Which one of the following reactions does NOT give benzene as the product?

To determine which reaction does not yield benzene, we can analyze each of the chemical reactions shown in the options:

Option 1: Decarboxylation of Sodium Benzoate
When sodium benzoate is heated with sodalime (a mixture of NaOH and CaO), it undergoes decarboxylation to produce benzene:
$C_6{H}_5\mathrm{COONa}+\mathrm{NaOH}\stackrel{\mathrm{CaO},\Delta }{\to }{C}_6{H}_6+{\mathrm{Na}}_2{\mathrm{CO}}_3$
Thus, this reaction produces benzene.

Option 2: Aromatization of n-Hexane
When n-hexane is heated at 773 K and 10 to 20 atm pressure in the presence of oxides of molybdenum, vanadium, or chromium (such as ${\mathrm{Mo}}_2{O}_3$), it undergoes dehydrogenation and cyclization (aromatization) to form benzene:
${\mathrm{CH}}_3{\left({\mathrm{CH}}_2\right)}_4{\mathrm{CH}}_3\stackrel{{\mathrm{Mo}}_2{O}_3,773K,10-20\mathrm{atm}}{\to }{C}_6{H}_6+4H_2$
Thus, this reaction produces benzene.

Option 3: Cyclic Polymerization of Ethyne
When ethyne (acetylene) gas is passed through a red hot iron tube at 873 K, it undergoes cyclic polymerization to form benzene:
$3\mathrm{HC}\equiv \mathrm{CH}\stackrel{\mathrm{Red}\mathrm{hot}\mathrm{Fe}\mathrm{tube},873K}{\to }{C}_6{H}_6$
Thus, this reaction produces benzene.

Option 4: Hydrolysis of Benzene Diazonium Chloride
When benzene diazonium chloride is warmed with water, it undergoes hydrolysis to form phenol rather than benzene, releasing nitrogen gas and hydrochloric acid:
$C_6{H}_5{N}_2^{+}{\mathrm{Cl}}^{-}+H_{2}O\to \mathrm{warm}{C}_6{H}_5\mathrm{OH}+N_2+\mathrm{HCl}$
Since this reaction yields phenol ($C_6{H}_5\mathrm{OH}$) and not benzene, it is the correct choice.

Therefore, the reaction that does NOT give benzene as the product is the reaction of benzene diazonium chloride with warm water (Option 4).