Unit 9: Haloarenes
Nomenclature and Isomerism
Haloarenes (Aryl Halides)
Haloarenes, also known as aryl halides, are aromatic compounds in which one or more hydrogen atoms of the benzene ring are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). The general formula is C6H5X where X = F, Cl, Br, I. According to IUPAC nomenclature, the halogen substituent is indicated by a prefix (fluoro‑, chloro‑, bromo‑, iodo‑) attached to the name of the parent aromatic hydrocarbon. For example:
C6H5Cl→ chlorobenzeneC6H5Br→ bromobenzeneC6H5I→ iodobenzeneC6H4Cl2(with two chlorine atoms) → dichlorobenzene (isomers discussed below)
When multiple substituents are present, the ring is numbered to give the lowest set of locants, and the halogen prefixes are listed alphabetically.
Isomerism: Position Isomerism
Because the benzene ring is symmetrical, substitution patterns give rise to position (or regio) isomers. For dihalogenated benzenes, three distinct isomers exist:
- Ortho (o‑): substituents on adjacent carbon atoms (1,2‑positions). Example:
1,2‑dichlorobenzene(o‑DCB). - Meta (m‑): substituents separated by one carbon atom (1,3‑positions). Example:
1,3‑dichlorobenzene(m‑DCB). - Para (p‑): substituents opposite each other (1,4‑positions). Example:
1,4‑dichlorobenzene(p‑DCB).
These isomers differ in physical properties such as melting point, boiling point, and solubility, and they exhibit different reactivities in electrophilic substitution reactions due to varying electronic effects.
Preparation
From Benzene: Electrophilic Aromatic Substitution
The most common laboratory method for preparing chlorobenzene and bromobenzene involves direct halogenation of benzene in the presence of a Lewis acid catalyst (FeCl₃, FeBr₃, or AlCl₃). The reaction proceeds via generation of an electrophilic halogen species.
General equation:
C6H6 + X2 \xrightarrow[FeX3]{} C6H5X + HX
Where: X = Cl or Br; FeX₃ = FeCl₃ for chlorination, FeBr₃ for bromination.
Example (chlorination):
C6H6 + Cl2 \xrightarrow{FeCl3} C6H5Cl + HCl
The reaction is typically carried out at temperatures between 40–80 °C. Excess halogen leads to polyhalogenated products, which can be minimized by controlling the stoichiometry and temperature.
From Benzene Diazonium Salt: Sandmeyer Reaction
Aryl halides can also be synthesized from aryl diazonium salts via the Sandmeyer reaction, which provides a reliable route to chlorobenzene, bromobenzene, and iodobenzene under mild conditions.
General equation:
Ar–N₂⁺X⁻ + CuY → Ar–Y + N₂ + CuX
Where: Ar = aryl group (e.g., C₆H₅–); X⁻ = counter‑anion of the diazonium salt (commonly Cl⁻); Y = nucleophilic halide (Cl⁻, Br⁻, CN⁻, etc.) supplied by CuY catalyst.
Example (chlorobenzene synthesis):
C6H5N₂⁺Cl⁻ + CuCl → C6H5Cl + N₂ + CuCl₂
The diazonium salt is prepared in situ by treating aniline with nitrous acid (generated from NaNO₂/HCl) at 0–5 °C. The Sandmeyer reaction is particularly valuable when direct halogenation of benzene is undesirable due to over‑reaction or sensitivity of substituents.
Physical Properties
- State and Appearance: Most mono‑halogenated benzenes are colourless liquids at room temperature (e.g., chlorobenzene, bp 132 °C; bromobenzene, bp 156 °C). Higher halogens (iodobenzene) are also liquids but may darken on exposure to light.
- Boiling Points: Haloarenes exhibit significantly higher boiling points than the corresponding haloalkanes of comparable molecular weight due to increased molecular mass, greater polarizability, and the ability to engage in dipole‑dipole interactions. For instance, chlorobenzene (C₆H₅Cl, M = 112.56 g mol⁻¹) boils at 132 °C, whereas chloroethane (C₂H₅Cl, M = 64.51 g mol⁻¹) boils at 12 °C.
- Density: Generally greater than water (chlorobenzene density ≈ 1.11 g cm⁻³).
- Solubility: Practically insoluble in water (<0.5 g L⁻¹) because the non‑polar aromatic ring dominates over the polar C–X bond. They are readily soluble in organic solvents such as ethanol, ether, benzene, carbon tetrachloride, and acetone.
- Refractive Index: Typically in the range 1.52–1.56 for chlorobenzenes and bromobenzenes.
Chemical Properties
Low Reactivity in Nucleophilic Substitution
The carbon–halogen bond in aryl halides is markedly stronger and less polarized than in alkyl halides. This results from resonance delocalization of the lone pair on the halogen into the aromatic π‑system, which imparts partial double‑bond character to the C–X bond.
Resonance illustration (chlorobenzene):
\displaystyle \overset{\cdot\cdot}{Cl}\!-\!C_6H_5 \leftrightarrow C_6H_5\!-\!Cl^{+}\!\leftrightarrow\!C_6H_5\!=\!Cl^{-}
Consequently, haloarenes resist typical SN1 and SN2 reactions under conditions that readily convert alkyl halides to alcohols, amines, etc. For example, aqueous NaOH does not hydrolyze chlorobenzene to phenol unless harsh conditions (high temperature, pressure) are applied.
