Haloarene

Halogen derivatives of arenes where the halo group is directly bonded to an aromatic ring are called haloarenes(aryl halides). It is represented by Ar-X where Ar is an aryl or phenyl group and X is F, Cl, Br or I.

Note: If the halogen atom is not directly bonded to the aromatic ring: Ar-CH₂X, they are not aryl halides. eg. Benzyl chloride.

Q: Write the structure of all possible isomers of C₇H₇Br.

General methods of preparation of chlorobenzene:

1. From Benzene: Benzene reacts with chlorine in presence of Lewis acid FeCl₃ at cold and dark conditions to give chlorobenzene.

2. Benzene diazonium chloride: Benzene diazonium chloride is reacted with cuprous chloride and HCl to give chlorobenzene. This reaction is called the Sandmeyer reaction.

Benzene diazonium chloride is reacted with copper and HCl to give chlorobenzene. This reaction is called the Gatterman reaction.

Diazotization reaction

Physical Properties

1. Chlorobenzene is a colorless oily liquid having sweet odour.
2. It is insoluble in water but soluble in the organic solvent.
3. It boils at 131 °C and freezes at -45 °C.

Chemical Properties

1. Nucleophilic Substitution Reaction

The low reactivity of haloarene than haloalkane.

Due to partial double bond development in the C-X bond of haloarene caused by resonance, the C-X bond becomes rigid and replacement of halogen by nucleophile becomes difficult. So, haloarenes are less reactive than haloalkanes.

i. Action with NaOH: Chlorobenzene is heated with NaOH at 350 °C and 300 atm to give sodium phenoxide which on acidification gives phenol. This reaction is called the Dow process.

ii. Action with ammonia: Chlorobenzene is heated with ammonia in presence of cuprous oxide to give aniline.

iii. Action with cuprous cyanide: Chlorobenzene is heated with cuprous cyanide in the presence of pyridine to give benzonitrile.

iv. Action with sodium

a. Fittig reaction: Chlorobenzene is reacted with sodium in presence of dry ether to give biphenyl.

b. Wurtz fittig reaction: Solution of chlorobenzene and chloromethane is reacted with sodium in presence of dry ether to give toluene.

v. Action with chloral: Chlorobenzene reacts with chloral (trichloro acetaldehyde) in presence of conc. H₂SO₄ to give dichlorodiphenyl trichloroethane (DDT). IUPAC: 2,2-Bis (4-chlorophenyl)-1,1,1-trichloroethane

2. Electrophilic Substitution Reaction

From above resonating structure, it is clear that -Cl group increases the electron density at ortho and para position. So, incoming electrophile prefers to attack at ortho and para position. Hence, -Cl group is ortho para directing group.
Chlorine atom withdraws electron from benzene ring due to electron withdrawing inductive effect. Since, inductive effect is stronger than resonace effect, -Cl is called deactivating group.

i. Chlorination:

ii. Nitration

iii. Sulphonation

iv. Friedel-craft alkylation:

v. Friedel-craft acylation:

Uses of chlorobenzene:

• To manufacture DDT, phenol, aniline, diphenyl, etc.

Some Important Questions

1. How do you prepare chlorobenzene from a. benzene b. aniline?
2. Why does chloroalkane undergoes nucleophilic substitution reaction easier than chlorobenzene?
3. Convert benzene into a. Benzoic acid b. Benzamide c. Benzyl amine
4. Convert chlorobenzene into a. bromobenzene b. DDT c. Toluene.
5. Chlorobenzene forms ortho para substituted products, explain.
6. Why chlorobenzene is less reactive than benzene in electrophilic substitution reaction?
7. Give an example of
   a. Fittig reaction b. Wurtz Fittig reaction c. Sandmayer reaction d. Gattermann reaction
8. Identify A and B

\begin{align*} a.\ Benzene\xrightarrow[]{Cl_{2}/FeCl_{3}}&A\xrightarrow[]{Cu_{2}(CN)_{2}}B\\ b.\ Aniline\xrightarrow[0-5\degree C]{NaNO_{2}+HCl}&A\xrightarrow[]{Cu_{2}(Cl)_{2}/HCl}B \end{align*}