Haloalkanes and Haloarenes

Replacing one or more hydrogen atoms of an aliphatic or aromatic hydrocarbon with halogen atoms (F, Cl, Br, I) yields haloalkanes (alkyl halides) and haloarenes (aryl halides), respectively.

1. Classification of Organohalogen Compounds (6.1)

• By Number of Halogens: Mono- (C₂H₅X), Di- (XCH₂CH₂X), or Polyhalogen (tri-, tetra-, etc.) compounds.
• Dihalides (Same Halogen):
  • Geminal dihalides (gem-dihalides): Both halogens on the same carbon. Common name: alkylidene halides (e.g., CH₃CHCl₂ = Ethylidene chloride / 1,1-Dichloroethane).
  • Vicinal dihalides (vic-dihalides): Halogens on adjacent carbons. Common name: alkylene dihalides (e.g., ClCH₂–CH₂Cl = Ethylene dichloride / 1,2-Dichloroethane).
• Compounds with sp³ C–X Bond:
  • Alkyl halides (R–X, C_nH_2n+1X): Classified as Primary (1°), Secondary (2°), or Tertiary (3°) based on the carbon holding X.
  • Allylic halides: X is bonded to an sp³ carbon adjacent to a C=C double bond (e.g., CH₂=CH–CH₂Br, 3-Bromopropene).
  • Benzylic halides: X is bonded to an sp³ carbon attached directly to an aromatic ring (e.g., C₆H₅CH₂Cl = Benzyl chloride).
• Compounds with sp² C–X Bond:
  • Vinylic halides: X is bonded directly to an sp² carbon of a C=C double bond (e.g., CH₂=CHCl, Chloroethene).
  • Aryl halides: X is bonded directly to an sp² carbon of an aromatic ring (e.g., Bromobenzene, p-Chlorotoluene).

2. Common & IUPAC Nomenclature Highlights (6.2, Table 6.1)

3. Nature of the C–X Bond (6.3, Table 6.2)

Because halogens are more electronegative than carbon, the C–X bond is polarised: C^δ+–X^δ−. Going down Group 17 (F → Cl → Br → I), atomic size increases, so C–X bond length increases while bond enthalpy decreases:

1. From Alcohols (Best Lab Route for Alkyl Halides, 6.4.1)

• R–OH + SOCl2 → R–Cl + SO2(g)↑ + HCl(g)↑ — Preferred method because both byproducts are escapable gases, yielding pure alkyl chloride.
• R–OH + HCl —[anhyd. ZnCl2]→ R–Cl + H2O — 1° and 2° alcohols need anhydrous ZnCl₂ catalyst; 3° alcohols react merely by shaking with conc. HCl at room temp. Reactivity of alcohols with HX: 3° > 2° > 1°.
• R–OH + NaBr + H2SO4 → R–Br + NaHSO4 + H2O (or constant-boiling 48% HBr).
• 3 R–OH + PX3 → 3 R–X + H3PO3 (X = Cl, Br)  |  R–OH + PCl5 → R–Cl + POCl3 + HCl  |  R–OH + red P/X2 → R–X (PX₃ generated in situ for Br₂, I₂).
• Why Phenols Don't React: Aryl halides cannot be prepared from phenols using HX/PX₃/SOCl₂ because the C–O bond in phenol has partial double-bond character and is too strong to break!

2. From Hydrocarbons (6.4.2)

• Free Radical Halogenation of Alkanes: CH3CH2CH2CH3 + Cl2 (UV / Δ) → 1-Chlorobutane + 2-Chlorobutane. Gives a complex mixture of mono- & polyhalo isomers (poor yield of single product). Example 6.3: (CH3)2CHCH2CH3 gives 4 monochloro isomers; Exercise 6.5: Cyclopentane (C₅H₁₀) gives a single monochloro product in sunlight.
• Addition of HX to Alkenes: Follows Markovnikov's rule (CH3CH=CH2 + HI → CH3CH(I)CH3 major + CH3CH2CH2I minor). With HBr + peroxide, anti-Markovnikov addition occurs (Example 6.4(iii): C6H5CH2CH=CH2 + HBr/peroxide → C6H5CH2CH2CH2Br).
• Addition of Halogens (Unsaturation Test): Passing alkene into reddish-brown Br₂ in CCl₄ discharges the colour, yielding colourless vic-dibromides.

