Haloalkanes and Haloarenes Class 12: Notes in One Read

These Haloalkanes and Haloarenes Class 12 notes take you through the chapter in order: classification and naming, the C-X bond, preparation, physical properties, SN1 and SN2 reactions, elimination, haloarene reactions and polyhalogen compounds. You will also get a revision list, important questions with model answers, and common mistakes to avoid.

Classification

Haloalkanes and haloarenes form when hydrogen atoms in hydrocarbons are replaced by halogen atoms (F, Cl, Br, I).

• Haloalkanes (alkyl halides): halogen attached to an sp³ hybridised carbon of an alkyl group.
• Haloarenes (aryl halides): halogen bonded directly to an sp² hybridised carbon of an aromatic ring.

They matter in medicine too: chloramphenicol is used for typhoid and chloroquine for malaria.

By number of halogens, they are monohalo, dihalo or polyhalo. Monohaloalkanes are primary (1°), secondary (2°) or tertiary (3°), depending on the carbon the halogen is attached to. Special types:

• Allylic halide: halogen on an sp³ carbon next to a C=C double bond.
• Benzylic halide: halogen on a carbon attached to an aromatic ring.
• Vinylic halide: halogen directly on an sp² carbon of a C=C double bond.

Naming and the C-X bond

Common names use the alkyl group + halide (ethyl chloride). IUPAC names use the prefix halo (chloroethane). Dihalides are geminal (both halogens on the same carbon) or vicinal (on adjacent carbons).

The C-X bond is polar: halogens are more electronegative than carbon, so carbon is δ+ and the halogen δ−. Going down from F to I, atomic size increases, so the bond gets longer and weaker.

Preparation

Haloalkanes from alcohols: react with HX, PX₃, PX₅ or thionyl chloride. SOCl₂ is preferred for chlorides because the by-products SO₂ and HCl are gases and escape, leaving pure alkyl halide:

CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ + HCl

From hydrocarbons: free radical halogenation of alkanes (gives mixtures), or addition of HX to alkenes. With unsymmetrical alkenes, Markovnikov's rule applies: the halogen attaches to the carbon with fewer hydrogens.

Halogen exchange:

• Finkelstein reaction: R-X + NaI → R-I, using alkyl chlorides or bromides with NaI in dry acetone.
• Swarts reaction: R-X + metallic fluoride → R-F.

Haloarenes:

• Electrophilic substitution: arene + Cl₂ or Br₂ with a Lewis acid such as Fe or FeCl₃, in the dark to avoid side-chain halogenation. Direct iodination is reversible, so an oxidising agent such as HNO₃ or HIO₄ is used to oxidise the HI formed.
• Sandmeyer reaction: a primary aromatic amine is treated with NaNO₂ and HCl at 273-278 K to form a benzene diazonium salt, which is then mixed with Cu₂Cl₂ or Cu₂Br₂. The diazonium group is replaced by the halogen and N₂ is released. For iodides, shaking the diazonium salt with KI is enough.

Physical properties

• Boiling point: higher than parent hydrocarbons, due to stronger dipole-dipole and van der Waals forces. For the same alkyl group: RI > RBr > RCl > RF. Branching lowers the boiling point because surface area decreases.
• Density: increases with more carbon atoms, more halogen atoms and heavier halogens. Bromo, iodo and polychloro derivatives are generally heavier than water.
• Solubility: very low in water, because haloalkanes cannot form hydrogen bonds with water; highly soluble in organic solvents.

SN2 and SN1 compared

SN2: the nucleophile attacks from the side opposite to the leaving group. The carbon flips like an umbrella turning inside out. Bulky groups block the attack, so methyl halides react fastest and tertiary halides slowest. Since C-I is the weakest bond, alkyl iodides are the most reactive.

SN1: step 1, the slow rate-determining step, is ionisation of C-X to form a carbocation. Step 2 is fast attack by the nucleophile. Tertiary carbocations are most stable (inductive effect and hyperconjugation). Allylic and benzylic halides are also very reactive in SN1, because their carbocations are resonance-stabilised. The carbocation is planar, so attack from either side is equally likely, giving a 50:50 mixture of two enantiomers.

