41Haloarenes

📚
INTRODUCTION
▢ Aromatic Halogen Compounds: Aromatic hydrocarbon derivatives formed by replacement of one or more H-atoms by halogen atoms.
▢ Types:

Table 1: Types of aromatic halogen compounds

Type
Other name
Halogen position
General example
Reactivity
Nuclear halogen substituted product
Haloarene / aryl halide
Halogen directly attached to benzene ring
\(Ar-X\), e.g. \(C_6H_5Cl\)
Less reactive
Side-chain halogen substituted product
Aralkyl halide
Halogen attached to side-chain carbon
\(Ar-CH_2X\), e.g. \(C_6H_5CH_2Cl\)
Very reactive; similar to haloalkanes
📚
PREPARATION OF ARYL HALIDES
▢ From Aromatic Hydrocarbons:
❖ Direct / Nuclear Halogenation:
  • •
    Best method for chloro and bromo derivatives.
  • •
    Electrophilic substitution reaction.
  • •
    Catalyst: Lewis acid such as \(FeCl_3\), \(AlCl_3\), \(FeBr_3\).
  • •
    Lewis acid carries/activates halogen for attack on aromatic ring.
  • •
    \(C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl\)
  • •
    \(C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr\)
❖ Iodination:
  • •
    Iodo derivatives require oxidizing agent.
  • •
    Oxidizing agents: iodic acid \((HIO_3)\), nitric acid \((HNO_3)\).
  • •
    Reason: removes HI and drives reaction forward.
  • •
    \(C_6H_6 + I_2 \xrightarrow{HIO_3/HNO_3} C_6H_5I\)
❖ Fluorination:
  • •
    Aryl fluorides cannot be prepared by direct fluorination.
  • •
    Reason: fluorine has very high affinity for hydrogen; reaction is very violent.
❖ Side-Chain Halogenation:
  • •
    Occurs in presence of sunlight / \(h\nu\).
  • •
    Forms aralkyl halides.
  • •
    Free radical substitution reaction.
  • •
    \(C_6H_5CH_3 + Cl_2 \xrightarrow{h\nu} C_6H_5CH_2Cl + HCl\)
  • •
    Further chlorination may form benzal chloride and benzotrichloride.
Mechanism difference
  • •
    Nuclear halogenation → electrophilic substitution.
  • •
    Side-chain halogenation → free radical substitution.
▢ From Benzene Diazonium Salt:
❖ Sandmeyer Reaction:
  • •
    Diazonium salt converted into aryl chloride/bromide/cyanide.
  • •
    Reagents: \(CuCl/HCl\), \(CuBr/HBr\), \(CuCN/KCN\).
  • •
    \(ArN_2^+Cl^- \xrightarrow{CuCl/HCl} ArCl + N_2\)
  • •
    \(ArN_2^+Cl^- \xrightarrow{CuBr/HBr} ArBr + N_2\)
❖ Gattermann Reaction:
  • •
    Modification of Sandmeyer reaction.
  • •
    Copper powder + corresponding halogen acid \((HCl/HBr)\) used instead of cuprous halide.
  • •
    \(ArN_2^+Cl^- \xrightarrow{Cu/HCl} ArCl + N_2\)
  • •
    \(ArN_2^+Cl^- \xrightarrow{Cu/HBr} ArBr + N_2\)
❖ Balz-Schiemann Reaction:
  • •
    Used for preparation of aryl fluorides.
  • •
    Diazonium salt reacts with fluoroboric acid.
  • •
    Aryl diazonium fluoroborate on heating gives aryl fluoride.
  • •
    \(ArN_2^+BF_4^- \xrightarrow{\Delta} ArF + BF_3 + N_2\)
▢ From Phenols:
  • •
    Phenol does not react normally with \(HX\), \(PX_3\) or \(SOCl_2\).
  • •
    With \(POCl_3\), chlorobenzene yield is very poor.
  • •
    Main product is triphenyl phosphate.
  • •
    \(3C_6H_5OH + POCl_3 \rightarrow (C_6H_5)_3PO_4 + 3HCl\)
▢ Raschig Process:
  • •
    Commercial method for chlorobenzene.
  • •
    Benzene heated with \(HCl\) and \(O_2\).
  • •
    Catalyst: \(CuCl_2\).
  • •
    Temperature: nearly \(200^\circ C\).
  • •
    \(C_6H_6 + HCl + \frac{1}{2}O_2 \xrightarrow{CuCl_2,200^\circ C} C_6H_5Cl + H_2O\)
▢ Hunsdiecker Reaction:
  • •
    Silver benzoate distilled with \(Br_2\) in \(CCl_4\).
  • •
    Product: bromobenzene.
  • •
    \(C_6H_5COOAg + Br_2 \xrightarrow{CCl_4} C_6H_5Br + CO_2 + AgBr\)
📚
REACTIONS OF ARYL HALIDES
▢ Two Main Reaction Types:

