Difficulty: Intermediate | Prerequisites: Functional groups, IUPAC naming conventions (Ch. 1–4), hydrogen bonding basics
Carboxylic acids are among the most important functional groups in organic chemistry, sitting at the crossroads of acid-base chemistry, synthesis, and biological processes. This section of Chapter 19 covers what the carboxyl group looks like, how to name carboxylic acids using IUPAC rules, and why their physical properties (especially boiling points) are unusually high compared to similar-weight molecules. You should already be comfortable with basic IUPAC nomenclature, functional group identification, and the concept of hydrogen bonding before diving in.
Carboxylic acids contain a -COOH group built around an sp2 carbon. They are named with the suffix "-oic acid" (or "-carboxylic acid" for ring-attached groups). Their boiling points are remarkably high because they form strong hydrogen-bonded dimers in solution and in the liquid phase.
Carboxylic acid (RCO₂H or RCOOH)
An organic compound containing a carbonyl group (C=O) bonded to a hydroxyl group (–OH), giving the functional group –COOH. In simple terms, it is the combination of a ketone-like C=O and an alcohol-like O–H on the same carbon.
Carboxyl group (–COOH)
The functional group that defines carboxylic acids: a carbon double-bonded to one oxygen and single-bonded to a hydroxyl (–OH). Think of it as the "acid end" of fatty acids, amino acids, and many other biomolecules.
sp2 carbon (in carboxyl group)
The central carbon of the –COOH group is sp2-hybridised, giving it trigonal planar geometry with approximately 120-degree bond angles. In simple terms, the carbon is flat, not tetrahedral, because of the double bond to oxygen.
Carboxylic acid derivatives
Compounds derived from carboxylic acids by replacing the –OH with another group. The main classes are: acid chlorides (RCOCl), esters (RCOOR'), amides (RCONR'₂), and anhydrides (RCO–O–COR). Think of these as "cousins" of carboxylic acids where the –OH has been swapped out.
Acid anhydride
Formed from two molecules of carboxylic acid with loss of one molecule of water (2 RCO₂H → RCO–O–COR + H₂O). In simple terms, two acid molecules stitched together by removing water.
Hydrogen-bonded dimer
A pair of carboxylic acid molecules held together by two intermolecular hydrogen bonds between their –COOH groups. This is the dominant association pattern in pure carboxylic acids and is responsible for their unusually high boiling points.
IUPAC suffix: "-oic acid"
The standard ending for naming carboxylic acids where the –COOH carbon is part of the parent chain. Think of it as: find the longest chain that includes the –COOH carbon, drop the "-e" from the alkane name, add "-oic acid."
IUPAC suffix: "-carboxylic acid"
Used when the –COOH group is attached to a ring (the ring is the parent, and the carboxyl carbon is not counted in the ring). Think of it as: name the ring, then bolt on "-carboxylic acid."
Diacid (dicarboxylic acid)
A molecule with two –COOH groups. Named with the suffix "-dioic acid" (e.g., nonanedioic acid). In simple terms, an acid at both ends of the chain.
The carboxyl group (–COOH) has a central carbon that is sp2-hybridised. This means three groups radiate from it in a trigonal planar arrangement at roughly 120-degree bond angles: a C=O (carbonyl oxygen), an O–H (hydroxyl), and the R group (the rest of the molecule).
Because the carbon is sp2, the entire –COOH unit is flat. This planarity matters for reactivity, resonance, and how the group interacts with other molecules.
Carboxylic acids sit at the centre of a family of related functional groups. Each derivative replaces the –OH of –COOH with something else:
Acid chloride (RCOCl): –OH replaced by –Cl
Ester (RCOOR'): –OH replaced by –OR'
Amide (RCONR'₂): –OH replaced by –NR'₂
Acid anhydride (RCO–O–COR): two acid molecules joined with loss of H₂O
All of these derivatives share the C=O (carbonyl) unit with carboxylic acids. The differences in the group attached to that carbonyl carbon determine each derivative's reactivity, which is a major theme of later chapters.
Naming follows the pattern: prefix – parent – suffix.
Find the longest continuous carbon chain that includes the carboxyl carbon.
The carboxyl carbon is always carbon 1 (you do not need to specify "1-" in the name).
Drop the "-e" from the parent alkane name and add "-oic acid."
Number substituents from the carboxyl end.
Worked examples from the source:
CH₃CH₂COOH → three-carbon chain → propane → propanoic acid
A nine-carbon diacid with two methyl groups on C5 → 5,5-dimethylnonanedioic acid (note the "-dioic acid" ending for two –COOH groups; number from whichever end gives the lowest locants to substituents)
A five-carbon chain with –OH on C3 and a ketone (C=O) on C4 → 3-hydroxy-4-oxo-pentanoic acid (the acid suffix takes priority; the ketone is indicated by "oxo-" as a prefix)
The ring is the parent. The carboxyl carbon is not counted in the ring size. Name the ring, then add "-carboxylic acid."
Worked examples:
–COOH on cyclohexane with a methyl group at the 4-position → 4-methylcyclohexanecarboxylic acid
–COOH on cyclopropane → cyclopropanecarboxylic acid
Tags: IUPAC nomenclature, naming carboxylic acids, "-oic acid" suffix, "-carboxylic acid" suffix, dioic acid, prefix-parent-suffix
Carboxylic acids have significantly higher boiling points than alcohols, ketones, and alkenes of comparable molecular weight. Two factors drive this:
Highly polar bonds in RCO₂H. The C=O and O–H bonds are both strongly polar, creating a large net dipole.