3. Halogen Exchange Reactions (6.4.3) — Guaranteed Exam Named Reactions

4. Preparation of Haloarenes (6.5)

• Electrophilic Substitution of Arenes: Toluene + X2 —[Fe / anhyd. FeCl3, dark]→ o-Halotoluene + p-Halotoluene (easily separated by large melting point difference). Iodination is reversible and requires an oxidising agent (HNO₃ or HIO₄) to oxidise HI; fluorination is avoided due to extreme reactivity.
• Sandmeyer's Reaction (from Primary Aromatic Amines):
  C6H5NH2 + NaNO2 + HX —[273–278 K]→ C6H5N2+X− (Benzene diazonium halide)
  C6H5N2+X− —[Cu2Cl2 or Cu2Br2]→ C6H5Cl (or C6H5Br) + N2↑
  For iodobenzene, simply shake the diazonium salt with aqueous KI (no Cu₂X₂ needed!).

Pure alkyl halides are colourless, though bromides and iodides develop colour when exposed to light. Many volatile halogen compounds possess a sweet smell. CH₃Cl, CH₃Br, C₂H₅Cl, and some chlorofluoromethanes are gases at room temperature; higher members are liquids or solids.

1. Boiling Points

• Vs Parent Hydrocarbons: Haloalkanes have considerably higher boiling points than hydrocarbons of comparable molecular mass due to greater polarity (dipole–dipole attractions) and stronger van der Waals forces.
• Effect of Halogen (Same R group): R–I > R–Br > R–Cl > R–F (size, electron cloud, and mass of halogen increase down the group → stronger van der Waals forces). Also increases with carbon chain length.
• Effect of Branching (Isomers): Boiling point decreases with branching because compact spherical shapes reduce surface area and van der Waals contact:
  1-Bromobutane (375 K) > 2-Bromobutane (364 K) > 2-Bromo-2-methylpropane (346 K)

2. Isomeric Dihalobenzenes: Boiling Point vs Melting Point

3. Density (Table 6.3) & Solubility

• Density: Bromo, iodo, and polychloro derivatives are heavier than water (density > 1 g mL^-1). Density rises with number of carbon atoms, number of halogen atoms, and halogen atomic mass:
  n-C3H7Cl (0.89) < n-C3H7Br (1.335) < n-C3H7I (1.747 g/mL)  |  CH2Cl2 (1.336) < CHCl3 (1.489) < CCl4 (1.595 g/mL).
• Solubility (Exercise 6.12(ii)): Haloalkanes are polar, yet very slightly soluble (immiscible) in water! Why? Dissolving requires breaking intermolecular attractions in R–X and strong hydrogen bonds in water, whereas the new dipole–water attractions formed release less energy than needed to break water's H-bonds. However, they dissolve freely in organic solvents where intermolecular forces have comparable strength.

Because the sp³ carbon bears a partial positive charge (C^δ+–X^δ−), electron-rich nucleophiles (Nu⁻) attack carbon and displace the halide leaving group (X⁻). In all nucleophilic substitutions of a given R group, leaving-group reactivity is: R–I > R–Br > R–Cl >> R–F (Example 6.6).

1. Key Reagents & Ambident Nucleophiles (Table 6.4 & Example 6.5)

Ambident nucleophiles possess two nucleophilic donor atoms and can link through either site:

Other vital Table 6.4 substitutions: aq. NaOH/KOH → ROH; NaOR' → ROR' (ether); NH3 → RNH2; R'COOAg → R'COOR (ester); LiAlH4 → RH.

2. S_N2 vs S_N1 Mechanisms Side-by-Side (Hughes & Ingold, 1937)

1. Optical Activity & The Polarimeter

• Passing ordinary light through a Nicol prism produces plane polarised light (PPL) vibrating in a single plane.
• Compounds whose solutions rotate the plane of PPL (measured in a polarimeter) are optically active:
  • Dextrorotatory (d or +): Rotates PPL to the right (clockwise).
  • Laevorotatory (l or −): Rotates PPL to the left (anticlockwise).

2. Chirality, Asymmetric Carbon & Enantiomers

• Asymmetric Carbon (Stereocentre): An sp³ tetrahedral carbon attached to four different atoms or groups.
• Chiral vs Achiral: Objects or molecules that are non-superimposable on their mirror images (e.g., hands, shoes, butan-2-ol, 2-chlorobutane, BrClCHI, 2-bromopropanoic acid, 2,3-dihydroxypropanal) are chiral (optically active). Molecules superimposable on their mirror images (e.g., propan-2-ol, Fig. 6.5 — rotated 180° matches identically!) are achiral (optically inactive).
• Enantiomers: Stereoisomers related as non-superimposable mirror images. They share identical melting point, boiling point, solubility, and refractive index, differing only in the direction they rotate PPL (+ vs −).
• Racemic Mixture (dl or ±) & Racemisation: An equimolar (50 : 50) mixture of (+) and (−) enantiomers has zero net optical rotation because one isomer's rotation cancels the other's (e.g., (±)-butan-2-ol). Converting an optically active enantiomer into a racemic mixture is called racemisation.

