CBSE Class 12 Chemistry Notes: Carboxylic Acids - General Methods of Preparation

Oxidation of alcohols, aldehydes and ketones

Illustration 1.

How will you prepare CH₃CH₂COOH from CH₃–CH=CH₂

Solution:

Illustration 2. Bring about the following transformations

Solution :

2. Oxidation of alkyl benzenes

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 (¾COOH) 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.

3. Carbonation of Grignard reagents

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.

*Illustration 1.*	(i)	Write A and B
	(ii)	Write A and B

*Solution:*	(i)
	(ii)

Illustration 2. Bring about the following transformations.

Solution:

Bring about the following transformations

4. Hydrolysis of nitriles

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.

*Illustration 1.*	*(i)*
		Write A, B and C.
	*(ii)*
		Write A and B.

*Solution:*	(i)
	(ii)

5. Use of alkoxide

6. Carbonylation of alkenes

7. Oxidative cleavage of alkenes, alkynes and cyclo alkenes

Illustration 2. Compound A, C₅H₈O₃, when heated with soda lime gives B which reacts with HCN to give C, C reacts with PCl₅ to give D which reacts with KCN to form E. E, on alkaline hydrolysis gives a salt which is isolated and heated with soda lime to produce n-butane. A, on careful oxidation with K₂Cr₂O₇ gives acetic acid and malonic acid. Give structural formulae of A to E.

Solution:

On Oxidation only two –COOH groups can be introduced, i.e., one to each carbon undergoing C–C fission, but in the resulting products we have three –COOH groups. Hence, one –COOH group is already there in compound A, the remaining portion C₄H₇O–, resembles with Keto substituted alkyl group

This indicates that the given organic compound A is Keto substituted acid. To assign position to Keto group in carbon chain, we know that keto acids on careful oxidation undergo C–C bond fission at a place where is situated, further Keto group is also converted into –COOH and remains with acid having small number of carbon atoms. From the above discussion it is clear that acetic acid is formed from: