloride, n-pentane 3, isopentane 4. C₅H₁₀ that doesn't react with Cl₂ in the dark but gives a single C₅H₉Cl in sunlight: cyclopentane.

Example (2026) · n-Butane → optically active monochloride P = 2-chlorobutane. Chlorinating P again (ignore stereoisomers): Cl on C1 (1,2-), C2 (2,2-), C3 (2,3-), C4 (1,3-) → 4 dichlorides.

Relative rates of H abstraction · Beyond NCERT · Chlorination per H, 3° : 2° : 1° ≈ 5 : 3.8 : 1. Multiply by the number of each H. Propane: 6 × 1 = 6 (1-chloro) and 2 × 3.8 = 7.6 (2-chloro), so 2-chloropropane ≈ 7.6/13.6 = 56%. Bromination is far more selective (almost only the 3° or 2° product).

From alkenes:

• HX addition: Markovnikov (H goes to the carbon with more H). Propene + HBr → mainly 2-bromopropane. With peroxide, HBr (only HBr) adds anti-Markovnikov → 1-bromopropane.
• X₂ addition: Br₂ in CCl₄ gives a colourless vic-dibromide. The loss of the red-brown colour is the lab test for C=C.

Halogen exchange:

• Finkelstein: NaCl or NaBr is insoluble in dry acetone and precipitates, which drives the reaction forward (Le Chatelier).
• Swarts is the best way to alkyl fluorides.

Allylic bromination with NBS · Beyond NCERT · N-bromosuccinimide with light or peroxide puts Br on the allylic (or benzylic) carbon and leaves the C=C alone: cyclohexene → 3-bromocyclohexene.

Example (2021) · 4-Bromostyrene + HBr with benzoyl peroxide → anti-Markovnikov: 1-bromo-4-(2-bromoethyl)benzene. Then CoF₂ (Swarts) replaces only the alkyl Br → 1-bromo-4-(2-fluoroethyl)benzene. The aryl Br stays.

5 · Physical properties

• Pure alkyl halides are colourless; bromides and iodides colour in light. Many volatile ones smell sweet.
• CH₃Cl, CH₃Br, C₂H₅Cl and some chlorofluoromethanes are gases at room temperature; higher members are liquids or solids.
• Boiling points are well above those of hydrocarbons of similar mass: polarity (dipole–dipole) plus larger mass (van der Waals).
  • Same alkyl group: RI > RBr > RCl > RF.
  • Larger chain: higher. CH₃CH₂CH₂I > CH₃CH₂I > CH₃I (2026).
  • Isomers: branching lowers the boiling point. Of the C₄H₉Br isomers, 2-bromo-2-methylpropane boils lowest.
• Dihalobenzenes: boiling points nearly equal, but the para isomer melts highest: it is symmetrical and packs better in the crystal.
• Solubility: very slight in water. Breaking water's H-bonds costs more energy than the new haloalkane–water attractions give back. They dissolve well in organic solvents, where the attractions broken and made are of similar strength.
• Density rises with the number of C atoms, the number of X atoms and the mass of X. Bromo, iodo and polychloro compounds are denser than water.

Compound	Density (g mL⁻¹)
n-C₃H₇Cl	0.89
n-C₃H₇Br	1.335
n-C₃H₇I	1.747
CH₂Cl₂	1.336
CHCl₃	1.489
CCl₄	1.595

Example (intext 6.6) · Increasing boiling point: chloromethane < bromomethane < dibromomethane < bromoform. And isopropyl chloride < 1-chloropropane < 1-chlorobutane.

6 · Nucleophilic substitution

A nucleophile (electron-rich) attacks the δ+ carbon; X leaves as X⁻ (the leaving group).

Reagent	Product	Class
NaOH or KOH (aq), H₂O	R–OH	Alcohol
NaOR′	R–O–R′	Ether
NaI	R–I	Alkyl iodide
NH₃	R–NH₂	1° amine
R′NH₂	R–NHR′	2° amine
R′R″NH	R–NR′R″	3° amine
KCN	R–CN	Nitrile (cyanide)
AgCN	R–NC	Isonitrile (isocyanide)
KNO₂	R–O–N=O	Alkyl nitrite
AgNO₂	R–NO₂	Nitroalkane
R′COOAg	R′COOR	Ester
LiAlH₄	R–H	Hydrocarbon
R′⁻M⁺	R–R′	Alkane

Ambident nucleophiles have two attacking atoms: cyanide (:C≡N: ↔ C=N:) through C or N, and nitrite (–O–N=O) through O or N.

