Chapter 6: Haloalkanes and Haloarenes | Summary

Common names of alkyl halides are derived by naming the alkyl group followed by the halide name. In the IUPAC system, alkyl halides are named as halosubstituted hydrocarbons with the halogen as a prefix. For monohalogen-substituted benzene, common and IUPAC names are identical. For dihalogen derivatives, o-, m-, p- prefixes are used in the common system while numerals 1,2; 1,3; 1,4 are used in the IUPAC system.

Gem-dihalides are named as alkylidene halides in common nomenclature and as dihaloalkanes in IUPAC. Vic-dihalides are named as alkylene dihalides in common nomenclature. The IUPAC naming follows standard rules of choosing the longest chain containing the halogen, with the lowest possible locant assigned to the halogen substituent.

Halogens are more electronegative than carbon, so the C–X bond is polarised with a partial positive charge on carbon and partial negative charge on halogen. The bond length increases from C–F (139 pm) to C–I (214 pm) as atomic size increases down the group. Correspondingly, bond enthalpies decrease from C–F (452 kJ/mol) to C–I (234 kJ/mol). The dipole moment of C–Cl bond is highest at 1.860 Debye.

The polarity of the C–X bond makes haloalkanes susceptible to nucleophilic attack at the electrophilic carbon. This polarity is fundamental to understanding the reactivity and reaction mechanisms of haloalkanes in nucleophilic substitution and elimination reactions.

Haloalkanes are best prepared from alcohols by reaction with concentrated halogen acids (HCl requires ZnCl2 catalyst for 1° and 2° alcohols; 3° alcohols react readily at room temperature), phosphorus halides (PX3, PCl5), or thionyl chloride (SOCl2, preferred as it gives pure alkyl halide with gaseous by-products SO2 and HCl). The reactivity order of alcohols is 3° > 2° > 1°.

From hydrocarbons, free radical halogenation of alkanes gives a complex mixture of mono- and polyhaloalkanes. Alkenes react with HX (per Markovnikov's rule) or X2 to give haloalkanes. Halogen exchange reactions include the Finkelstein reaction (R–X + NaI in dry acetone → R–I) and the Swarts reaction (R–Cl/Br + metallic fluoride such as AgF → R–F), enabling preparation of alkyl iodides and fluorides respectively.

Aryl chlorides and bromides are prepared by electrophilic substitution of arenes using Cl2 or Br2 in the presence of Lewis acid catalysts (Fe or FeCl3). Iodination is reversible and requires an oxidising agent (HNO3 or HIO4). Fluorine compounds are not made this way due to fluorine's high reactivity.

The Sandmeyer's reaction involves treating a primary aromatic amine with NaNO2 in cold aqueous mineral acid to form a diazonium salt, which is then reacted with CuCl or CuBr to introduce –Cl or –Br. For iodo compounds, the diazonium salt is simply shaken with KI. This method is important for introducing halogens that are otherwise difficult to introduce by direct methods.

Alkyl halides are colourless when pure; bromides and iodides develop colour on exposure to light. Lower members (methyl chloride, methyl bromide, ethyl chloride) are gases at room temperature while higher members are liquids or solids. Boiling points are higher than corresponding hydrocarbons due to strong dipole-dipole and van der Waals forces. For the same alkyl group, boiling points follow: RI > RBr > RCl > RF. Branching decreases boiling point; para-isomers of dihalobenzenes have higher melting points due to greater crystal lattice symmetry.

Haloalkanes are only slightly soluble in water because insufficient energy is released when new haloalkane–water interactions replace the stronger original hydrogen bonds in water. However, they dissolve readily in organic solvents due to similar intermolecular forces. Density increases with the number and atomic mass of halogen atoms; bromo, iodo, and polychloro derivatives are denser than water.

Haloalkanes undergo three main types of reactions: nucleophilic substitution (SN1 and SN2), elimination, and reaction with metals. In SN2 reactions (bimolecular, second-order kinetics), the nucleophile attacks the carbon from the back, forming a transition state with five bonds to carbon, resulting in inversion of configuration (Walden inversion). Reactivity order: CH3X > 1° > 2° > 3°. In SN1 reactions (unimolecular, first-order kinetics), a carbocation intermediate is formed in the slow step; the nucleophile then attacks from either face, leading to racemisation. Reactivity order: 3° > 2° > 1°. Allylic and benzylic halides are highly reactive in SN1 due to resonance-stabilised carbocations.

