(a) Name the starting material used in the industrial preparation of phenol.
(b) Write complete reaction for the bromination of phenol in aqueous and non-aqueous medium.
(c) Explain why Lewis acid is not required in bromination of phenol?
(a) The starting material used in the industrial preparation of phenol is cumene.
(b) Phenols when treated with bromine water gives polyhalogen derivatives in which all the hydrogen atoms present at ortho and para positions with respect to - OH group are replaced by bromine atoms.
However, in non-aqueous medium such as $\mathrm{CS}_2, \mathrm{CCl}_4, \mathrm{CHCl}_3$, monobromophenols are obtained.
In aqueous solution, phenol ionises to form phenoxide ion. This ion activates the benzene ring to a very large extent and hence the substitution of halogen takes place at all three positions.
On the other hand, in non-aqueous solution ionisation of phenol is greatly suppressed. Therefore, ring is activated slightly and hence monosubstitution occur.
(c) Lewis acid is an electron deficient molecule. In bromination of benzene, Lewis acid is used to polarise $\mathrm{Br}_2$ to form $\mathrm{Br}^{+}$electrophile.
In case of phenol, oxygen atom of phenol itself polarises the bromine molecule to form $\mathrm{Br}^{+}$ion (electrophile). So, Lewis acid is not required in the bromination of phenol.
How can phenol be converted to aspirin?
Phenol is converted into salicylic acid. This salicylic acid is treated with acetic anhydride. On acetylation of salicylic acid, aspirin (acetyl salicylic acid) is formed.
Explain a process in which a biocatalyst is used in industrial preparation of a compound known to you.
Enzymes are biocatalyst. These biocatalysts (enzymes) are used in the industrial preparation of ethanol. Ethanol is prepared by the fermentation of molasses - a dark brown coloured syrup left after crystallisation of sugar which still contains about $40 \%$ of sugar.
The process of fermentation actually involves breaking down of large molecules into simple ones in the presence of enzymes. The source of these enzymes is yeast. The various reactions taking place during fermentation of carbohydrates are
$\underset{\text { Sucrose }}{\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}}+\mathrm{H}_2 \mathrm{O} \xrightarrow{\text { Invertase }} \underset{\text { Glucose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}+\underset{\text { Fructose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}$
$$\mathrm{\mathop {{C_6}{H_{12}}{O_6}}\limits_{Glu\cos e} \buildrel {Zymase} \over \longrightarrow \mathop {2{C_2}{H_5}OH}\limits_{Ethyl\,alcohol} + 2C{O_2}}$$
In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact and fermentation starts. Fermentation takes place in anaerobic conditions i.e., in absence of air. $\mathrm{CO}_2$ gas is released during fermentation.
The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 per cent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.