• KCN is ionic, so free CN⁻ attacks through C (the C–C bond is more stable than C–N) → nitrile.
• AgCN is mainly covalent (Ag–C bond), so only N is free to attack → isocyanide.
• Likewise KNO₂ → alkyl nitrite (through O); AgNO₂ → nitroalkane (through N).

Example (2025) · CH₃CH₂CH₂Br + AgNO₂ → CH₃CH₂CH₂NO₂; + AgCN → CH₃CH₂CH₂NC.

Example (2025) · 1-Bromo-5-(chloromethyl)-3-iodobenzene + AgCN: only the benzylic C–Cl reacts; the aryl Br and I don't → 1-bromo-3-(isocyanomethyl)-5-iodobenzene.

SN2: substitution, nucleophilic, bimolecular

CH₃Cl + OH⁻ → CH₃OH + Cl⁻ is second order:

• One step, no intermediate. The nucleophile attacks from the side opposite the leaving group. The new bond forms as the old one breaks.
• In the transition state the carbon is bonded to five groups (Nu and X partly); the three other bonds lie in one plane. It cannot be isolated.
• The configuration inverts, like an umbrella turned inside out by wind (Walden inversion).
• Bulky groups on or near the carbon block the nucleophile: CH₃X > 1° > 2° > 3°.
• Hughes and Ingold proposed the mechanism (1937).

SN2 has one hump (a transition state). SN1 has two, with the carbocation in the valley, and the first hump is higher: carbocation formation is the slow step (asked in 2019).

SN1: substitution, nucleophilic, unimolecular

(CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻ is first order:

1. Step I (slow, reversible): the C–Br bond breaks to a carbocation and Br⁻. The energy comes from solvation of the halide ion by the protic solvent.
2. Step II (fast): the nucleophile attacks the carbocation.

• Usually run in polar protic solvents (water, alcohol, acetic acid).
• The rate depends only on [RX], so [Nu⁻] doesn't matter.
• The more stable the carbocation, the faster: 3° > 2° > 1° > CH₃X.
• Allylic and benzylic halides are fast in SN1: their carbocations are resonance-stabilised.
• For one alkyl group, both mechanisms follow R–I > R–Br > R–Cl >> R–F (the leaving group).

Example (NCERT Example 6.6) · Faster SN2: a 1° halide over a 2° or 3° one; an iodide over a chloride (I⁻ is the better leaving group, being large).

Example (NCERT Example 6.7) · SN1: CH₃CH₂CH₂CH₂Br < (CH₃)₂CHCH₂Br < CH₃CH₂CHBrCH₃ < (CH₃)₃CBr. SN2 is the reverse. Of the two 1° bromides, (CH₃)₂CH– pushes more electrons (+I), so its cation is more stable. With phenyls: C₆H₅C(CH₃)(C₆H₅)Br > C₆H₅CH(C₆H₅)Br > C₆H₅CH(CH₃)Br > C₆H₅CH₂Br in SN1 (two phenyls stabilise more); SN2 is the reverse (phenyl is bulkier than methyl).

Example (2024) · Which won't show SN1: allyl chloride, CH₃CH=CHCl, benzyl chloride, isopropyl chloride? CH₃CH=CHCl: it is vinylic.

Beyond the book: what decides SN1 vs SN2 · Beyond NCERT

Factor	Favours SN2	Favours SN1
Substrate	CH₃, 1° (unhindered)	3°, allylic, benzylic
Nucleophile	strong, high concentration	weak (H₂O, ROH: solvolysis)
Solvent	polar aprotic (DMSO, DMF, acetone)	polar protic (H₂O, ROH, AcOH)