Hydrogen bonding, and lots of it. Carboxylic acids form hydrogen-bonded dimers: two molecules pair up through two simultaneous O–H···O=C hydrogen bonds (one from each molecule). This dimer structure effectively doubles the apparent molecular weight in the liquid phase and in non-polar solvents.
Acetic acid (CH₃CO₂H, MW ~60) boils far higher than 2-propanol (MW ~60), acetone (MW ~58), or propene (MW ~42). The dimer hydrogen bonding in acetic acid is the key difference.
Small carboxylic acids (up to about four carbons) are water-soluble because the –COOH group hydrogen-bonds effectively with water. As the hydrocarbon chain lengthens, the non-polar tail dominates and solubility drops.
Tags: boiling point, hydrogen bonding, carboxylic acid dimer, physical properties, solubility, polarity
Students often think the –OH in a carboxylic acid behaves the same as an alcohol –OH. It does not. The adjacent C=O withdraws electron density and makes the O–H bond much more polar and acidic than in an alcohol.
Students sometimes number the chain starting from the wrong end. The carboxyl carbon is always C1. You do not state "1-" in the name because it is implied.
Students confuse the "-oic acid" suffix with the "-carboxylic acid" suffix. Use "-oic acid" when the –COOH carbon is part of the longest chain. Use "-carboxylic acid" when the –COOH is attached to a ring and the ring is the parent.
Students occasionally forget that carboxylic acids form dimers, not just single hydrogen bonds, and underestimate their boiling points as a result.
⚠️ Naming carboxylic acids is a staple exam question. Be ready to draw a structure from a name and vice versa, including diacids and ring-attached acids.
⚠️ Expect a question comparing the boiling points of a carboxylic acid, an alcohol, a ketone, and an alkene of similar molecular weight. Know that dimer hydrogen bonding explains why the acid wins.
⚠️ The distinction between "-oic acid" and "-carboxylic acid" suffixes is a common short-answer trap.
⚠️ Know the five major classes of carboxylic acid derivatives (acid, acid chloride, ester, amide, anhydride) and be able to draw the general structure for each. This is groundwork for later chapters on nucleophilic acyl substitution.
True or False: The carbon of the –COOH group is sp3-hybridised. (False, it is sp2.)
Fill in the blank: Carboxylic acids form hydrogen-bonded ______ in solution, which raises their boiling points. (dimers)
True or False: When –COOH is attached to a ring, you use the suffix "-oic acid." (False, you use "-carboxylic acid.")
Fill in the blank: The bond angles around the carboxyl carbon are approximately ______ degrees. (120)
True or False: An acid anhydride is formed from two molecules of carboxylic acid with loss of H₂O. (True)
Q: Draw the structure of 4-methylcyclohexanecarboxylic acid and explain why the suffix is "-carboxylic acid" rather than "-oic acid."
A: The structure is a cyclohexane ring with a –COOH group directly attached and a methyl group at the 4-position. The suffix is "-carboxylic acid" because the –COOH carbon is not part of the ring (the ring is the parent); when the carboxyl carbon is external to the parent structure, we use "-carboxylic acid."
Q: Rank the following in order of increasing boiling point: propene (CH₃CH=CH₂), acetone (CH₃COCH₃), 2-propanol ((CH₃)₂CHOH), acetic acid (CH₃CO₂H). Briefly explain your ranking.
A: Propene < acetone < 2-propanol < acetic acid. Propene has only weak van der Waals forces. Acetone has a dipole but no hydrogen-bond donor. 2-Propanol can hydrogen-bond (one H-bond per molecule). Acetic acid forms hydrogen-bonded dimers (two H-bonds per pair), effectively doubling its apparent molecular weight in the liquid phase.
Q: Name the compound: HO₂C–CH₂–C(CH₃)₂–CH₂–CO₂H (a five-carbon diacid with two methyl groups on C3).
A: 3,3-dimethylpentanedioic acid. Both –COOH groups are on the chain, so use "-dioic acid." The two methyl substituents are both on carbon 3.
Q: Why is a carboxylic acid –OH more acidic than an alcohol –OH?
A: The adjacent C=O group is strongly electron-withdrawing. It polarises the O–H bond, making the hydrogen more easily lost as H⁺. In addition, the resulting carboxylate anion (RCO₂⁻) is stabilised by resonance: the negative charge is delocalised equally over both oxygens. An alkoxide (RO⁻) has no such resonance stabilisation.
This material connects directly to nucleophilic acyl substitution (the reactions of acid chlorides, esters, amides, and anhydrides), which is the next major block in most organic chemistry courses. Understanding the carboxyl group's geometry and polarity is essential groundwork.
The hydrogen-bonding behaviour here ties back to intermolecular forces from general chemistry and early organic chemistry. If the dimer concept feels unfamiliar, revisit your notes on hydrogen bonding in alcohols and water.
The naming conventions follow the same IUPAC logic used for alcohols and ketones: longest chain, lowest locants, appropriate suffix. Confidence with those earlier naming rules makes carboxylic acid nomenclature straightforward.
Carboxylic acid, RCOOH, RCO₂H, carboxyl group, –COOH, sp2 carbonyl, acid chloride, ester, amide, anhydride, hydrogen-bonded dimer, boiling point comparison, IUPAC nomenclature, "-oic acid," "-carboxylic acid," diacid, dicarboxylic acid, prefix-parent-suffix, Chapter 19, organic chemistry, Ohio State, functional group families