Carboxylic Acids — Nomenclature, Structure & Preparation

The carboxyl group: a fused functional group

Carbon compounds containing a carboxyl functional group, –COOH, are called carboxylic acids. The name itself records the construction: a carbonyl group ($\ce{C=O}$) joined to a hydroxyl group ($\ce{-OH}$) on the same carbon. Carboxylic acids may be aliphatic ($\ce{RCOOH}$) or aromatic ($\ce{ArCOOH}$) depending on whether an alkyl or aryl group is attached to the carboxyl carbon.

The group is biologically and industrially central. Higher members from $\ce{C12}$ to $\ce{C18}$ — the fatty acids — occur in natural fats as esters of glycerol, and the carboxyl group serves as the launch point for an entire family of derivatives: anhydrides, esters, acid chlorides and amides. The crucial idea for this note is that –COOH does not behave like a simple ketone’s $\ce{C=O}$ nor like an alcohol’s $\ce{-OH}$; the two merge into a single group with its own character, and that character flows directly from resonance.

Structure and resonance of –COOH

In carboxylic acids the three bonds to the carboxyl carbon lie in one plane and are separated by about 120°, exactly as expected for an $sp^2$-hybridised carbon. The carboxyl carbon, however, is less electrophilic than the carbonyl carbon of an aldehyde or ketone. The reason is a resonance interaction unavailable to a plain carbonyl: a lone pair on the hydroxyl oxygen can be donated towards the carbon, spreading electron density across the $\ce{O-C-O}$ unit.

The practical consequences are direct. Because the –OH oxygen feeds electron density into the carbon, nucleophilic attack on the carboxyl carbon is harder than on a ketone — carboxylic acids react chiefly by losing the acidic O–H proton or by substituting the –OH group, not by simple addition. The same delocalisation, extended into the conjugate base, is what makes carboxylic acids unusually acidic; that thread is developed in the dedicated note on acidity of carboxylic acids.

Common names and natural sources

Carboxylic acids were among the earliest organic compounds isolated from nature, so a large set carry common names that end in –ic acid and recall their Latin or Greek source. Formic acid ($\ce{HCOOH}$) was first obtained from red ants (Latin formica), acetic acid ($\ce{CH3COOH}$) from vinegar (acetum), and butyric acid ($\ce{CH3CH2CH2COOH}$) from rancid butter (butyrum). These names are still examined, so they must be memorised alongside the systematic ones.

IUPAC nomenclature

In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending –e of the corresponding alkane with –oic acid. While numbering the carbon chain, the carboxyl carbon is always carbon-1; because that position is fixed, it is never cited in the name. Substituents take the lowest possible locants and are prefixed in alphabetical order.

For compounds carrying more than one carboxyl group, the alkyl chain bearing the carboxyl groups is numbered and the count is shown by the multiplicative suffix — dioic acid, tricarboxylic acid and so on. For straight-chain di-acids the suffix is built directly onto the parent alkane name (the final –e is retained), so $\ce{HOOC-CH2-COOH}$ is propanedioic acid. When carboxyl groups hang off a ring, the suffix carboxylic acid is used, with the ring carbon bearing –COOH numbered one; thus benzoic acid is systematically benzenecarboxylic acid and phthalic acid is benzene-1,2-dicarboxylic acid.

Preparation: the six NEET routes

NCERT Section 8.7 organises the synthesis of carboxylic acids into a compact set of methods. Read as a map, they fall into two families: oxidative routes that climb the oxidation ladder of an existing carbon (alcohols, aldehydes, alkylbenzene side chains), and carbon-building or hydrolytic routes that either add a carbon (Grignard, nitriles) or unmask a carboxyl already present in a derivative (amides, acyl halides, anhydrides, esters). The figure below lays out the whole network around the central –COOH.

From primary alcohols and aldehydes

Primary alcohols are readily oxidised to carboxylic acids by common oxidising agents — potassium permanganate ($\ce{KMnO4}$) in neutral, acidic or alkaline media, or potassium dichromate ($\ce{K2Cr2O7}$) and chromium trioxide ($\ce{CrO3}$) in acidic media (Jones reagent). The alcohol is taken all the way up the ladder, passing through the aldehyde and not stopping there.

$$\ce{CH3CH2CH2CH2OH ->[\text{KMnO4 / H+}] CH3CH2CH2COOH}$$

Aldehydes themselves, sitting one rung below the acid, are oxidised even by mild reagents such as Tollens’ or Fehling’s reagent (and the stronger ones above) to the corresponding acid:

$$\ce{R-CHO ->[\text{[O]}] R-COOH}$$

From alkylbenzenes

Aromatic carboxylic acids are prepared by vigorous oxidation of alkylbenzenes with chromic acid or acidic/alkaline potassium permanganate. The defining feature, and a perennial NEET favourite, is that the entire side chain is oxidised down to a single carboxyl group irrespective of the length of the side chain. Toluene, ethylbenzene and propylbenzene all collapse to benzoic acid.

$$\ce{C6H5-CH2CH2CH3 ->[\text{KMnO4, OH-, $\Delta$}][\text{then H+}] C6H5-COOH}$$

Primary and secondary alkyl groups are oxidised in this way; a tertiary alkyl group, lacking a benzylic C–H bond, is not affected. Suitably substituted alkenes are likewise cleaved to carboxylic acids by these reagents, which is why cyclohexene oxidises to hexane-1,6-dioic acid (adipic acid).

From nitriles and amides

Nitriles are hydrolysed in the presence of acid ($\ce{H+}$) or base ($\ce{OH-}$) first to amides and then to acids. Mild conditions allow the reaction to be stopped at the amide stage, which is why amide hydrolysis appears as a closely related route — an amide is simply the half-way intermediate carried the rest of the way.

$$\ce{R-C#N ->[\text{H2O, H+}] R-CONH2 ->[\text{H2O, H+}] R-COOH}$$

Because nitriles are made from alkyl halides plus cyanide ($\ce{R-X + CN- -> R-CN}$), this route — like the Grignard route below — adds one carbon, converting an alkyl halide into an acid with one more carbon (ascending the series). NIOS also notes that a cyanohydrin (formed from an aldehyde and HCN) hydrolyses to a 2-hydroxy carboxylic acid by the same chemistry.

From Grignard reagents

Grignard reagents react with carbon dioxide (typically as solid dry ice) to form the magnesium salt of a carboxylic acid, which on acidification with mineral acid gives the free acid. The new carboxyl carbon comes entirely from $\ce{CO2}$, so this method too ascends the series by one carbon.

$$\ce{R-MgX + CO2 ->[\text{dry ether}] R-COO-MgX ->[\text{H3O+}] R-COOH}$$

The intermediate $\ce{R-COO^- Mg^+ X}$ is exactly the species NEET 2022 asked about. Since both Grignard reagents and nitriles are accessible from alkyl halides, these two routes together are the standard textbook way of stepping up the homologous series.

From acyl halides, anhydrides and esters

The acid derivatives all carry a masked carboxyl, and hydrolysis returns the parent acid. Acyl (acid) chlorides hydrolyse with water to carboxylic acids, or more readily with aqueous base to carboxylate ions that give the acid on acidification:

$$\ce{R-COCl + H2O -> R-COOH + HCl}$$

Anhydrides hydrolyse with water to the corresponding acid (or pair of acids):

$$\ce{(RCO)2O + H2O -> 2 R-COOH}$$

Esters give the acid directly on acidic hydrolysis; basic hydrolysis (saponification) gives the carboxylate, which is then acidified:

$$\ce{R-COOR' + H2O ->[\text{H+}] R-COOH + R'-OH}$$

Read in the other direction, this is the same nucleophilic acyl substitution network that builds the derivatives from the acid; that interconversion, along with esterification, anhydride formation and amide formation, is treated in reactions of carboxylic acids.