8.7.1. 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).

Primary alcohols, which have the -OH group attached to the first carbon atom of the chain, can be converted to carboxylic acids through oxidation. This process involves the addition of oxygen or the removal of hydrogen. Common oxidizing agents like potassium permanganate or potassium dichromate facilitate this transformation, often in acidic conditions to ensure the alcohol fully converts to the carboxylic acid. The essential idea is that the alcohol loses hydrogen atoms which combine with the oxidizing agent to form water, while the alcohol remains as a carbonyl group in the carboxylic acid.

Think of primary alcohols like a sponge soaked with water. Oxidation is like squeezing out some of that water (removing hydrogen) to create a firmer sponge (the carboxylic acid). Just as squeezing too hard can damage a sponge, if the oxidation is too strong, it can also cause unwanted side reactions.

Carboxylic acids are also prepared from aldehydes by the use of mild oxidising agents.

Aldehydes can be converted into carboxylic acids using mild oxidizing agents. Since aldehydes already contain a carbonyl group (C=O), they require less rigorous treatment compared to alcohols. This transformation typically involves mild oxidants that can facilitate the addition of an -OH group to form the carboxylic acid. For instance, substances like Tollens' reagent serve this purpose without over-oxidizing the aldehyde.

Imagine you have a ripe fruit (the aldehyde). Giving it a bit of tenderness with very gentle care (mild oxidization) adds more flesh (the -OH group), turning it into a fruit jam (the carboxylic acid) without overcooking it. You still retain the fruity flavor, but now it’s richer.

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.

When alkyl groups attached to a benzene ring (alkylbenzenes) undergo vigorous oxidation, the entire side chain is converted into a carboxylic acid. This oxidation is typically more aggressive than that of alcohols, and it leads to the transformation of even long alkyl chains into shorter carboxylic acids. Tertiary alkyl groups resist this oxidation and remain intact, highlighting a difference in reactivity among the types of carbon chains.

Consider an alkylbenzene like a tangled ball of yarn (the alkyl group) attached to a wall (the benzene), and vigorous oxidation is like pulling on the yarn hard enough that it all unravels into short strands (the carboxylic acid). However, if the yarn is really thick (tertiary), it doesn’t unravel as easily.

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.

In the conversion process involving nitriles, the nitrile group (-C≡N) is treated with water, typically in acidic or basic conditions, leading first to the formation of an amide and subsequently to a carboxylic acid. This two-step hydrolysis allows the reaction to be carefully controlled to yield the desired product, particularly allowing for the stopping at the amide stage if needed.

Imagine you are preparing a layered cake (the carboxylic acid), and you can choose to stop at the frosting layer (the amide), which is made from mixing the ingredients in a specific order (the hydrolysis). You control the layers you want to end up with.

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 are highly reactive organomagnesium compounds that can react with carbon dioxide to create carboxylate salts. Upon adding an acid, such as hydrochloric acid, these salts are then converted into the corresponding carboxylic acids. This reaction is highly valuable in organic synthesis as it introduces carboxylic acid functionality to molecules derived from Grignard reagents.

Think of the Grignard reagent as a sponge that can soak up carbon dioxide (like soaker pads absorbing water) to form a stable bridge (the carboxylate salt). When you add vinegar (an acid), it converts this bridge into a solid structure (the carboxylic acid).

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