Haloalkanes and Haloarenes — NEET Notes

Classification of haloalkanes & haloarenes

Halogen-containing organic compounds are classified along two axes — by the number of halogen atoms they carry, and by the nature of the carbon bearing the halogen. The first axis divides them into mono-, di-, tri-, and polyhalogen compounds. The second is where NEET focuses, because reactivity depends almost entirely on which kind of carbon holds the halogen.

The carbon-halogen carbon can be sp³ or sp², and within sp³ it can be alkyl, allylic, or benzylic. Within sp² it can be vinylic or aryl. Each kind behaves differently in substitution and elimination — so this opening classification is not pedantry, it is the map of the entire chapter.

NEET 2023 asked exactly this — given a structure with halogen on an sp³ carbon adjacent to a C=C bond, identify the class. The answer is allylic halide. Vinylic would require the halogen on the sp² carbon itself. Benzylic would need an aromatic ring instead of an alkene. Memorising this matrix solves a free mark.

IUPAC nomenclature

Common names of alkyl halides simply name the alkyl group followed by halide — methyl chloride, ethyl bromide, tert-butyl bromide. IUPAC names them as halosubstituted hydrocarbons. The halogen takes a locant, the lowest-numbered position. The parent chain is chosen as the longest carbon chain containing the halogen-bearing carbon. Common names of dihaloalkanes give the bonding type explicitly — alkylidene halide for gem-dihalides, alkylene dihalide for vic-dihalides. IUPAC writes both as dihaloalkanes with numerical locants.

For benzene-ring substitution, common system uses o-, m-, p- prefixes for the disubstituted derivatives; IUPAC uses numerical locants 1,2; 1,3; 1,4. NCERT Table 6.1 is the canonical reference: tert-butyl bromide = 2-bromo-2-methylpropane; chloroform = trichloromethane; CCl₄ = tetrachloromethane; methylene chloride = dichloromethane; allyl bromide = 3-bromopropene; vinyl chloride = chloroethene. Memorise the common-to-IUPAC pairs — questions on naming are basic NCERT-spotters.

Nature of the C–X bond

Halogens are more electronegative than carbon, so the C–X bond is polarised: carbon carries a δ⁺ partial charge and halogen carries δ⁻. This polarity is the engine of every nucleophilic substitution reaction in this chapter — nucleophiles attack the electron-deficient carbon, the polarised bond breaks heterolytically, and the halogen leaves as a halide ion.

Down the halogen group, atomic size rises sharply. Fluorine is the smallest, iodine the largest. Three consequences follow.

Reactivity in both SN1 and SN2 follows the same order — R–I > R–Br > R–Cl >> R–F. The driving factor is the leaving-group ability of the halide ion, which improves down the group (weaker C–X bond, more stable halide ion in solution).

Methods of preparation — haloalkanes

NCERT lays out three routes to haloalkanes. Each has a signature reagent and a signature reaction name that NEET examiners love to test.

1. From alcohols (the workhorse route)

Concentrated HX, phosphorus halides (PCl₅, PBr₃, PI₃), or thionyl chloride (SOCl₂) all replace the –OH group with halogen. SOCl₂ is preferred industrially because the by-products are gaseous SO₂ and HCl — both escape, leaving pure alkyl chloride. Reactivity order of alcohols with HX is 3° > 2° > 1°. With 1° and 2° alcohols + HCl, anhydrous ZnCl₂ catalyst is needed; with 3° alcohols, simply shaking with conc. HCl works at room temperature (basis of the Lucas test).

For alkyl iodides, mixing alcohol with NaI/KI in 95% orthophosphoric acid gives good yields. Note: H₂SO₄ is not used here — it oxidises HI back to I₂, defeating the reaction.

2. From hydrocarbons

(a) Free-radical halogenation of alkanes — alkane + Cl₂/Br₂ under sunlight or heat. Gives a complex mixture of mono- and polyhalogenated products, useful in industry but poor in selectivity. (b) Addition of HX to alkenes — Markovnikov's rule applies: H adds to the carbon with more H atoms, halogen to the more substituted one. (c) Addition of X₂ to alkenes — Br₂ in CCl₄ giving vic-dibromide is the standard laboratory test for unsaturation (the red-brown bromine is decolorised).