• Allylic and benzylic halides are fast in SN2 too: the ring's π system stabilises the transition state (2024, assertion and reason both true).
• A 2° halide goes SN2 with a high concentration of a strong nucleophile, SN1 in a large excess of ethanol (2024, both statements true).
• Neopentyl halides are 1° but crowded: very slow in SN2; in SN1 they rearrange.
• CH₃OCH₂Cl goes SN1 although it is 1°: the oxygen lone pair stabilises the cation (2025).
• Nucleophilicity: for the same attacking atom it follows basicity: ⁻OH > PhO⁻ > CH₃COO⁻ > ClO₄⁻ (2026). In a protic solvent, small ions are held by H-bonds, so I⁻ beats F⁻: I⁻ > C₂H₅O⁻ > C₆H₅O⁻ > F⁻ with CH₃Br in methanol (2026).
• Carbocations rearrange (H or CH₃ shifts to a more stable cation) in SN1 and E1.
• Neighbouring groups: Et₂N–CH₂–CH₂–Cl hydrolyses faster than Et₂CH–CH₂–Cl; nitrogen's lone pair displaces Cl inside the molecule first (2025).
• NaI speeds the hydrolysis of RCl: I⁻ is a good nucleophile and a good leaving group (2023).

Example (2023) · Isobutyl bromide (CH₃)₂CHCH₂Br with C₂H₅O⁻: SN2 → isobutyl ethyl ether. With C₂H₅OH alone: SN1, the 1° cation shifts H⁻ to the 3° cation → tert-butyl ethyl ether.

Example (2021) · 3-Bromo-2,2-dimethylbutane in ethanol: 2° cation, CH₃ shift to 3° → 2-ethoxy-2,3-dimethylbutane.

AgNO₃ test · Beyond NCERT · Alcoholic AgNO₃ pulls X⁻ off and precipitates AgX (AgCl white, AgBr pale yellow, AgI yellow). It goes as fast as the carbocation forms: 3°, allylic and benzylic halides quickly; vinyl and aryl halides, CHCl₃ and CCl₄ not at all. (CH₃)₃CCl gives the precipitate (2019). 7-Bromocyclohepta-1,3,5-triene ionises at once to the aromatic tropylium ion, so it does too (2019).

7 · Stereochemistry

• Plane-polarised light comes from passing ordinary light through a Nicol prism. A polarimeter measures how far a compound rotates it.
• Dextrorotatory (d, +): turns it clockwise. Laevorotatory (l, −): anticlockwise. The two are optical isomers.
• Pasteur (1848) found crystals that were mirror images, with equal and opposite rotations. Van't Hoff and Le Bel (1874) proposed the tetrahedral carbon: four different groups make an asymmetric carbon (stereocentre).
• Chiral: not superimposable on its mirror image (like hands); optically active. Achiral: superimposable; inactive.
• Propan-2-ol is achiral (two CH₃ on the central carbon); butan-2-ol is chiral. Other chiral molecules NCERT names: 2-chlorobutane, 2,3-dihydroxypropanal (OHC–CHOH–CH₂OH), BrClCHI, 2-bromopropanoic acid.
• Enantiomers: non-superimposable mirror images. Same m.p., b.p. and refractive index; they differ only in the direction of rotation.
• Racemic mixture: a 50 : 50 mix of enantiomers; zero rotation; written dl or (±). Turning one enantiomer into a racemic mixture is racemisation.
• The sign of rotation is not tied to the actual configuration.

Three outcomes when a bond at the stereocentre breaks:

Outcome	Product	Where
Inversion	configuration flipped	SN2
Retention	same arrangement	no bond to the stereocentre broken
Racemisation	50 : 50 of both	SN1 (planar sp² carbocation, attacked from both faces)

• SN2: (−)-2-bromooctane + NaOH → (+)-octan-2-ol, OH opposite where Br was.
• SN1: optically active 2-bromobutane hydrolyses to (±)-butan-2-ol.
• Retention: (−)-2-methylbutan-1-ol + conc. HCl → (+)-1-chloro-2-methylbutane. No bond to the stereocentre breaks, so the configuration stays; the sign still changes, because a different compound can rotate light differently.

Example (2020) · Which shows retention on substitution by OH⁻? CH₃–CH(C₂H₅)–CH₂Br: the Br is on CH₂, not on the stereocentre.

Example (2024) · SN2 is stereospecific (one stereoisomer) and SN1 generally gives a racemic mixture: both true. In practice SN1 gives slightly more inversion, as the leaving ion shields one face, but "racemisation" is the answer JEE expects.

8 · Elimination

Heating a haloalkane that has a β-hydrogen with alcoholic KOH removes H from the β-carbon and X from the α-carbon (the carbon holding X): β-elimination (dehydrohalogenation). The product is an alkene.

Saytzeff (Zaitsev) rule (1875): the major alkene is the one with more alkyl groups on the double-bond carbons. 2-Bromopentane → mainly pent-2-ene (over pent-1-ene).

