Halogen-bearing organic compounds are the workhorses of organic synthesis — they are the bridge that converts hydrocarbons into alcohols, amines, nitriles, ethers and Grignard reagents. For NEET, this chapter is a reliable source of questions on reactivity order, SN1 vs SN2 mechanisms, stereochemistry, and named reactions, so precision matters more than bulk memorisation here.
Classification and Naming
A halogen-containing organic compound is simply one in which one or more hydrogens of a hydrocarbon have been replaced by F, Cl, Br or I. They are sorted in three useful ways.
By number of halogens: mono-, di-, tri-, ... polyhalogen compounds. Dihalides are further described as gem-dihalides (both halogens on the same carbon, e.g. ethylidene chloride, CH₃CHCl₂) or vic-dihalides (halogens on adjacent carbons, e.g. ethylene dichloride, CH₂Cl–CH₂Cl).
By the nature of the carbon carrying the halogen:
- Alkyl halides (haloalkanes): halogen on sp³ carbon — further subdivided as primary, secondary or tertiary depending on how many carbons are attached to that carbon.
- Allylic halides: halogen on the sp³ carbon next to a C=C.
- Benzylic halides: halogen on the sp³ carbon attached to a benzene ring.
- Vinylic halides: halogen directly on an sp² carbon of a double bond.
- Aryl halides (haloarenes): halogen directly on an sp² carbon of a ring.
Nomenclature: In IUPAC names the halogen is a substituent prefix (fluoro-, chloro-, bromo-, iodo-) with the lowest possible locant set, and prefixes are cited alphabetically. Common (older) names treat the group as an "alkyl halide". So (CH₃)₃C–Br is 2-bromo-2-methylpropane (tert-butyl bromide), and CH₃CH₂CHClCH₃ is 2-chlorobutane (sec-butyl chloride). Note that the dihalogenated common names are traps: ethylene dichloride is the 1,2-isomer while ethylidene chloride is the 1,1-isomer.
Methods of Preparation
1. From alcohols (the most general laboratory route):
- With concentrated HX (HCl needs anhydrous ZnCl₂ as catalyst for 1° and 2° alcohols; 3° alcohols react with HCl alone).
- With PCl₅, PCl₃ or SOCl₂. Thionyl chloride is preferred because the by-products (SO₂ and HCl) are gases that escape, leaving a pure alkyl chloride.
- Bromides and iodides are best made with red phosphorus + Br₂ or I₂ (which generates PBr₃/PI₃ in situ), since HBr/HI tend to reduce or rearrange.
2. From hydrocarbons:
- Free-radical halogenation of alkanes gives mixtures (poor preparative value) — ease of substitution follows 3° H > 2° H > 1° H.
- Electrophilic substitution on arenes with X₂/Lewis acid (Fe or FeX₃) gives aryl halides; chlorination/bromination give mainly ortho and para products because halogens are o,p-directors.
- Side-chain (benzylic) halogenation of alkylbenzenes occurs in sunlight/heat with no Lewis acid.
3. Addition to alkenes and alkynes: HX adds following Markovnikov's rule (peroxide effect with HBr reverses it); X₂ addition gives vic-dihalides — the decolourisation of bromine in CCl₄ is the classic unsaturation test.
4. Halogen exchange:
- Finkelstein reaction: R–X + NaI in dry acetone → R–I (NaCl/NaBr precipitate drives it forward).
- Swarts reaction: R–X + metallic fluoride (AgF, Hg₂F₂, CoF₂, SbF₃) → R–F.
5. Sandmeyer route to aryl halides: benzenediazonium chloride (from aniline + NaNO₂/HCl at 273–278 K) treated with CuCl/HCl or CuBr/HBr gives chloro- or bromobenzene; with KI, no copper salt is needed. This is the only convenient way to place a halogen exactly where an amino group was.
