Example 8.5

Example . Write chemical reactions to affect the following transformations: Butan- -ol to butanoic acid Benzyl alcohol to phenylethanoic acid (iii) -Nitrobromobenzene to -nitrobenzoic acid (iv) -Methylacetophenone to benzene- , -dicarboxylic acid (v) Cyclohexene to hexane- , -dioic acid (vi) Butanal to butanoic acid. . Show how each of the following compounds can be converted to benzoic acid. Ethylbenzene Acetophenone (iii) Bromobenzene Phenylethene (Styrene) (iii) (v) (vi) Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents. Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. The solubility decreases with increasing number of carbon atoms. Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, etc. The reaction of carboxylic acids are classified as follows: Acidity Reactions with metals and alkalies The carboxylic acids like alcohols evolve hydrogen with electropositive metals and form salts with alkalies similar to phenols. However, unlike phenols they react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound. Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion. . . Reactions Involving Cleavage of O–H Bond