Example 7.5

Example . . Kolbe’s reaction Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product. . Reimer-Tiemann reaction On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann reaction . The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde. . Reaction of phenol with zinc dust Phenol is converted to benzene on heating with zinc dust. . Oxidation Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones. . Give structures of the products you would expect when each of the following alcohol reacts with (a) HCl –ZnCl (b) HBr and (c) SOCl . (i) Butan- -ol (ii) -Methylbutan- -ol . Predict the major product of acid catalysed dehydration of (i) -methylcyclohexanol and (ii) butan- -ol . Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of the corresponding phenoxide ions. . Write the equations involved in the following reactions: (i) Reimer - Tiemann reaction (ii) Kolbe’s reaction Methanol and ethanol are among the two commercially important alcohols. . Methanol Methanol, CH OH, also known as ‘wood spirit’, was produced by destructive distillation of wood. Today, most of the methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr O catalyst. Methanol is a colourless liquid and boils at K. It is highly poisonous in nature. Ingestion of even small quantities of methanol can cause blindness and large quantities causes even death. Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde. . Ethanol Ethanol, C H OH, is obtained commercially by fermentation, the oldest method is from sugars. The sugar in molasses, sugarcane or fruits such as grapes is converted to glucose and fructose, (both of which have the formula C H O ), in the presence of an enzyme, invertase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase, which is found in yeast. 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. Carbon dioxide is released during fermentation. The action of zymase is inhibited once the percentage of alcohol formed exceeds percent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks. Ethanol is a colourless liquid with boiling point K. It is used as a solvent in paint industry and in the preparation of a number of carbon compounds. The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate (to give it a colour) and pyridine (a foul smelling liquid). It is known as denaturation of alcohol. Nowadays, large quantities of ethanol are obtained by hydration of ethene (Section . ).