Carboxyl group: nomenclature and structure

What is the carboxyl group?

In plain words: –COOH looks like a carbonyl group with an –OH stuck on, but the two parts change each other. The –OH’s lone pair feeds the C=O, so the carbon is less hungry for nucleophiles than an aldehyde’s, and the O–H is acidic, which the next pages explore.

• Many carboxylic acids occur in nature. NCERT highlights C₁₂–C₁₈ fatty acids, found in natural fats as glycerol esters (page 244). This is a common range, not the definition: the IUPAC Gold Book includes aliphatic monocarboxylic acids derived from fats, oils or waxes over a wider range, including shorter and longer chains.
• Carboxylic acids are the starting materials for anhydrides, esters, acid chlorides, amides and other compounds (NCERT page 244).

Nomenclature

Common names end in –ic acid and come from the Latin or Greek names of the acids’ natural sources (NCERT page 244):

• formic acid, HCOOH, first obtained from red ants (Latin formica, ant);
• acetic acid, CH₃COOH, from vinegar (Latin acetum);
• butyric acid, CH₃CH₂CH₂COOH, from rancid butter (Latin butyrum).

IUPAC names (NCERT page 244), following the Class 11 rules for functional groups:

1. Replace the final –e of the alkane name by –oic acid.
2. For a simple open-chain monoacid, choose the parent chain containing the principal –COOH group and number its carboxyl carbon C1.
3. For more than one –COOH, NCERT describes naming the alkyl chain that carries the groups and adding dicarboxylic acid, tricarboxylic acid and so on, with numbers for their positions; its own table does not do this for chain diacids (see the NCERT slip below).

NCERT Table 8.3 (pages 244–245), common name, IUPAC name:

• HCOOH: formic acid, methanoic acid

• CH₃COOH: acetic acid, ethanoic acid

• CH₃CH₂COOH: propionic acid, propanoic acid

• CH₃CH₂CH₂COOH: butyric acid, butanoic acid

• (CH₃)₂CHCOOH: isobutyric acid, 2-methylpropanoic acid

• HOOC–COOH: oxalic acid, ethanedioic acid

• HOOC–CH₂–COOH: malonic acid, propanedioic acid

• HOOC–(CH₂)₂–COOH: succinic acid, butanedioic acid

• HOOC–(CH₂)₃–COOH: glutaric acid, pentanedioic acid

• HOOC–(CH₂)₄–COOH: adipic acid, hexanedioic acid

• HOOC–CH₂–CH(COOH)–CH₂–COOH: tricarballylic acid, propane-1,2,3-tricarboxylic acid

• C₆H₅COOH: benzoic acid, benzenecarboxylic acid (benzoic acid)

• C₆H₅CH₂COOH: phenylacetic acid, 2-phenylethanoic acid

• o-C₆H₄(COOH)₂: phthalic acid, benzene-1,2-dicarboxylic acid

• Two styles. The table names the open-chain diacids with –dioic acid: the chain includes both carboxyl carbons, so HOOC–COOH, with two carbons, is ethanedioic acid. The –carboxylic acid style counts the chain without the carboxyl carbons, and the table uses it for the triacid, whose three –COOH groups sit on a propane chain, and for the acids with –COOH on the benzene ring (reasoned from NCERT Table 8.3).

• NCERT slip (page 244): the text gives the –carboxylic acid style to every compound with “more than one carboxyl group”, but its own Table 8.3 names all five open-chain diacids as –dioic acids. By the text’s rule, succinic acid would be ethane-1,2-dicarboxylic acid, and oxalic acid, with no carbon besides its two carboxyl carbons, would have no chain to name. Beyond the NCERT text: IUPAC names a chain ended by –COOH groups with the suffix –oic acid (–dioic acid for two), and keeps –carboxylic acid for more than two –COOH groups on one chain or for –COOH on a ring (IUPAC Recommendations 2013, P-65.1.2). In an exam, give the name NCERT’s table gives: a –dioic acid for a chain with –COOH at both ends.

Structure of carboxyl group

• The bonds to the carboxyl carbon lie in one plane, about 120° apart (NCERT page 245), as expected for an sp² carbon (see hybridisation of carbon; reasoned).
• The carboxyl carbon is less electrophilic than the carbonyl carbon of a comparable aldehyde or ketone because of resonance: a lone pair of the –OH oxygen is shared with the carbon, as in the structure –C(–O⁻)=O⁺–H (NCERT page 245).

Solved examples

Common mistakes to avoid

Quick revision summary

• –COOH = carbonyl + hydroxyl; RCOOH (aliphatic) or ArCOOH (aromatic); common C₁₂–C₁₈ fatty acids occur as glycerol esters, but fatty acids are not restricted to this range.
• Common names from natural sources: formic (ants), acetic (vinegar), butyric (butter).
• IUPAC: –e → –oic acid; COOH carbon is C1; diacids as –dioic acids (HOOC–COOH ethanedioic acid); three COOH on a chain as –tricarboxylic acid.
• Structure: planar, about 120°; the carboxyl carbon is less electrophilic than a comparable aldehyde or ketone carbonyl carbon (resonance).

