8.7. Methods of Preparation of Carboxylic Acids

1. From primary alcohols and aldehydes Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media (Jones reagent).

This method involves oxidizing primary alcohols, which are alcohols where the hydroxyl (–OH) group is attached to a carbon atom that is only connected to one other carbon atom. The common oxidizing agents, such as potassium permanganate (KMnO4) or chromium compounds, facilitate this transformation to produce carboxylic acids. For instance, if we start with ethanol (a primary alcohol) and oxidize it using potassium permanganate, the result will be acetic acid (ethanoic acid).

Think of the oxidation of alcohols like the aging process of fruits. Just as a ripe banana can turn brown when exposed to air (a form of oxidation), primary alcohols become carboxylic acids when they undergo a similar process using strong chemicals. This transformation is crucial in various industries, such as making vinegar from fermented alcohol.

2. From alkylbenzenes Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain. Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected.

This preparation method focuses on aromatic compounds, particularly alkyl benzenes, which are compounds where a hydrocarbon chain is attached to a benzene ring. When these compounds are treated with strong oxidizers like chromic acid, the entire side chain is converted to a carboxylic acid. For example, toluene (methylbenzene) can be oxidized to benzoic acid. However, tertiary alkyl groups don’t undergo this reaction because they are resistant to oxidation.

Imagine a tree (the benzene ring) with branches (the alkyl side chains). If you cut all the branches back to their connection to the tree trunk (the benzene), you effectively create a clean stump (the carboxylic acid). This method is significant in chemical manufacturing, especially in the production of various fragrances and dyes that contain carboxylic acids.

3. From nitriles and amides Nitriles are hydrolysed to amides and then to acids in the presence of H+ or OH− as catalyst. Mild reaction conditions are used to stop the reaction at the amide stage.

This method involves converting nitriles, which are compounds containing a –C≡N group, into carboxylic acids. First, the nitrile undergoes hydrolysis to form an amide. This process can be facilitated by either acidic (H+) or basic (OH−) conditions. If we want to produce the acid, we can continue to hydrolyze the amide, but if we desire to stop at the amide, we would use milder conditions. For example, converting benzonitrile first yields benzamide, and with continued hydrolysis, produces benzoic acid.

Consider a garden where a seed (nitrile) can grow into a small plant (amide) and, with proper conditions, it matures into a full-fledged tree (carboxylic acid). This method is vital in organic synthesis, serving as a pathway to create many pharmaceuticals and agrochemicals.

4. From Grignard reagents Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which in turn give corresponding carboxylic acids after acidification with mineral acid.

Grignard reagents, which are organomagnesium halides, react vigorously with carbon dioxide to form a carboxylate salt. Once these salts are treated with a mineral acid, they are converted into the corresponding carboxylic acids. For example, phenylmagnesium bromide (a Grignard reagent) reacts with carbon dioxide to form benzoate, which turns into benzoic acid when treated with hydrochloric acid.

Imagine baking a cake (the Grignard reagent) where adding chocolate chips (carbon dioxide) creates a delicious chocolate cake (the carboxylic acid). This creative method is widely used in organic chemistry labs to produce complex carboxylic acids needed for various synthetic pathways.

5. From acyl halides and anhydrides Acid chlorides when hydrolysed with water give carboxylic acids or more readily hydrolysed with aqueous base to give carboxylate ions which on acidification provide corresponding carboxylic acids. Anhydrides on the other hand are hydrolysed to corresponding acid(s) with water.

This method involves acid chlorides (a type of acyl halide) reacting with water to produce carboxylic acids directly or, if treated with a base first, yielding carboxylate ions that can also form carboxylic acids upon acidification. Anhydrides, when hydrolyzed with water, will directly yield carboxylic acids as well. For instance, if acetyl chloride (an acyl halide) is treated with water, acetic acid is formed.

You can visualize this process like watering plants (the acid chlorides or anhydrides). Just as water transforms dry soil into nourishing mud (the acids), the hydrolysis of these compounds leads to the formation of useful carboxylic acids in our chemical garden.

6. From esters Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.

Esters can be converted into carboxylic acids through hydrolysis. In acidic conditions, this hydrolysis yields carboxylic acids directly. In basic hydrolysis, esters are first converted into carboxylate ions, which can then be acidified to produce the carboxylic acids. For example, treating ethyl acetate under acidic hydrolysis yields acetic acid.

Think of sugar dissolving in water (the ester hydrolysis) to create sweet syrup (the carboxylic acid). This method is widely utilized in food and beverage industries, as it is fundamental in producing flavorful carboxylic acids from naturally occurring esters.

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