Substitution vs elimination (NCERT's box):

• A 1° halide prefers SN2.
• A 2° halide goes SN2 or elimination, depending on how strong the base or nucleophile is.
• A 3° halide goes SN1 or elimination, depending on the stability of the carbocation or the more substituted alkene.
• A bulky base or nucleophile prefers to pull off a proton (elimination).
• Aqueous KOH → substitution (alcohol); alcoholic KOH → elimination (alkene). Alkoxide in alcohol is a stronger base, and the less polar solvent favours elimination (2026).

Example (2024, numerical) · CH₃CH₂Br + NaOH in ethanol → CH₂=CH₂ (4 H); in water → CH₃CH₂OH (6 H). Total 10.

Example (2024) · 1-Bromo-1-methylcyclopentane + OH⁻ in ethanol → 1-methylcyclopentene (Saytzeff, the ring double bond), not the alcohol or methylenecyclopentane.

Example (2025, numerical) · 151 g 2-bromopentane (151 g mol⁻¹, so 1 mol) → alc KOH → pent-2-ene, 80% → 0.8 mol. + Br₂, 100% → 2,3-dibromopentane C₅H₁₀Br₂ (230 g mol⁻¹): 0.8 × 230 = 184 g.

Example (2023) · Most alkenes on dehydrohalogenation (no rearrangement): 2-bromopentane: pent-1-ene, cis- and trans-pent-2-ene = 3.

E2 and E1 · Beyond NCERT

• E2: one step, rate = k[RX][base], strong base; the H and X must be anti-periplanar (180° apart).
• E1: via the carbocation, competes with SN1; rearrangement possible.
• A bulky base (t-BuO⁻K⁺) gives the less substituted (Hofmann) alkene and more elimination. With CH₃CH₂CH₂Br, t-BuO⁻ gives more elimination than C₂H₅O⁻ (2020). (CH₃)₃CCl + t-BuO⁻K⁺ → 2-methylpropene (2022).
• In E2 the better leaving group goes: 3-bromo-2-fluoropentane loses HBr, not HF (2020).
• Vic- or gem-dihalides with excess alc KOH then NaNH₂ lose two HX and give alkynes.

Example (2026) · Optically active C₄H₉Br (2-bromobutane) → alc KOH → but-2-ene → Br₂ → 2,3-dibromobutane → alc KOH, NaNH₂ → but-2-yne → HgSO₄, dil. H₂SO₄, 333 K → butan-2-one, confirmed by the iodoform (haloform) test (it has CH₃CO–).

9 · Reaction with metals

Grignard reagent: R–X + Mg in dry ether → RMgX (Victor Grignard, 1900; Nobel 1912, shared with Sabatier). The C–Mg bond is covalent but highly polar (C is δ−); Mg–X is essentially ionic.

• Grignard reagents take a proton from any proton source (water, alcohols, amines) and become hydrocarbons. So even traces of moisture must be kept out; that is why the ether is dry.
• This is also a way to turn a halide into a hydrocarbon. (Grignard additions to carbonyls: see the Grignard Reagent and Carbonyl chapters.)

Wurtz reaction: two alkyl halides join, doubling the carbon count:

Two different halides give a mixture of three alkanes, so Wurtz suits symmetrical alkanes only. · Beyond NCERT

The eight reactions of R–X that come up most. Aqueous vs alcoholic KOH and K vs Ag salts are the pairs to keep apart.

10 · Haloarenes

NCERT puts aryl halides in this unit; Plus4 also files their questions under Aromatic Compounds & Aryl Halides.

Preparation

• From arenes (electrophilic substitution): Cl₂ or Br₂ with a Lewis acid (Fe or FeCl₃). Toluene gives o- and p-chlorotoluene, easily separated by their very different melting points.
• Iodination is reversible: it needs an oxidising agent (HNO₃, HIO₄) to remove the HI formed.
• Fluorination this way is too violent; fluoro compounds aren't made by it.
• Sandmeyer: a 1° aromatic amine in cold mineral acid + NaNO₂ gives the diazonium salt. With CuCl or CuBr, N₂⁺ is replaced by –Cl or –Br.
• Iodobenzene: shake the diazonium salt with KI; no copper needed.

Gattermann and Balz–Schiemann (NCERT has them in the Amines unit) · Gattermann uses Cu powder + HX instead of Cu₂X₂. Fluorobenzene: ArN₂⁺BF₄⁻ heated (Balz–Schiemann). So of F, Cl, Br, I-benzene, only 2 (Cl, Br) come from Sandmeyer (2024, numerical).

Why aryl halides resist nucleophilic substitution

1. Resonance: X's lone pairs conjugate with the ring, giving C–X partial double-bond character. Harder to break.
2. sp² carbon: more s-character, more electronegative, holds the bond pair tighter. C–Cl is 169 pm in chlorobenzene vs 177 pm in a haloalkane.
3. Phenyl cation isn't resonance-stabilised, so SN1 is ruled out.
4. The electron-rich ring repels the electron-rich nucleophile.

Vinyl halides resist for the same first two reasons. C–Cl in CH₂=CHCl is stronger than in CH₃CH₂Cl (2026).

Example (2026) · Chlorobenzene vs chlorocyclohexane: C–Cl has partial double-bond character, is less polar, and uses an sp² orbital of C. True: B, C, E. The bond is shorter, not longer, and Cl carries less negative charge.

Replacement by –OH (Dow process):

(Asked in 2021: 623 K and 300 atm, no Cu.)

–NO₂ at ortho or para makes it easy (SNAr: addition, then loss of Cl⁻):

Substrate	Conditions to the phenol
1-chloro-4-nitrobenzene	NaOH, 443 K, then H⁺
1-chloro-2,4-dinitrobenzene	NaOH, 368 K, then H⁺
2,4,6-trinitrochlorobenzene	warm water (→ picric acid); NCERT gives no temperature, 323 K is Beyond NCERT

• The nucleophile adds to the C–Cl carbon, making a carbanion. Its negative charge sits on the carbons ortho and para to Cl, where an o- or p-NO₂ takes it over by resonance.
• meta-NO₂ has no effect: no resonance form puts the charge on the carbon carrying it.
• More o/p nitro groups, faster reaction.

Benzyne · Beyond NCERT · Chlorobenzene + NaNH₂ in liquid NH₃ eliminates HCl to benzyne, which adds NH₂⁻ at either end. A label on the ring ends up spread over two positions (cine substitution).

Electrophilic substitution: halogen is slightly deactivating but o, p-directing.

• –I effect pulls electrons from the whole ring, so the rate drops (harsher conditions than benzene).
• +R effect (lone-pair resonance) raises electron density at o and p, so the electrophile goes there.
• Reactivity is controlled by the stronger –I; orientation by the resonance (NCERT Example 6.9).

Reaction	Reagent	Products (p major)
Halogenation	Cl₂, anhyd. FeCl₃	1,2- and 1,4-dichlorobenzene
Nitration	conc. HNO₃ + conc. H₂SO₄	1-chloro-2- and 1-chloro-4-nitrobenzene
Sulphonation	conc. H₂SO₄	2- and 4-chlorobenzenesulphonic acid
FC alkylation	CH₃Cl, anhyd. AlCl₃	2- and 4-chlorotoluene
FC acylation	CH₃COCl, anhyd. AlCl₃	2- and 4-chloroacetophenone

With sodium:

11 · Polyhalogen compounds

Compound	Uses	Harm
CH₂Cl₂ (methylene chloride)	solvent, paint remover, aerosol propellant, metal cleaning	CNS; hearing, vision; skin burns
CHCl₃ (chloroform)	solvent for fats, alkaloids, I₂; makes freon R-22; once an anaesthetic	CNS; liver (→ phosgene), kidneys
CHI₃ (iodoform)	once an antiseptic (from the I₂ it frees)	objectionable smell
CCl₄	refrigerants, propellants, CFC feedstock, solvent; once a fire extinguisher	liver, nerves; depletes ozone
Freons (CFCs of CH₄, C₂H₆)	aerosols, refrigeration, air-conditioning	radical chain reactions destroy stratospheric ozone
DDT	insecticide (malaria mosquito, typhus lice)	not biodegradable; stored in fat; toxic to fish

• Chloroform is slowly oxidised by air in light to phosgene, which is very poisonous. So it is kept in dark, closed bottles filled to the top.

• Freon 12 is CCl₂F₂, made from CCl₄ by the Swarts reaction. Freons are stable, unreactive, non-toxic, non-corrosive and easily liquefied.
• DDT is p,p′-dichlorodiphenyltrichloroethane.

