Refer AEP & AK
Although benzene and alkane are quite unreative towards the usual oxidizing agents (KMnO4, K2Cr2O7 etc). The benzene ring renders an aliphatic side chain quite susceptible to oxidation. The side chain is oxidised down to the ring and only a carboxyl group (⎯COOH) remains to indicate the position of the original side chain. Potassium permanganate is generally used for this purpose, although potassium dichromate or dilute nitric acid can also be used. (Oxidation of a side chain is more difficult, however, than oxidation of an alkene and requires prolonged treatment with hot KMnO4)
This reaction is used for two purposes (a) synthesis of carboxylic acids and (b) identification of alkyl benzenes.
The Grignard synthesis of a carboxylic acid is carried out by bubbling gaseous CO2 into the ether solution of the Grignard reagent or by pouring the Grignard reagent on crushed dry ice (solid CO2). In the latter method dry ice serves not only as reagent but also as cooling agent.
The Grignard reagent adds to the carbon – oxygen double bond of CO2 just as in the reaction with aldehydes and keotnes. The product is the magnesium salt of the carboxylic acid, from which the free acid is liberated by treatment with mineral acid.
The Grignard’s reagent can be prepared from primary, secondary, tertiary or aromatic halides. The method is limited only by the presence of other reactive group in the molecule. The following synthesis illustrate the application of this method.
Aliphatic nitriles are prepared by treatment of alkyl halides with sodium cyanide in a solvent that will dissolve both reactants. In dimethyl sulfoxide (DMSO), reaction occurs rapidly and exothermically at room temperature. The resulting nitrile is then hydrolysed to the acid by boiling with aqueous alkali or acid.