Physical Properties
The C–X bond is polar because halogens are more electronegative than carbon, so these molecules have permanent dipoles. Bond length increases and bond enthalpy decreases down the group: C–F is the shortest and strongest, C–I the longest and weakest. This single fact controls most reactivity trends — iodides react fastest in substitution, fluorides slowest.
- Boiling points rise with molecular size: for the same alkyl group, RI > RBr > RCl > RF, and within a series they rise with chain length. Haloalkanes boil higher than the parent alkanes because of dipole–dipole plus stronger van der Waals forces.
- Branching lowers boiling point (less surface contact): n-butyl bromide > isobutyl bromide > tert-butyl bromide.
- Among isomeric dihalobenzenes, boiling points are close, but the para isomer melts much higher because its symmetry lets it pack tightly in the crystal lattice.
- Solubility: they are only slightly soluble in water — energy is needed to break water's H-bonds and the new interactions formed are weaker. They dissolve well in organic solvents.
- Density: bromides, iodides and polychloro compounds are heavier than water.
Reactions of Haloalkanes
Nucleophilic substitution
The polarised carbon is electrophilic, so nucleophiles displace the halide (a good leaving group). Two limiting mechanisms exist.
SN2 (bimolecular): the nucleophile attacks from the side opposite the leaving group in one concerted step through a five-coordinate transition state. Rate = k[RX][Nu⁻]. Because attack is from the back, the configuration is inverted — if the carbon is a stereocentre, an optically pure substrate gives the product of opposite configuration (Walden inversion). Steric crowding kills this pathway, so the rate order is:
CH₃X > 1° > 2° > 3°
SN1 (unimolecular): the C–X bond ionises first (slow, rate-determining) to a carbocation, which is then captured by the nucleophile. Rate = k[RX] only. Polar protic solvents (aqueous ethanol) help by solvating the ions. Since carbocation stability governs the rate:
3° > 2° > 1° > CH₃X
The planar carbocation can be attacked from either face, so an optically active substrate gives a racemic mixture (racemisation), often with slight excess of the inverted product.
Allylic and benzylic halides are exceptionally reactive in SN1 because the cation is resonance-stabilised. Vinylic and aryl halides are essentially unreactive (see below).
Useful substitutions to remember: aqueous KOH → alcohol; alcoholic NaOR → ether (Williamson); KCN → nitrile (C-attack, since CN⁻ is ambidentate); AgCN → isocyanide (N-attack); KNO₂ → alkyl nitrite; AgNO₂ → nitroalkane; NH₃ → amine; AgSH → thiol; R'COOAg → ester; LiAlH₄ → alkane.
Elimination (dehydrohalogenation)
With a strong base such as alcoholic KOH, a β-hydrogen and the halogen are removed to give an alkene (β-elimination). When more than one alkene is possible, Saytzeff's rule applies: the more substituted (more stable) alkene predominates. Substitution and elimination compete — bulky bases, higher temperature, and more substituted substrates favour elimination, while good nucleophiles in polar protic solvents favour substitution.
Reactions with metals
- Grignard reagents: R–X + Mg in dry ether → RMgX. The C–Mg bond is highly polar with carbon carrying negative charge, making it a powerful base and nucleophile. Grignards are destroyed by even traces of moisture, giving alkanes — hence the compulsory "dry ether".
- Wurtz reaction: 2 R–X + 2Na in dry ether → R–R, used to make symmetrical alkanes.
- Fittig / Wurtz–Fittig: aryl halide + Na gives biaryl; aryl halide + alkyl halide + Na gives an alkylarene.
Stereochemistry note
A carbon bearing four different groups is a stereocentre; the molecule is chiral and non-superimposable on its mirror image (enantiomers). Enantiomers rotate plane-polarised light equally in opposite directions; an equimolar mixture (racemate) is optically inactive by external compensation. This is exactly why SN1 and SN2 outcomes are experimentally distinguishable.
Haloarenes: Why They Are Different
Aryl (and vinyl) halides resist nucleophilic substitution for several reasons acting together:
- Resonance: the halogen's lone pair conjugates with the ring, giving the C–X bond partial double-bond character; it is shorter and stronger, hence harder to break.
