Haloalkanes and Haloarenes

In this chapter: classification and nomenclature, nature of the C-X bond, methods of preparation, physical properties, nucleophilic substitution (SN1 and SN2) and its stereochemistry, elimination and Zaitsev's rule, reactions with metals, reactions of haloarenes, and uses and effects of polyhalogen compounds.

Classification and nomenclature

Haloalkanes (alkyl halides) have halogen attached to an sp³ carbon; haloarenes (aryl halides) have halogen attached to an sp² carbon of an aromatic ring. By the number of halogen atoms they are mono-, di- or polyhalogen compounds.

Dihalides with both halogens on the same carbon are geminal (gem-dihalides, e.g. CH₃CHCl₂, ethylidene chloride); on adjacent carbons they are vicinal (vic-dihalides, e.g. ClCH₂CH₂Cl, ethylene dichloride).

Nature of the C-X bond

Halogens are more electronegative than carbon, so the C-X bond is polar (C^δ+-X^δ−). Down the group, the size of the halogen increases, so the C-X bond length increases (C-F < C-Cl < C-Br < C-I) and the bond enthalpy decreases. The dipole moments of the methyl halides are CH₃Cl (1.860 D) > CH₃F (1.847 D) > CH₃Br (1.830 D) > CH₃I (1.636 D); fluorine's very short bond offsets its high electronegativity.

Preparation

From alcohols

Thionyl chloride is preferred because both by-products are gases that escape, leaving fairly pure alkyl chloride. For HCl, the reactivity of alcohols is 3° > 2° > 1°; tertiary alcohols react with concentrated HCl even without ZnCl₂.

From hydrocarbons

• Free radical halogenation of alkanes gives a mixture of isomeric mono- and polyhalogen products, so it is not a good laboratory method.
• Electrophilic substitution of arenes with Cl₂ or Br₂ in the presence of a Lewis acid (Fe, FeCl₃, FeBr₃) in the dark. Toluene gives a mixture of ortho and para halotoluenes.
• Sandmeyer reaction: a benzenediazonium salt with Cu₂Cl₂ or Cu₂Br₂ gives chloro- or bromobenzene; iodobenzene is made by shaking the diazonium salt with KI.
• Addition to alkenes: HX adds by Markovnikov's rule (propene + HBr gives 2-bromopropane); in the presence of peroxide, HBr adds anti-Markovnikov. Br₂ in CCl₄ adds to give a vic-dibromide; loss of the reddish brown bromine colour is a test for unsaturation.

Halogen exchange

• Finkelstein reaction: R-Cl or R-Br + NaI in dry acetone → R-I + NaCl or NaBr. The reaction goes forward because NaCl and NaBr are insoluble in dry acetone and precipitate out (Le Chatelier's principle).
• Swarts reaction: an alkyl chloride or bromide heated with a metallic fluoride such as AgF, Hg₂F₂, CoF₂ or SbF₃ gives the alkyl fluoride.

Physical properties

Boiling points are higher than those of the parent hydrocarbons and increase with molar mass: RI > RBr > RCl > RF for the same alkyl group. Among isomers, boiling point falls with branching (more spherical molecules have less surface contact). Among dihalobenzene isomers, the para isomer has the highest melting point because its symmetry lets it pack best in the crystal. Bromo, iodo and polychloro derivatives are denser than water. Haloalkanes are only very slightly soluble in water, since the energy released in forming new attractions with water is less than that needed to break its hydrogen bonds.

Nucleophilic substitution

In R-X, the carbon carries a partial positive charge, so a nucleophile can attack it and displace X⁻ (the leaving group). Examples: with aqueous KOH an alcohol forms; with NaI, an iodide; with NH₃, an amine; with R′O⁻Na⁺, an ether (Williamson synthesis).

Ambident nucleophiles have two donor sites. KCN is ionic, so the more stable C-C bond forms and the product is an alkyl cyanide (R-CN). AgCN is mainly covalent, so nitrogen donates its lone pair and the product is an isocyanide (R-NC). In the same way KNO₂ gives an alkyl nitrite (R-O-N=O) and AgNO₂ a nitroalkane (R-NO₂).

SN2 mechanism

Substitution, nucleophilic, bimolecular. The nucleophile attacks the carbon from the side opposite the leaving group, and the new bond forms while the old one breaks, in a single step through a transition state in which carbon is bonded partly to both. The other three groups flip over like an umbrella in a strong wind, so the configuration is inverted. Rate = k[RX][Nu⁻]. Because the nucleophile must reach the back of the carbon, bulky groups hinder the reaction: CH₃X > 1° > 2° > 3°.

SN1 mechanism

Substitution, nucleophilic, unimolecular. It occurs in two steps, usually in polar protic solvents (water, alcohols, acetic acid). First the C-X bond ionises slowly to give a planar carbocation; then the nucleophile attacks it quickly. Rate = k[RX], independent of the nucleophile. The more stable the carbocation, the faster the reaction: 3° > 2° > 1° > CH₃X. Allylic and benzylic halides react readily by SN1 because their carbocations are stabilised by resonance.

For a given alkyl group, reactivity in both mechanisms is R-I > R-Br > R-Cl > R-F, since the C-I bond is the weakest.

Stereochemistry

A carbon bonded to four different groups is chiral (asymmetric); its molecule is not superimposable on its mirror image. The two mirror-image forms are enantiomers. Compounds that rotate plane polarised light are optically active: clockwise rotation is dextrorotatory (d or +), anticlockwise is laevorotatory (l or −). An equimolar mixture of the two enantiomers, a racemic mixture, shows zero rotation. Butan-2-ol is a simple chiral molecule; its C-2 carries H, OH, CH₃ and C₂H₅.