Example (2023) · Most Cl atoms: gammaxene (C₆H₆Cl₆, 6). Chloral CCl₃CHO and chloropicrin CCl₃NO₂ have 3, Freon-12 has 2.

12 · Named reactions

Name	Reagents	Product
Finkelstein	R–Cl/Br + NaI, dry acetone	R–I
Swarts	R–Cl/Br + AgF, Hg₂F₂, CoF₂ or SbF₃	R–F
Wurtz	2 R–X + 2 Na, dry ether	R–R
Wurtz–Fittig	Ar–X + R–X + 2 Na, dry ether	Ar–R
Fittig	2 Ar–X + 2 Na, dry ether	Ar–Ar
Sandmeyer	ArN₂⁺X⁻ + CuCl or CuBr (CuCN)	Ar–Cl, Ar–Br (Ar–CN)
Gattermann (Amines unit)	ArN₂⁺X⁻ + Cu, HX	Ar–X
Balz–Schiemann (Amines unit)	ArN₂⁺BF₄⁻, heat	Ar–F
Grignard	R–X + Mg, dry ether	RMgX
Dow	C₆H₅Cl + NaOH, 623 K, 300 atm	phenol (after H⁺)
Lucas reagent (Alcohols)	conc. HCl + ZnCl₂	R–Cl

Example (2025) · Swarts → ethyl fluoride; Sandmeyer → cyanobenzene; Wurtz–Fittig → ethylbenzene; Finkelstein → ethyl iodide.

13 · Traps

Trap	What goes wrong	Do this
KCN vs AgCN	Swapping nitrile and isocyanide	K (ionic) → C attacks → R–CN; Ag (covalent) → N attacks → R–NC
Aq vs alc KOH	Writing an alcohol with alcoholic KOH	Alcoholic → alkene; aqueous → alcohol
SN1 in ethanol	Writing the unrearranged ether	Make the cation; shift H or CH₃ to a 3° one first (2021, 2023)
Allyl vs vinyl	Calling allylic carbon sp²	Allylic X is on sp³ next to C=C; vinylic X is on the sp² carbon
Benzyl in SN2	Ranking benzyl halides slow in SN2	Fast in both SN1 and SN2
Neopentyl	Treating it as a normal 1° halide	Very slow SN2 (steric)
Dipole moment	Putting CH₃F first	CH₃Cl > CH₃F > CH₃Br > CH₃I
Aryl halide in a mixed molecule	Substituting the ring X	Only the alkyl or benzylic X reacts (2021, 2025)
meta-NO₂	Thinking any NO₂ helps SNAr	Only o/p; meta does nothing
Retention	Calling every SN2 product inverted	No bond to the stereocentre broken → retention (2020)
Sign of rotation	Assuming (+) stays (+)	Retention can flip the sign; sign ≠ configuration
Bulky base	Saytzeff with t-BuO⁻	Bulky base: less substituted alkene, more elimination
H₂SO₄ with KI	Using it to make R–I	Use H₃PO₄; H₂SO₄ oxidises HI to I₂

14 · Numbers worth knowing

• C–Cl: 177 pm in a haloalkane, 169 pm in chlorobenzene. Bond lengths and enthalpies: the table in section 3.
• Dow: 623 K, 300 atm. Nitro-chlorobenzenes: 443 K (one NO₂), 368 K (two), warm water (three; 323 K is Beyond NCERT).
• HBr for bromides: 48%. H₃PO₄ for iodides: 95%. Alkyne hydration: HgSO₄, dil. H₂SO₄, 333 K.

15 · For the boards only

JEE Main doesn't ask this, but the CBSE paper can.

• Natural and medical halogen compounds: chloramphenicol (from microbes, for typhoid), thyroxine (iodine hormone; its lack causes goitre), chloroquine (malaria), halothane (anaesthetic). Fully fluorinated compounds are being studied as blood substitutes.
• Halogenated compounds persist: soil bacteria can't break them down.
• Nicol (1768–1851) made the first polarising prism; Van't Hoff won the first chemistry Nobel (1901). Grignard first took a maths degree.
• DDT was first made in 1873; Paul Müller (Geigy, Switzerland) found it was an insecticide in 1939 and won the 1948 Nobel Prize in Medicine. Many insects became resistant. The US banned it in 1973.
• Ozone loss raises UV exposure: skin cancer, eye disease, weaker immunity.