3. Halogen exchange — Finkelstein & Swarts

Preparation of haloarenes

Haloarenes cannot be prepared from phenols because the C–O bond in phenol has partial double-bond character and is too strong to cleave. Two routes work instead.

(i) Electrophilic substitution. Benzene + Cl₂/Br₂ in the presence of a Lewis acid (Fe, FeCl₃, FeBr₃) gives chlorobenzene or bromobenzene. The Lewis acid generates the electrophile (Cl⁺ or Br⁺), which attacks the ring. Iodination needs an oxidising agent (HNO₃, HIO₄) to consume HI as it forms. Fluorination cannot be done this way — fluorine is too reactive. This is the route NEET 2022 tested directly: chlorobenzene is made from benzene + Cl₂ + anhydrous FeCl₃.

(ii) From amines — Sandmeyer's reaction. Primary aromatic amine + NaNO₂ + cold dilute HCl gives a diazonium salt; treating that diazonium salt with CuCl, CuBr replaces –N₂⁺ with –Cl or –Br. For iodination, plain KI is enough (no copper salt needed).

Physical properties

Lower haloalkanes (methyl chloride, methyl bromide, ethyl chloride, chlorofluoromethanes) are gases at room temperature; higher members are liquids or solids. Boiling points are higher than the parent hydrocarbon because of stronger dipole-dipole and van der Waals forces. For the same alkyl group: R–I > R–Br > R–Cl > R–F (rises with halogen mass and polarisability). Branching lowers the boiling point — among isomeric C₄H₉Br, tert-butyl bromide has the lowest b.p. Bromo, iodo and polychloro compounds are denser than water. Haloalkanes are nearly insoluble in water (their dipoles cannot replace water's hydrogen-bond network) but dissolve freely in organic solvents.

For dihalobenzenes, the para-isomer melts at the highest temperature while o-, m-, and p- have similar boiling points. The para-isomer's symmetry packs the crystal lattice efficiently, raising m.p. — a classic NEET reasoning question (NCERT exercise 6.18).

Nucleophilic substitution — the central reaction

The δ⁺ carbon of a haloalkane is the electrophilic site. A nucleophile (Nu⁻) — anything with a lone pair and an inclination to share it — attacks the carbon, displaces the halide ion as the leaving group, and a new C–Nu bond replaces the old C–X bond. This is the nucleophilic substitution reaction. The bond that breaks (C–X) is broken heterolytically: both bonding electrons leave with the halide. The same set of substrates can be converted into alcohols (with OH⁻), ethers (with OR⁻), amines (with NH₃), nitriles (with CN⁻), nitroalkanes (with NO₂⁻), and so on — Table 6.4 in NCERT.

Cyanide (CN⁻) and nitrite (NO₂⁻) are ambident nucleophiles — they can attack from either of two atoms. KCN (ionic) gives mainly alkyl cyanides via C-attack; AgCN (covalent) gives mainly isocyanides via N-attack. The rule: ionic salt → C-attack → cyanide; covalent silver salt → N-attack → isocyanide.

SN1 vs SN2 — the most-tested distinction in organic chemistry

Substitution can proceed by either of two mechanisms, and they are opposite in almost every dimension. NEET tests this contrast every year — sometimes by direct comparison, sometimes hidden inside a reactivity-order or stereochemistry question.

SN2 mechanism — one step, inversion

Hughes and Ingold (1937) proposed the SN2 mechanism for the reaction of methyl chloride with hydroxide ion. The nucleophile attacks the δ⁺ carbon from the side opposite the leaving group. As the new C–OH bond forms, the C–Cl bond simultaneously weakens. In the transition state the carbon is partly bonded to five atoms — three to the original substituents (which start moving away like an umbrella catching wind), one to the incoming OH⁻, and one to the departing Cl⁻. As the new bond fully forms and the old one fully breaks, the three other substituents "flip" through the plane of the carbon. The configuration inverts — Walden inversion.

Because the transition state involves the nucleophile attaching close to the carbon, steric bulk kills SN2. Methyl halides are fastest (only three small hydrogens). Tertiary halides are dead — the three alkyl groups block the back-side approach. Hence the order: CH₃X > 1° > 2° > 3°.

SN1 mechanism — two steps, carbocation, racemisation

Because the rate depends only on the C–X bond breaking, anything that stabilises the carbocation accelerates SN1. Tertiary carbocations are the most stable (three alkyl groups donating electrons inductively and through hyperconjugation), so 3° haloalkanes are fastest. Allylic and benzylic halides are also very fast because their carbocations enjoy resonance stabilisation — the positive charge delocalises onto the double bond or aromatic ring.

Stereochemistry of substitution & the Walden inversion

If the substrate carries a stereocentre — a carbon with four different groups — the difference between SN1 and SN2 becomes geometrically visible. Three outcomes are possible at a stereocentre when a bond is broken: retention (configuration preserved), inversion (configuration flipped), or racemisation (50:50 mixture). SN2 always gives inversion. SN1 always gives racemisation (because the planar carbocation is attacked equally from both faces).

The classic example: (–)-2-bromooctane + NaOH gives (+)-octan-2-ol via SN2 — the –OH ends up on the opposite face from where Br was. The optical rotation has flipped sign because the configuration has flipped. This is the Walden inversion, the stereochemical fingerprint of SN2.

The carbon is turned inside out like an umbrella caught in a strong wind.

NCERT description of Walden inversion in SN2

For SN1: (±)-butan-2-ol forms when optically active 2-bromobutane is hydrolysed under SN1 conditions. The carbocation intermediate is flat; the nucleophile attacks from either face; the product mixture rotates light through equal and opposite angles — net zero rotation. Racemisation. Inversion = SN2; racemisation = SN1. Memorise it as a two-line rule.

Elimination reactions — E1 & E2

When a haloalkane carrying a β-hydrogen is heated with alcoholic KOH, hydrogen leaves the β-carbon, halogen leaves the α-carbon, and a C=C double bond forms between them. An alkene is produced. Because the H is lost from the β-position, this is called β-elimination or dehydrohalogenation.

α-carbon: the carbon bearing the halogen. β-carbon: the carbon adjacent to it. β-hydrogen: any hydrogen on a β-carbon — only these are eliminated.

Saytzeff (Zaitsev) rule

When more than one β-hydrogen is available, more than one alkene can form. The Russian chemist Alexander Saytzeff (also spelled Zaitsev) noted in 1875 that the major product is the more substituted alkene — the one with more alkyl groups on the doubly bonded carbons. This is the more stable alkene (greater hyperconjugation, greater inductive donation). Example: 2-bromopentane gives pent-2-ene (di-substituted, major) over pent-1-ene (mono-substituted, minor). NEET 2021 asked this directly; NEET 2020 asked it as part of a statement-correctness question.

Substitution vs elimination — which wins?

A haloalkane with a β-hydrogen has a choice: substitution (SN1 or SN2) or elimination (E1 or E2). The outcome depends on substrate, base, and conditions. The default rules:

• Primary haloalkanes prefer SN2 (back-side attack easy, no carbocation). With strong bulky base — elimination.
• Secondary haloalkanes are the most ambiguous — SN2, E2, or E1 depending on conditions. Strong base + heat favours E2.
• Tertiary haloalkanes avoid SN2 (steric blockade); they choose between SN1 and E1 depending on conditions. With heat and strong base, E2.
• Aqueous KOH favours substitution → alcohol. Alcoholic KOH favours elimination → alkene. NEET 2016 tested this exact reagent contrast.

Reactions with metals — Grignard, Wurtz

Haloalkanes are the starting point for organometallic chemistry. With Mg in dry ether, R–X gives an alkyl magnesium halide R–MgX — the Grignard reagent (Victor Grignard, 1900; Nobel 1912). The C–Mg bond is highly polar — carbon carries a partial negative charge and behaves as a nucleophilic carbanion. Grignard reagents react with carbonyls, epoxides, CO₂, and many other electrophiles to forge new C–C bonds. They are how organic chemists build up molecules from small fragments.

The catch: Grignard reagents are destroyed by even trace moisture. Water, alcohols, amines, terminal alkynes — anything with a proton acidic enough to be plucked — protonates the carbanion, converting R–MgX into R–H (the hydrocarbon). Hence the rule: Grignard reactions must be run under strictly anhydrous conditions, in dry ether.