Elimination and reactions with metals

β-elimination: heating with alcoholic KOH removes a hydrogen from the β-carbon and the halogen from the α-carbon, forming a double bond. Zaitsev's rule: the more substituted alkene is the major product. For 2-bromopentane, pent-2-ene (81%) is major and pent-1-ene (19%) is minor.

Grignard reagents: haloalkanes react with magnesium in dry ether to form RMgX. They react with even traces of water to form hydrocarbons, so they must be made under anhydrous conditions. The C-Mg bond is covalent but highly polar.

Wurtz reaction: 2R-X + 2Na → R-R + 2NaX (in dry ether), giving an alkane with double the number of carbon atoms.

Reactions of haloarenes

Haloarenes are much less reactive than haloalkanes towards nucleophilic substitution, for three reasons:

1. Resonance gives the C-X bond partial double bond character.
2. Hybridisation: the sp² carbon makes the bond shorter and stronger.
3. Unstable phenyl cation, which rules out SN1.

Electron-withdrawing groups like -NO₂ at the ortho and para positions increase reactivity, by stabilising the intermediate carbanion through resonance.

Haloarenes undergo electrophilic substitution: halogenation, nitration, sulphonation and Friedel-Crafts reactions. The halogen is deactivating but ortho/para directing, because resonance increases electron density at those positions.

Polyhalogen compounds

• Dichloromethane: solvent and paint remover; harms the central nervous system.
• Chloroform: once an anaesthetic, now mainly used to make Freon refrigerant R-22. In air and light it oxidises to phosgene (COCl₂), a poisonous gas, so it is stored in dark, completely filled bottles.
• Iodoform: was used as an antiseptic because it releases free iodine.
• Carbon tetrachloride: used in refrigerants and as a solvent; linked to liver cancer and ozone depletion.
• Freons (CFCs): stable and non-toxic but deplete the ozone layer in the stratosphere.
• DDT: a powerful insecticide that persists in the environment and builds up in fatty tissues, which led to its ban in many countries.

Remember this

• C-X bond strength: C-F strongest, C-I weakest.
• Boiling point: RI > RBr > RCl > RF; branching lowers it.
• SN2: 1° > 2° > 3°, inversion. SN1: 3° > 2° > 1°, racemisation.
• Zaitsev: more substituted alkene is major.
• Halogen on benzene: deactivating, ortho/para directing.

Important questions with model answers

1. Why does C-I have the lowest bond enthalpy among methyl halides? Iodine is the largest halogen, so the C-I bond is the longest. Longer bonds are weaker and need less energy to break.

2. Why is SOCl₂ preferred for making alkyl chlorides from alcohols? Its by-products, SO₂ and HCl, are gases that escape, leaving pure alkyl chloride.

3. Arrange in increasing boiling point: 1-chloropropane, isopropyl chloride, 1-chlorobutane. Isopropyl chloride < 1-chloropropane < 1-chlorobutane. Isopropyl chloride is branched, and 1-chlorobutane has a higher molecular mass than 1-chloropropane.

4. Why are tertiary halides least reactive in SN2? Bulky alkyl groups around the carbon bearing the halogen block the approach of the nucleophile.

5. Which reacts faster by SN1: 2-bromobutane or 2-bromo-2-methylpropane? 2-bromo-2-methylpropane, because it forms a tertiary carbocation, which is more stable than the secondary carbocation from 2-bromobutane.

6. Why must Grignard reagents be prepared in anhydrous conditions? They react with even traces of moisture to form alkanes, which destroys the reagent.

7. Why is chlorobenzene less reactive than methyl chloride towards nucleophilic substitution? Resonance gives the C-Cl bond in chlorobenzene partial double bond character, making it stronger and harder to break.

8. Why is chloroform stored in dark, completely filled bottles? To prevent its oxidation by air in light into phosgene, an extremely poisonous gas.

Common mistakes to avoid

• Mixing up allylic (next to C=C) and vinylic (on C=C) halides.
• Reversing the reactivity orders of SN1 and SN2.
• Writing inversion for SN1; SN1 gives racemisation.
• Forgetting the conditions: dry acetone (Finkelstein), dry ether (Grignard, Wurtz), alcoholic KOH (elimination), 273-278 K (diazotisation).

To practise mechanisms and name reactions until they feel easy, study this chapter with Joy.