Elimination reactions occur when haloalkanes with β-hydrogen are treated with alcoholic KOH; the major product follows Zaitsev's rule (more substituted alkene preferred). Grignard reagents (RMgX) are prepared by reacting haloalkanes with Mg in dry ether; they are highly reactive and react with any proton source. The Wurtz reaction (2RX + 2Na → RR + 2NaX in dry ether) doubles the carbon chain. Ambident nucleophiles like CN– and NO2– can attack through two different atoms, giving different products depending on the reagent used (KCN gives nitrile; AgCN gives isonitrile).

Haloarenes are far less reactive than haloalkanes towards nucleophilic substitution for four reasons: resonance gives the C–X bond partial double-bond character (harder to break); sp2 carbon holds the bonding electrons more tightly than sp3 carbon (C–Cl bond length 169 pm vs 177 pm in haloalkane); phenyl cation formed in SN1 is not resonance-stabilised; and electron-rich nucleophiles are repelled by the electron-rich arene ring. Electron-withdrawing groups (–NO2) at ortho/para positions markedly increase reactivity by stabilising the carbanion intermediate through resonance.

Haloarenes readily undergo electrophilic substitution reactions (halogenation, nitration, sulphonation, Friedel-Crafts). Halogen is o,p-directing because resonance stabilises the intermediate carbocation at ortho and para positions, although its inductive effect causes net deactivation of the ring. Reactions with metals include the Wurtz-Fittig reaction (aryl halide + alkyl halide + Na → alkylarene) and the Fittig reaction (two aryl halides + Na → diaryl compound).

Optically active compounds rotate plane-polarised light. A molecule is chiral (optically active) if it has an asymmetric carbon (four different substituents) and is non-superimposable on its mirror image. Enantiomers are non-superimposable mirror images; they have identical physical properties but differ in the direction of rotation of plane-polarised light. A racemic mixture (dl or ±) contains equal amounts of both enantiomers and is optically inactive.

SN2 reactions proceed with complete inversion of configuration at the chiral centre (Walden inversion). SN1 reactions produce racemic mixtures because the planar carbocation intermediate is attacked equally from both faces. Retention of configuration occurs when no bond to the stereocentre is broken during the reaction.

Dichloromethane (methylene chloride, CH2Cl2) is a widely used solvent, paint remover, and aerosol propellant, but harms the central nervous system. Chloroform (CHCl3) is used as a solvent and in production of freon R-22; it depresses the CNS and is stored in dark bottles as it oxidises to poisonous phosgene (COCl2) in air and light. Iodoform (CHI3) was used as an antiseptic (action due to free iodine released) but replaced due to objectionable odour. Carbon tetrachloride (CCl4) was widely used as a solvent and fire extinguisher but causes liver cancer and depletes the ozone layer.

Freons (chlorofluorocarbons, e.g., Freon-12, CCl2F2) are stable, non-toxic, non-corrosive gases used as refrigerants and aerosol propellants, prepared by Swarts reaction. They diffuse into the stratosphere and initiate radical chain reactions that deplete ozone, increasing UV exposure. DDT (p,p'-dichlorodiphenyltrichloroethane), originally synthesised in 1873 and recognised as an insecticide in 1939, was effective against malaria-spreading mosquitoes but was banned in the US in 1973 due to high stability, fat solubility, bioaccumulation, and toxicity to fish.

Haloalkanes and haloarenes form a crucial class of organic compounds whose chemistry is governed by the polarity and nature of the carbon–halogen bond. The chapter integrates classification, nomenclature, preparation methods, physical properties, and a wide spectrum of reactions—nucleophilic substitution (SN1 and SN2), elimination, and organometallic reactions—with mechanistic and stereochemical insight. The distinction between haloalkanes and haloarenes in reactivity, particularly towards nucleophilic substitution, arises fundamentally from differences in hybridisation, resonance, and bond strength.

For the CBSE board examination, this chapter is high-yield because it connects multiple concepts tested frequently: reaction mechanisms with stereochemistry, Grignard reagent preparation and properties, Sandmeyer's and halogen exchange reactions, the directing influence of halogens in electrophilic substitution, and the environmental impact of polyhalogen compounds such as freons and DDT. Mastery of SN1 vs SN2 reactivity trends, Zaitsev's rule, and the properties of common polyhalogen compounds is essential for scoring well.