13. Benzeneand its homologues may bepreparedby the following methods.

By the decarboxylation of sodium benzoate.

By heating phenol with zinc.

By the polymerization of ethyne (acetylene)

By the reduction ofbenzenediazonium chloride.

By the hydrolysis of sulphonic acid.

14. Haloarenes are chemically less reactive than haloalkanes. They can undergo the following reactions:

Replacement by hydroxy group (formation of phenol)

When aryl halides are heated at 623 K and under pressure (300 atm), with aqueous solution of sodium hydroxide, the halogen atom is replaced by hydroxyl group forming phenol. Firstly sodium phenoxide is formed, which on acidification gives phenol.



This reaction forms the basis for the manufacture of phenol by Dow's process.

Substitution by amino group (formation of aniline)

The halogen atom is replaced by amino group when aryl halides are heated with aqueous ammonia in the presence of cuprous oxide (catalyst) at 475 K and under a pressure of 60 atm.,



Substitution by cyano group (formation of cyanobenzene)

The halogen atom is replaced by cyano group on heating with anhydrous cuprous cyanide in the presence of pyridine or dimethylformamide (DMF).



The cyanobenzene can be converted into other useful compounds like Benzamide, benzoic aid and benzylamine under different conditions.

Effect of substituents in haloarenes (aryl halides) on the reactivity

The reactivity of haloarenes is markedly affected by the presence of certain groups at certain positions of the ring. The presence of electron withdrawing groups such as -NO2,-CN, -COOH, etc. at 'o' and 'p' positions to the halogen atom, greatly activates the halogen towards nucleophilic substitution reactions. For example, chlorobenzene is converted into phenol by aqueous NaOH only at temperatures above 573 K, whereas 'p' chloronitrobenzene is converted into nitrophenol by aqueous NaOH at a lower temperature of 433K. As the number of ortho and para nitro groups on the ring is increased the reactivity increases.