• Retention: the spatial arrangement of bonds at the chiral centre is kept.
• Inversion: the product has the opposite arrangement. SN2 reactions of optically active halides always give inversion.
• Racemisation: an optically active compound gives a racemic mixture. SN1 reactions of optically active halides are accompanied by racemisation.

Elimination and reactions with metals

Heated with alcoholic KOH, a haloalkane with a β-hydrogen loses HX to form an alkene (β-elimination or dehydrohalogenation). When more than one alkene is possible, Zaitsev's (Saytzeff's) rule applies: the major product is the alkene with the greater number of alkyl groups on the double-bonded carbons. 2-Bromobutane gives mainly but-2-ene, with but-1-ene as the minor product.

Substitution and elimination compete. Primary halides with a good nucleophile tend to substitute; tertiary halides and strong, bulky bases favour elimination. In practice, aqueous KOH gives alcohols and alcoholic KOH gives alkenes.

Worked example: Predict the main organic product when 2-bromopentane is heated with (a) aqueous KOH and (b) alcoholic KOH.
Solution: (a) OH⁻ acts as a nucleophile and substitutes Br, giving pentan-2-ol, CH₃CH(OH)CH₂CH₂CH₃. (b) OH⁻ acts as a base. H can be removed from C-1 or C-3; removal from C-3 gives the more substituted alkene, so by Zaitsev's rule the major product is pent-2-ene, CH₃CH=CHCH₂CH₃, with pent-1-ene as minor product.

• Grignard reagent: R-X + Mg in dry ether → R-MgX. The C-Mg bond is highly polar, so the carbon behaves as a carbanion. Grignard reagents react with any source of protons (water, alcohols, amines) to give hydrocarbons: RMgX + H₂O → RH + Mg(OH)X. They must therefore be made and kept under anhydrous conditions.
• Wurtz reaction: 2R-X + 2Na → R-R + 2NaX, in dry ether. It gives alkanes with double the number of carbon atoms of the halide.

Reactions of haloarenes

Low reactivity towards nucleophilic substitution

• Resonance: a lone pair of the halogen is delocalised into the ring, giving the C-Cl bond partial double bond character, which is harder to break.
• Hybridisation: the carbon is sp², with more s character, so the C-X bond is shorter and stronger than in haloalkanes.
• Unstable phenyl cation: the SN1 path is ruled out because the phenyl cation is not stabilised by resonance.
• Repulsion: the electron-rich ring repels the approaching nucleophile, and backside attack is blocked by the ring.

Chlorobenzene is converted to phenol only under drastic conditions: heating with aqueous NaOH at 623 K and 300 atm gives sodium phenoxide, which is acidified (Dow's process). An electron-withdrawing group such as -NO₂ at the ortho or para position makes substitution much easier, because it stabilises the negatively charged intermediate by resonance: 4-nitrochlorobenzene reacts at 443 K, 2,4-dinitrochlorobenzene at 368 K and 2,4,6-trinitrochlorobenzene with warm water. A nitro group at the meta position has no such resonance effect.

Electrophilic substitution

Halogen is deactivating but ortho, para-directing. Its −I effect withdraws electron density from the whole ring and slows the reaction, while its +R effect increases electron density at the ortho and para positions more than at meta. The para product is usually major because the ortho positions are more crowded.

• Halogenation (Cl₂, anhydrous FeCl₃): 1,2- and 1,4-dichlorobenzene.
• Nitration (conc. HNO₃ + conc. H₂SO₄): 1-chloro-2-nitrobenzene (minor) and 1-chloro-4-nitrobenzene (major).
• Sulphonation (conc. H₂SO₄): 2- and 4-chlorobenzenesulphonic acid.
• Friedel-Crafts alkylation (CH₃Cl, anhydrous AlCl₃): 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene; acylation (CH₃COCl) gives 2- and 4-chloroacetophenone.

Reactions with sodium

Wurtz-Fittig reaction: an aryl halide and an alkyl halide with sodium in dry ether give an alkylarene (C₆H₅Cl + CH₃Cl → C₆H₅CH₃). Fittig reaction: two aryl halides with sodium give a biaryl (2C₆H₅Cl → C₆H₅-C₆H₅, diphenyl).

Polyhalogen compounds

Common mistakes: (1) Writing SN1 reactivity order for SN2 questions, or the reverse. (2) Saying SN1 gives pure inversion; it gives mainly racemisation. (3) Forgetting that KCN gives cyanides but AgCN gives isocyanides. (4) Calling halogen an activating group because it is ortho, para-directing. It deactivates the ring. (5) Making a Grignard reagent in the presence of any trace of water or alcohol.

JEE and NEET focus

• Preparation reactions with reagents and conditions: SOCl₂, PCl₅, Finkelstein, Swarts, Sandmeyer, Markovnikov and peroxide effect.
• SN1 vs SN2: rate laws, mechanism, stereochemistry and reactivity orders, including allylic and benzylic halides.
• Zaitsev's rule and substitution vs elimination with aqueous and alcoholic KOH.
• Reasons for the low reactivity of haloarenes and the effect of ortho and para nitro groups.
• Electrophilic substitution products of chlorobenzene; Wurtz, Wurtz-Fittig and Fittig reactions.
• Uses and environmental effects of CHCl₃, CHI₃, CCl₄, freons and DDT.

Practice questions