Haloarenes or Arylhalides

Haloarene are the aromatic compound in which at least one hydrogen atom is replaced by halogen atom.

Eg.

If there are diff group are attached with the benzene ring then the priority of the group is

General preparation of Haloarenes

a) From benzene

Benzene react with halogen in presence of corresponding lewis acid gives haloarenes.

Raschig process

Industrially chlorobenzene is prepared by passing a mixture of benzene vapour, HCl and atmospheric oxygen over the cupric chloride gives chlorobenzene.

From benzenediazonium salt (chloride)

Aniline react with nitrous acid at cold condition gives benzenediazonium chloride. This rxn is known as diazotization reaction.

Sand-meyer’s rxn

Benzenediazonium chloride when warmed with cuprous chloride and bromide in presence of corresponding halogen acid gives haloarene.

Modified Sandmeyer’s rxn (Gattermann’s rxn)

Benzenediazonium chloride when warmed with copper powder in presence of HCl or HBr gives haloarene.

Iodo benzene can be prepared by warming the benzenediazonium chloride with KI soln.

Fluorobenzene can be obtained by the rxn of benzenediazonium chloride with fluroboric acid.

Properties of Haloarene

• Haloarenes are low boiling crystalline solid or liquid with sweet smell.
• They are insoluble in water but soluble in organic solvent like ether, alcohol etc.

Resonating Structure of chlorobenzene

The source shows resonance structures of chlorobenzene with delocalisation of the lone pair of chlorine into the benzene ring.

1) Haloarene are less reactive than haloalkane toward nucleophilic substitution rxn. why?

Haloarenes are less reactive than haloalkane toward nucleophilic substitution rxn due to following facts:

1) Resonating effect

According to the resonating structure the bond b/w carbon and halogen atom is partially double bond which is shorter and stronger than the bond in haloalkane.

2) Hybridization effect

The C atom of haloarene is sp² hybridized but in haloalkane carbon atom is sp³ hybridized. sp³ hybridization is highly reactive than sp².

2) Haloarene are ortho para director towards electrophilic substitution rxn. why?

• Electrophile are the electron deficient particle, they always attack on electron rich centre.
• According to the resonating structure of haloarene ortho and para position contain −ve charge. That mean electron rich centre.
• Therefore incoming electrophile attack on ortho and para position to give ortho para disubstituted product.
• Hence haloarene are called ortho-para director.

Electrophilic Substitution Rxn

i) Halogenation

Chlorobenzene react with chlorine in presence of lewis acid gives ortho and para dichloro benzene.

ii) Nitration

Chlorobenzene react with nitrating mixture (conc HNO₃ + conc H₂SO₄) gives ortho and para nitro chlorobenzene.

iii) Sulphonation rxn

Chlorobenzene react with fuming H₂SO₄ gives ortho or para chlorobenzene sulphonic acid.

iv) Friedel’s Craft’s rxn

a) Friedel’s craft’s alkylation

Chlorobenzene react with alkyl halide in presence of lewis acid gives ortho and para chlorotoluene.

b) Friedel’s craft’s acylation

Chlorobenzene react with acid halid in presence of lewis acid gives aromatic ketone.

Nucleophilic Substitution rxn

i) Dow’s process

Chlorobenzene when fused with NaOH gives sodium phenoxide which on acidification gives phenol. This is the industrial preparation of phenol.

ii) Ammonolysis

Chlorobenzene react with ammonia in presence of Cu₂O gives aniline.

iii) Formation of cyanide

Chlorobenzene react with CuCN in presence of pyridine gives phenylcyanide.

iv) Fittig rxn (Rxn with Na metal)

Chlorobenzene when heated with Na metal in presence of dry ether gives diphenyl.

v) Wurtz Fittig rxn

Chlorobenzene when react with alkyl halide in presence of sodium metal and the dry ether gives alkyl benzene.

vi) Rxn with mg-metal

Chlorobenzene react with mg metal in presence of dry ether gives aromatic grignard reagent.

vii) Redⁿ

Chlorobenzene undergoes redⁿ in presence of Ni−Al alloy in alkaline medium gives benzene.

Imp Reaction with chloral

Chloral react with chlorobenzene in presence of conc H₂SO₄ gives dichlorodiphenyltrichloro ethane (DDT) which is an insecticide.

IX Ullmann rxn

* Phenol into DDT

6* Benzoin condensation

Benzaldehyde when react with alk. cyanide gives benzoin and their rxn is called benzoin condensation.