- Hybridisation: the carbon is sp² — more electronegative and holding electrons more tightly than the sp³ carbon of an alkyl halide, so the bond is shorter and stronger.
- Phenyl cation instability: the aryl cation is not resonance-stabilised, so SN1 is out of the question.
- Repulsion: the electron-rich ring repels the approaching nucleophile.
Substitution can be forced only under drastic conditions (chlorobenzene + NaOH at 623 K and 300 atm → phenol) or when strongly electron-withdrawing groups sit ortho/para to the halogen (e.g. 2,4-dinitrochlorobenzene hydrolyses with warm aqueous NaOH), because such groups stabilise the intermediate carbanion.
In electrophilic substitution, halogens are a curious case: they are deactivating (the −I effect withdraws electron density, so haloarenes react slower than benzene) yet ortho–para directing (resonance donation stabilises the o/p intermediates specifically). Chlorobenzene therefore gives mainly o- and p-products, with para usually dominating on steric grounds.
Polyhalogen Compounds Worth Knowing
- Dichloromethane (CH₂Cl₂): solvent, paint remover, aerosol propellant; harms the central nervous system and eyes at high exposure.
- Chloroform (CHCl₃): once a general anaesthetic; slowly oxidises in air and light to the poisonous gas phosgene (COCl₂); it is therefore stored in dark, completely filled, air-tight bottles with a small amount of ethanol added, which converts any phosgene formed into the harmless diethyl carbonate.
- Iodoform (CHI₃): a pale yellow, antiseptic solid once used to dress wounds; formed whenever a methyl ketone, ethanol, or any compound oxidisable to a methyl ketone is treated with I₂/NaOH (the iodoform test), making it a diagnostic test for the CH₃CO– group and for CH₃CH(OH)– alcohols.
- Carbon tetrachloride (CCl₄): once a common dry-cleaning solvent and fire extinguisher fluid, now restricted because of liver toxicity and its role as an ozone-depleting substance.
- Freons (chlorofluorocarbons, CFCl₃ and CF₂Cl₂ type compounds): extremely stable, non-toxic, non-flammable gases once used widely as refrigerants and aerosol propellants; their inertness in the troposphere lets them drift intact into the stratosphere, where UV radiation liberates chlorine radicals that catalytically destroy ozone — the reason their production is now phased out under international agreements.
- DDT (dichlorodiphenyltrichloroethane): the first major synthetic pesticide; its usefulness is undercut by its chemical stability and fat solubility, which allow it to bioaccumulate up food chains and persist in the environment for years.
Common Mistakes and Exam Traps
- Applying SN1/SN2 reasoning to haloarenes. Aryl and vinyl halides do not undergo either mechanism under ordinary conditions because the C–X bond has partial double-bond character and the aryl/vinyl cation is not resonance-stabilised; students who forget this try to rank haloarene reactivity using the same 3° > 2° > 1° or CH₃X > 1° > 2° > 3° logic that applies only to alkyl halides.
- Forgetting that halogens are deactivating yet still o,p-directing in electrophilic substitution. It is tempting to assume a deactivating group must also be meta-directing, but halogens are the standard exception — their inductive withdrawal slows the reaction down while their resonance donation still directs incoming electrophiles to ortho and para positions.
- Mixing up gem- and vic-dihalide naming with the wrong common-name compound. Ethylene dichloride (1,2-, a vic-dihalide) and ethylidene chloride (1,1-, a gem-dihalide) are frequently swapped in objective questions because the names sound similar but describe different substitution patterns.
- Predicting racemisation for every substitution without checking the mechanism. Complete inversion (Walden inversion) is the SN2 signature and full racemisation is the SN1 signature; a substrate that is only moderately hindered can go through a mixture of both pathways, giving partial (not complete) racemisation — a nuance NEET sometimes tests with allylic or secondary substrates.