Questions students ask

What is the difference between the carbonyl group and the carboxyl group?

The carbonyl group is the >C=O unit of aldehydes and ketones, while the carboxyl group, –COOH, is a carbonyl group with a hydroxyl group on the same carbon, the functional group of carboxylic acids (NCERT pages 227 and 244). So carboxylic acids do contain a carbonyl group, and NCERT counts them among the carbonyl compounds (page 227), but the two parts change each other: the –OH makes the carbon less electrophilic (NCERT page 245), and the O–H is acidic enough to give CO₂ with sodium hydrogencarbonate, which aldehydes and ketones do not (NCERT page 249). In names, a carbonyl group gives –al or –one, a carboxyl group –oic acid (NCERT pages 229 and 244).

Why do carboxylic acids not give the 2,4-DNP test or the other addition reactions of aldehydes and ketones?

Carboxylic acids do not normally give the stable addition products or 2,4-DNP derivatives characteristic of aldehydes and ketones under the standard test conditions, because their carboxyl carbon is less electrophilic, for the resonance reason shown under its structure (NCERT page 245). Basic reagents also take the acid’s proton first, as ammonia does (NCERT page 252), and the carboxylate ion formed, its charge spread over two oxygen atoms (NCERT page 250), attracts a nucleophile even less (reasoned). Esters and amides also normally fail this test. This does not mean that nucleophiles can never attack an acid derivative: nucleophilic acyl substitution can proceed through a tetrahedral addition intermediate followed by elimination.

What is the difference between a carboxyl group, a carboxylic acid and a carboxylate ion?

The carboxyl group is the functional group –COOH, a carboxylic acid is a whole compound that carries one or more of these groups, such as ethanoic acid, CH₃COOH, and a carboxylate ion, RCOO⁻, is what is left when the acid loses the proton of its O–H (NCERT pages 244 and 249). Salts are named from the carboxylate, with –oic acid changed to –oate, as in sodium ethanoate, CH₃COONa (reasoned). In an isolated carboxylate ion, or a symmetric environment, the two oxygen atoms are equivalent, because two equivalent resonance structures share the negative charge between them (NCERT page 250), so its two C–O bonds have equal lengths in that model. Unequal coordination or hydrogen bonding in a specific crystal can remove this equivalence.

What is the general formula of carboxylic acids?

Saturated acyclic monocarboxylic acids with no other functional groups have the general formula C_nH_2nO₂: ethanoic acid, CH₃COOH, is C₂H₄O₂ (reasoned from NCERT Table 8.3, page 244). Saturated open-chain diacids such as oxalic acid, C₂H₂O₄, follow C_nH_2n−2O₄ (reasoned). The formula alone does not prove an acid, as esters share it: propanoic acid, methyl ethanoate and ethyl methanoate are all C₃H₆O₂, functional group isomers (reasoned).

How many carboxyl groups does citric acid have, and what is its IUPAC name?

Citric acid has three carboxyl groups and one hydroxyl group, HOOC–CH₂–C(OH)(COOH)–CH₂–COOH, C₆H₈O₇, and its IUPAC name is 2-hydroxypropane-1,2,3-tricarboxylic acid, a point beyond the NCERT text (PubChem). It is named in the –carboxylic acid style that NCERT Table 8.3 uses for tricarballylic acid, propane-1,2,3-tricarboxylic acid (page 244): the three carboxyl carbons are not counted in the propane chain, so its six carbons are three in the chain and three in the –COOH groups (reasoned). Citric acid is thus tricarballylic acid with an –OH on the middle carbon of the chain (reasoned).

What is the difference between carbolic acid, carbonic acid and carboxylic acids?

Carbolic acid, carbonic acid and carboxylic acids are different substances with similar names: carbolic acid is another name for phenol, C₆H₅OH (NCERT Unit 7, page 201), and carboxylic acids are the organic compounds with a –COOH group (NCERT page 244), while carbonic acid, H₂CO₃, is an inorganic acid of carbon, a point beyond the NCERT text (Wikipedia, “Carbonic acid”). Phenol is not a carboxylic acid, since its acidic –OH sits on a benzene ring with no C=O beside it (reasoned). In water, CO₂(aq) and H₂CO₃ are in equilibrium, with CO₂(aq) predominating over molecular H₂CO₃. Their relationship to hydrogencarbonate and carbonate also depends on pH.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is the carboxyl carbon less electrophilic than the carbonyl carbon of an aldehyde?

   Show answerHide answer

   A lone pair of the –OH oxygen is delocalised onto the carboxyl carbon by resonance, which reduces its positive character (NCERT page 245).

2. Q2. From which natural sources do the names formic, acetic and butyric acid come?

   Show answerHide answer

   Formic from red ants (formica), acetic from vinegar (acetum) and butyric from rancid butter (butyrum) (NCERT page 244).