Reduction
Catalytic hydrogenation of haloarenes over metals such as Ni, Pd, or Pt leads to saturation of the aromatic ring, yielding cyclohexane derivatives. The halogen is typically removed as hydrogen halide.
General equation:
C6H5X + 3 H_2 \xrightarrow[Ni,\,\Delta]{} C6H12 + HX
Example:
C6H5Cl + 3 H_2 \xrightarrow{Ni,\,200^\circ C} C6H12 + HCl
Thus, chlorobenzene can be reduced to cyclohexane, a reaction exploited in the industrial production of cyclohexane from benzene via chlorination followed by hydrogenation.
Electrophilic Aromatic Substitution (EAS)
Despite the deactivating nature of the halogen substituent (due to –I effect), halogens are ortho‑para directors because of resonance donation (+M). Consequently, electrophilic substitution on haloarenes yields a mixture of ortho‑ and para‑products, with para often predominating due to steric hindrance at the ortho positions.
Nitration:
C6H5Cl + HNO_3 \xrightarrow{H_2SO_4,\,50^\circ C} o\text{-}NO_2C_6H_4Cl + p\text{-}NO_2C_6H_4Cl + H_2O
The nitro group is strongly deactivating; further nitration is difficult.
Sulphonation:
C6H5Cl + H_2SO_4 \xrightarrow{\Delta} o\text{-}ClC_6H_4SO_3H + p\text{-}ClC_6H_4SO_3H + H_2O
Sulphonic acid groups can be removed by heating with aqueous acid, making this a useful protective‑group strategy.
Halogenation (further halogenation):
C6H5Cl + Cl_2 \xrightarrow{FeCl_3} o\text{-}C_6H_4Cl_2 + p\text{-}C_6H_4Cl_2 + HCl
Polyhalogenated benzenes (e.g., dichlorobenzenes, trichlorobenzenes) are important intermediates for dyes and agrochemicals.
Fittig Reaction
The Fittig reaction couples two aryl halide molecules in the presence of sodium metal to form a biaryl compound (biphenyl). This reaction exemplifies the ability of aryl halides to undergo oxidative coupling under strongly reducing conditions.
General equation:
2\,ArX + 2\,Na \xrightarrow{\Delta} Ar–Ar + 2\,NaX
Example (formation of biphenyl):
2\,C_6H_5Cl + 2\,Na \xrightarrow{300^\circ C} C_6H_5–C_6H_5 + 2\,NaCl
The reaction proceeds via radical intermediates; the sodium metal donates electrons to the carbon–halogen bond, generating aryl radicals that combine.
Wurtz‑Fittig Reaction
A mixed coupling between an aryl halide and an alkyl halide in the presence of sodium yields an alkyl‑aryl product. This reaction is a variant of the Wurtz reaction and is useful for synthesizing toluene and related compounds.
General equation:
ArX + R'X + 2\,Na \xrightarrow{\Delta} Ar–R' + 2\,NaX
Example (toluene synthesis):
C_6H_5Cl + CH_3Cl + 2\,Na \xrightarrow{250^\circ C} C_6H_5–CH_3 + 2\,NaCl
The product, toluene, is a key precursor in the manufacture of benzene, xylenes, and various solvents.
Action with Chloral: DDT Formation
One of the most historically significant reactions of chlorobenzene is its condensation with chloral (trichloroacetaldehyde) in the presence of concentrated sulfuric acid to produce dichloro‑diphenyl‑trichloroethane (DDT), a potent insecticide.
Overall equation:
C_6H_5Cl + CCl_3CHO \xrightarrow{H_2SO_4} C_{14}H_9Cl_5 + H_2O
Where: C₁₄H₉Cl₅ is DDT (specifically, p,p′‑DDT).
The mechanism involves electrophilic aromatic substitution of the chlorobenzene ring by the chloral‑derived carbocation, followed by a second substitution on another chlorobenzene molecule, yielding the symmetrical diphenyl structure. DDT’s effectiveness and environmental persistence led to its widespread use in the mid‑20th century, though its ecological impact eventually resulted in restrictions.
Uses
- Solvents: Chlorobenzene and bromobenzene are employed as solvents for resins, oils, rubber, and in the formulation of paints and inks due to their good solvating power and moderate polarity.
- Intermediates in Synthesis: Aryl halides serve as precursors for phenols (via NaOH under high temperature/pressure), amines (via ammonolysis), and organometallic compounds (e.g., Grignard reagents: C₆H₅MgBr from bromobenzene).
- Pesticides: DDT, derived from chlorobenzene, was historically used extensively for agricultural pest control and malaria vector management.
- Refrigerants: Certain chloro‑ and fluorobenzenes (e.g., chlorotrifluorobenzene) have been investigated as low‑global‑warming‑potential refrigerants, although their use is limited.
- Chemical Industry: Polychlorinated benzenes (e.g., 1,2,4‑trichlorobenzene) are used as dielectric fluids in transformers and as intermediates in the production of dyes, pharmaceuticals, and agrochemicals.