Wurtz reaction. Alkyl halide + sodium in dry ether → alkane with double the number of carbons. Two R–X molecules couple: 2 R–X + 2 Na → R–R + 2 NaX. Useful for symmetric higher alkanes; gives mixtures when two different R–X are used.

Reactions of haloarenes

Haloarenes resist the SN1 and SN2 reactions that haloalkanes love. Four reasons:

1. Resonance. The halogen's lone pair conjugates with the ring's π-electrons, putting partial double-bond character on the C–X bond — harder to break.
2. Hybridisation. The ring carbon is sp² (33% s-character) vs sp³ in haloalkane (25% s-character). More s-character means more electronegative — the bonding pair is held more tightly. C–Cl bond length: chlorobenzene 169 pm, methyl chloride 177 pm.
3. Phenyl cation instability. SN1 is impossible — the phenyl cation cannot be stabilised by resonance (the empty orbital is orthogonal to the π-system).
4. Repulsion. The electron-rich nucleophile is repelled by the electron-rich aromatic ring.

Forcing conditions (NaOH, 623 K, 300 atm) convert chlorobenzene to phenol. An electron-withdrawing group (e.g. –NO₂) at ortho/para position increases reactivity by stabilising the intermediate carbanion (Meisenheimer complex). At meta — no effect.

Electrophilic substitution of haloarenes

Haloarenes still undergo the standard electrophilic substitutions — halogenation, nitration, sulphonation, Friedel-Crafts — but more slowly than benzene. The halogen is slightly deactivating (electron-withdrawing through induction) but ortho-, para-directing (electron-donating through resonance for the σ-complex stabilisation). This dichotomy is the most asked reasoning question in haloarene chemistry: reactivity controlled by the inductive effect, orientation controlled by resonance.

Wurtz-Fittig & Fittig reactions

A mixture of alkyl halide + aryl halide + Na in dry ether gives an alkylarene — Wurtz-Fittig reaction. Two aryl halides + Na in dry ether give a biaryl — Fittig reaction. Bromobenzene + Na gives biphenyl (Fittig); bromobenzene + methyl bromide + Na gives toluene (Wurtz-Fittig).

Polyhalogen compounds — uses & concerns

Compounds with two or more halogen atoms have outsized industrial importance — and outsized environmental cost. The six NCERT-named members:

• Dichloromethane (CH₂Cl₂, methylene chloride): solvent, paint remover, aerosol propellant, drug-manufacture solvent. Harms CNS; high exposure causes dizziness, nausea, numbness; direct skin contact burns.
• Trichloromethane (CHCl₃, chloroform): solvent for fats, alkaloids, iodine; historically a general anaesthetic, now replaced by safer ones. Major industrial use: feedstock for freon R-22. Slowly oxidised by air + light to phosgene (COCl₂) — extremely poisonous. Stored in dark bottles, completely filled, with a little ethanol as stabiliser.
• Triiodomethane (CHI₃, iodoform): formerly antiseptic, replaced now. Its antiseptic action came from liberated iodine, not the molecule itself. Used in the iodoform test (yellow precipitate identifies CH₃CO– or CH₃CH(OH)– groups).
• Tetrachloromethane (CCl₄): feedstock for chlorofluorocarbons, pharmaceutical manufacture, general solvent. Once used as cleaning fluid and fire extinguisher. Damages the liver and CNS; releases to atmosphere deplete the ozone layer.
• Freons (chlorofluorocarbons, CFCs): CFCl₃, CCl₂F₂ etc. Stable, non-toxic, non-flammable, easily liquefiable — perfect refrigerants and propellants. Freon-12 (CCl₂F₂) is manufactured from CCl₄ by Swarts reaction. Released CFCs reach the stratosphere and initiate radical chain reactions that destroy ozone — banned globally under the Montreal Protocol.
• DDT (p,p'-dichlorodiphenyltrichloroethane): the first chlorinated organic insecticide. Paul Müller won the 1948 Nobel Prize in Medicine for showing its effectiveness against malaria-carrying mosquitoes. But DDT is fat-soluble, persistent, and bioaccumulates — banned in the US in 1973; still used in malaria control elsewhere.

The shared concern across these compounds: persistence. The chlorinated and fluorinated C–C and C–Cl bonds resist biodegradation. CFCs and CCl₄ deplete ozone. DDT bioaccumulates up the food chain. Modern green chemistry replaces them where possible.