3. Stereochemical Outcomes in Substitution Reactions

1. β-Elimination (Dehydrohalogenation) & Saytzeff / Zaitsev Rule (1875)

When a haloalkane carrying a β-hydrogen (on the carbon adjacent to the α-carbon holding X) is heated with alcoholic KOH (or NaOEt/EtOH), H is eliminated from the β-carbon and X from the α-carbon to form an alkene.

2. Elimination vs Substitution Competition (Exercise 6.20)

• Solvent Effect: Aqueous KOH provides hydrated OH⁻ favouring Substitution (Alcohol); Alcoholic KOH generates stronger alkoxide base (RO⁻) favouring β-Elimination (Alkene).
• Steric Bulk & Substrate: A bulky base like (CH3)3CO− prefers abstracting an outer β-proton (Elimination) rather than squeezing to carbon; 1° halides favour S_N2, 2° depend on base strength/size, 3° halides favour S_N1 or Elimination.

3. Reaction with Metals: Grignard Reagents & Wurtz Reaction

• Grignard Reagent (Victor Grignard, 1900; Nobel Prize 1912):
  CH3CH2Br + Mg —[dry ether]→ CH3CH2MgBr (Alkyl magnesium halide, R^δ−–Mg^δ+–X^δ−).
  Here, the C–Mg bond is covalent yet highly polar (carbon pulls electrons from electropositive Mg), while Mg–X is essentially ionic.
• Why Anhydrous Conditions / Dry Ether? (Exercise 6.12(iii)): Grignard reagents are so reactive they react violently with any proton source—even traces of water, alcohols, or amines—to give hydrocarbons:
  RMgX + H2O → R–H + Mg(OH)X (and with D₂O → R–D, Intext 6.9!).
• Wurtz Reaction: Alkyl halides react with sodium in dry ether to yield symmetrical alkanes with double the carbon atoms: 2 R–X + 2 Na —[dry ether]→ R–R + 2 NaX.

1. Why Are Haloarenes Unreactive Toward Nucleophilic Substitution? (4 NCERT Reasons)

1. Resonance Effect (Partial Double-Bond Character): Lone pairs on Cl conjugate with the benzene π-ring across four resonance structures, giving the C–Cl bond partial double-bond character—harder to break than a single bond.
2. Difference in Hybridisation of Carbon: Haloarenes bind X to an sp² carbon (33.3% s-character, more electronegative → holds electron pair tightly), whereas haloalkanes use sp³ carbon (25% s-character). Hence C–Cl bond length is shorter and stronger in chlorobenzene (169 pm) than in haloalkanes (177 pm)! (This also explains Exercise 6.12(i): chlorobenzene has a lower dipole moment than cyclohexyl chloride!)
3. Instability of Phenyl Cation: Self-ionisation gives a phenyl cation unstabilised by resonance → S_N1 mechanism is completely ruled out.
4. Electronic Repulsion: An incoming electron-rich nucleophile faces electrostatic repulsion from the electron-rich benzene π-cloud.

2. Replacement by –OH (Dow's Conditions) & Effect of –NO₂ Groups

Why ortho/para–NO₂ works while meta–NO₂ has NO effect: During nucleophilic attack, the negative charge in the resonance-stabilised carbanion intermediate appears only at ortho and para carbons relative to Cl. Thus o/p–NO₂ groups pull and delocalise that negative charge, whereas a meta–NO₂ sits on a carbon that never bears the negative charge!

3. Electrophilic Aromatic Substitution (Example 6.9)

Chlorine is deactivating yet ortho, para-directing! Why?

• Reactivity is controlled by the stronger –I (inductive) effect: Cl withdraws net electron density, destabilising the carbocation intermediate and making EAS slower than benzene (needs harsher conditions).
• Orientation is controlled by the +R (resonance) effect: Resonance pushes electron density and stabilises the carbocation specifically at ortho and para positions (para isomer is ALWAYS major due to minimal steric hindrance).
• 4 Classic EAS Reactions of Chlorobenzene: Halogenation (Cl2 / anhyd. FeCl3 → 1,4-dichlorobenzene major), Nitration (HNO3 + conc. H2SO4 → 1-chloro-4-nitrobenzene major), Sulphonation (conc. H2SO4, Δ → 4-chlorobenzenesulphonic acid major), Friedel-Crafts Alkylation/Acylation (CH3Cl or CH3COCl + anhyd. AlCl3 → 4-chloro derivative major).

4. Reaction with Metals: Wurtz-Fittig & Fittig Reactions

Hydrocarbons carrying more than one halogen atom are called polyhalogen compounds. While hugely useful in industry, medicine, and agriculture, many resist breakdown by soil bacteria and harm human health or the environment.