Table 1: Reaction types of aryl halides

Reaction type
Meaning
Important point
Nucleophilic substitution
Replacement of halo group by other nucleophile
Aryl halides are less reactive than alkyl halides
Electrophilic ring substitution
Substitution on benzene ring
Halogen is deactivating but ortho-para directing
▢ Low Reactivity Towards Nucleophilic Substitution:
  • •
    Carbon bearing halogen is \(sp^2\)-hybridized.
  • •
    \(sp^2\) carbon is more electronegative than \(sp^3\) carbon.
  • •
    \(C-X\) bond becomes less polar.
  • •
    \(C-X\) bond gets partial double bond character due to resonance.
  • •
    Resonance makes cleavage of \(C-X\) bond difficult.
  • •
    Aryl halide molecule is resonance stabilized.
▢ Ring Substitution Behaviour:
  • •
    Haloarenes undergo electrophilic substitution.
  • •
    Haloarenes are less reactive than benzene due to \(-I\) effect of halogen.
  • •
    Halogens are deactivating but ortho-para directors.
  • •
    Ortho-para direction occurs due to \(+R\) effect of halogen.
📚
IMPORTANT REACTIONS OF CHLOROBENZENE
▢ Replacement of Halogen by \(-OH\): Dow Process:
  • •
    Chlorobenzene treated with aqueous \(NaOH\).
  • •
    Conditions: \(350^\circ C\), \(300\ atm\).
  • •
    Sodium phenoxide formed first.
  • •
    Acidification gives phenol.
  • •
    \(C_6H_5Cl + NaOH \xrightarrow{350^\circ C,300atm} C_6H_5ONa + NaCl\)
  • •
    \(C_6H_5ONa + HCl \rightarrow C_6H_5OH + NaCl\)
▢ Replacement by \(-NH_2\): Formation of Aniline:
  • •
    Chlorobenzene reacts with ammonia under high temperature and pressure.
  • •
    Conditions: nearly \(200^\circ C\), \(600\ atm\).
  • •
    \(C_6H_5Cl + NH_3 \rightarrow C_6H_5NH_2 + HCl\)
▢ Replacement by \(-CN\): Formation of Benzonitrile:
  • •
    Chlorobenzene reacts with cuprous cyanide.
  • •
    Condition: nearly \(200^\circ C\).
  • •
    \(C_6H_5Cl + CuCN \xrightarrow{200^\circ C} C_6H_5CN + CuCl\)
  • •
    Phenyl cyanide = benzonitrile.
▢ Conversions of Benzonitrile:

Table 1: Useful conversions of benzonitrile

Reaction
Reagent / condition
Product
Partial hydrolysis
Conc. HCl or alkaline \(H_2O_2\)
Benzamide \((C_6H_5CONH_2)\)
Complete hydrolysis
Dil. HCl or dil. NaOH
Benzoic acid \((C_6H_5COOH)\)
Complete reduction
\(LiAlH_4\)
Benzylamine \((C_6H_5CH_2NH_2)\)
Mendius reduction
Na / ethanol
Benzylamine
Stephen reduction
\(SnCl_2/HCl\), then hydrolysis
Benzaldehyde \((C_6H_5CHO)\)
▢ Reaction with Sodamide in Liquid Ammonia:
  • •
    Chlorobenzene + \(NaNH_2\) in liquid \(NH_3\).
  • •
    Temperature: about \(-77^\circ C\).
  • •
    Intermediate: benzyne.
  • •
    Final product: aniline.
  • •
    \(C_6H_5Cl \xrightarrow{NaNH_2/liquid\ NH_3} C_6H_5NH_2\)
▢ Reduction of Aryl Halides:
  • •
    Aryl halides reduced to parent hydrocarbons.
  • •
    Reagents: \(LiAlH_4\), Ni-Al/NaOH, Na-Hg/\(H_2O\).
  • •
    \(C_6H_5X \rightarrow C_6H_6\)
▢ Wurtz-Fittig Reaction:
  • •
    Aryl halide + alkyl halide + Na in dry ether.
  • •
    Product: alkyl benzene.
  • •
    \(ArX + RX + 2Na \xrightarrow{dry\ ether} ArR + 2NaX\)
▢ Fittig Reaction:
  • •
    Aryl halides only + Na in dry ether.
  • •
    Product: biaryl.
  • •
    \(2ArX + 2Na \xrightarrow{dry\ ether} Ar-Ar + 2NaX\)
▢ Reaction with Magnesium:
  • •
    Aryl halides do not easily form Grignard reagent in ordinary ether.
  • •
    Aryl halides form Grignard reagent in tetrahydrofuran / THF.
  • •
    THF is an organic solvent.
  • •
    \(ArX + Mg \xrightarrow{THF} ArMgX\)
▢ Reaction with Lithium:
  • •
    Aryl halides react with lithium to form aryl lithium compounds.
  • •
    \(ArX + 2Li \rightarrow ArLi + LiX\)
▢ Ullmann Reaction:
  • •
    Aryl iodide heated with copper powder gives biaryl.
  • •
    \(2C_6H_5I + 2Cu \xrightarrow{\Delta} C_6H_5-C_6H_5 + 2CuI\)
  • •
    Given mainly by iodobenzene.
  • •
    Not generally given by chloro- and bromobenzene in given note.
📚
ELECTROPHILIC SUBSTITUTION REACTIONS OF HALOARENES
▢ General:
  • •
    Halogen is ortho-para directing.
  • •
    Ring is deactivated.
  • •
    Haloarenes are less reactive than benzene.
  • •
    Para product usually major due to steric hindrance at ortho position.
▢ Halogenation:
  • •
    \(C_6H_5Cl + Cl_2 \xrightarrow{FeCl_3} o\text{-}C_6H_4Cl_2 + p\text{-}C_6H_4Cl_2\)
  • •
    Para isomer generally major.
▢ Nitration:
  • •
    \(C_6H_5Cl + HNO_3 \xrightarrow{conc.H_2SO_4} o\text{-}NO_2C_6H_4Cl + p\text{-}NO_2C_6H_4Cl\)
  • •
    With fuming \(HNO_3\), uncontrolled nitration may occur.
▢ Sulphonation:
  • •
    \(C_6H_5Cl + SO_3/H_2SO_4 \rightarrow o\text{-}chlorobenzenesulphonic\ acid + p\text{-}chlorobenzenesulphonic\ acid\)
  • •
    Para product generally major.
▢ Friedel-Crafts Alkylation:
  • •
    \(C_6H_5Cl + RCl \xrightarrow{anhyd.AlCl_3} o\text{-}alkylchlorobenzene + p\text{-}alkylchlorobenzene\)
▢ Friedel-Crafts Acylation:
  • •
    \(C_6H_5Cl + RCOCl \xrightarrow{anhyd.AlCl_3} o\text{-}acylchlorobenzene + p\text{-}acylchlorobenzene\)
📚
DDT
▢ Full Form: Dichlorodiphenyl trichloroethane.
▢ IUPAC Name: 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane.
▢ Preparation:
  • •
    Chlorobenzene reacts with chloral in presence of conc. \(H_2SO_4\).
  • •
    \(2C_6H_5Cl + CCl_3CHO \xrightarrow{conc.H_2SO_4} DDT + H_2O\)
▢ Properties:
  • •
    White powder.
  • •
    Insoluble in water.
  • •
    Non-biodegradable.
▢ Uses:
  • •
    Effective insecticide.
  • •
    Used against mosquitoes.
  • •
    Used against flies.
  • •
    Used against crop pests.
📚
HIGH-YIELD POINTS
▢ Must Remember:
  • •
    Haloarene = halogen directly attached to aromatic ring.
  • •
    Aralkyl halide = halogen attached to side-chain carbon.
  • •
    Aralkyl halides are more reactive than aryl halides.
  • •
    Direct halogenation of benzene gives chlorobenzene/bromobenzene in presence of Lewis acid.
  • •
    Direct fluorination is not used because reaction is violent.
  • •
    Iodination requires oxidizing agent.
  • •
    Nuclear halogenation = electrophilic substitution.
  • •
    Side-chain halogenation = free radical substitution.
  • •
    Aryl halides are less reactive towards nucleophilic substitution due to resonance and \(sp^2\) C-X bond.
  • •
    Halogens are ring-deactivating but ortho-para directing.
  • •
    Dow process converts chlorobenzene into phenol.
  • •
    Benzyne intermediate forms in reaction of chlorobenzene with sodamide in liquid ammonia.
  • •
    Wurtz-Fittig gives alkyl benzene.
  • •
    Fittig gives biaryl.
  • •
    Ullmann reaction is mainly shown by iodobenzene.
  • •
    Aryl halides form Grignard reagent in THF.
  • •
    DDT is non-biodegradable insecticide.
Q1.
Which of the following is a nuclear halogen substituted aromatic compound?
Q2.
Nuclear halogenation of benzene occurs by
Q3.
Side-chain halogenation of toluene occurs by
Q4.
Aryl fluorides are best prepared by
Q5.
Chlorobenzene is converted into phenol by
Q6.
The intermediate in reaction of chlorobenzene with sodamide in liquid ammonia is
Q7.
Halogens in haloarenes are
Q8.
Aryl halides are less reactive than alkyl halides towards nucleophilic substitution mainly due to
Q9.
Wurtz-Fittig reaction gives
Q10.
Fittig reaction gives
Q11.
Ullmann reaction is mainly given by
Q12.
DDT is