Haloalkanes and Haloarenes: Preparation, SN1 and SN2 Reactions

What are they?

Replacing a hydrogen atom of a hydrocarbon by a halogen gives a halogen derivative.

• In haloalkanes (alkyl halides, R–X), the halogen is attached to an sp³ carbon of an alkyl group.

• In haloarenes (aryl halides, Ar–X), it is attached to an sp² carbon of an aromatic ring.

Haloalkanes are classified as primary (1°), secondary (2°) or tertiary (3°) depending on whether the carbon bearing X is attached to one, two or three other carbon atoms.

Special types: allylic halides (X on a carbon next to a C=C), benzylic halides (X on a carbon next to a benzene ring), and vinylic halides (X directly on a C=C carbon).

Naming. CH₃CH₂CH(Cl)CH₃ is 2-chlorobutane; (CH₃)₃CBr is 2-bromo-2-methylpropane (tert-butyl bromide).

The C–X bond

Halogens are more electronegative than carbon, so the C–X bond is polar: the carbon carries a partial positive charge. This makes it a target for nucleophiles.

Bond length increases and bond strength decreases from C–F to C–I, so reactivity is R–I > R–Br > R–Cl > R–F.

Preparation

From alcohols (the most common method):

• R–OH + HX (with ZnCl₂ for HCl) → R–X + H₂O. The ZnCl₂ + HCl mixture is Lucas reagent.

• 3ROH + PX₃ → 3RX + H₃PO₃

• R–OH + SOCl₂ → R–Cl + SO₂ + HCl: the best method, because both by-products are gases that escape, leaving pure product.

From hydrocarbons:

• Free-radical halogenation of alkanes gives mixtures, so it is less useful.

• Addition of HX to alkenes (Markovnikov's rule; with peroxide, HBr adds anti-Markovnikov).

Halogen exchange:

• Finkelstein reaction: R–Cl/Br + NaI (in dry acetone) → R–I + NaCl/NaBr. It works because NaCl and NaBr are insoluble in acetone and precipitate.

• Swarts reaction: heating an alkyl chloride/bromide with a metal fluoride such as AgF or Hg₂F₂ gives an alkyl fluoride.

Haloarenes: by electrophilic substitution of arenes with Cl₂/Br₂ and a Lewis acid catalyst (FeCl₃), or by the Sandmeyer reaction from diazonium salts: ArN₂⁺Cl⁻ + CuCl → ArCl + N₂.

Physical properties

Boiling points are higher than those of the parent hydrocarbons (greater mass and polarity). For the same alkyl group: R–I > R–Br > R–Cl > R–F. Among isomers, boiling point decreases with branching.

Haloalkanes are only slightly soluble in water: the energy released in forming new attractions with water is less than that needed to break the existing hydrogen bonds between water molecules.

Nucleophilic substitution

A nucleophile (Nu⁻) replaces the halogen: R–X + Nu⁻ → R–Nu + X⁻. With KCN this gives nitriles (R–CN); with aqueous KOH, alcohols; with AgCN, isocyanides (R–NC), because AgCN is mainly covalent and attacks through nitrogen.

There are two mechanisms:

Allylic and benzylic halides react readily by SN1 because their carbocations are stabilised by resonance.

Optical activity and chirality

A carbon attached to four different groups is a chiral (asymmetric) carbon. A molecule with one chiral carbon is not superimposable on its mirror image; the two mirror images are enantiomers. They rotate plane-polarised light equally in opposite directions: dextrorotatory (+) and laevorotatory (−). An equal mixture of both is a racemic mixture, which shows no rotation.

In an SN2 reaction of a chiral haloalkane, the product has the opposite configuration; in SN1, a racemic mixture forms.

Elimination

Heating a haloalkane with alcoholic KOH removes HX and gives an alkene (β-elimination, or dehydrohalogenation).

Zaitsev's (Saytzeff's) rule: when more than one alkene can form, the more substituted alkene (with more alkyl groups on the double-bond carbons) is the major product. 2-bromobutane gives mainly but-2-ene.

Substitution and elimination compete: aqueous KOH favours substitution; alcoholic KOH and tertiary halides favour elimination.

Reactions with metals

• Grignard reagents: R–X + Mg (in dry ether) → R–MgX. They are very reactive and must be kept dry, because water converts them to alkanes: RMgX + H₂O → RH + Mg(OH)X.

• Wurtz reaction: 2R–X + 2Na (dry ether) → R–R + 2NaX.

• Wurtz-Fittig reaction: aryl halide + alkyl halide + Na → alkylbenzene.

• Fittig reaction: two aryl halides + Na → diphenyl.

Reactions of haloarenes

Haloarenes are much less reactive towards nucleophilic substitution than haloalkanes, because:

• resonance gives the C–X bond partial double-bond character,

• the carbon is sp² hybridised, holding the bond more tightly,

• the phenyl cation is unstable, so SN1 is not possible,

• the electron-rich ring repels nucleophiles.

Substitution happens only under harsh conditions (chlorobenzene + NaOH at 623 K and 300 atm gives phenol), unless electron-withdrawing –NO₂ groups are present at the ortho or para positions, which make it much easier.

The halogen is ortho-para directing but deactivating in electrophilic substitution (nitration, halogenation, sulphonation, Friedel-Crafts).

Polyhalogen compounds

Summary

• R–X reactivity: R–I > R–Br > R–Cl; SOCl₂ is best for making alkyl chlorides.

• SN2: one step, primary substrates, inversion. SN1: carbocation, tertiary substrates, racemisation.

• Alcoholic KOH gives alkenes (Zaitsev's rule); aqueous KOH gives alcohols.

• Haloarenes resist nucleophilic substitution unless activated by ortho/para –NO₂ groups.

• CFCs damage ozone; DDT biomagnifies.