Carboxylic Acids: The –COOH Group

The carboxyl group

The carboxyl group (–COOH) combines a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. This pairing creates the most acidic common organic functional group.

The acidity has a structural explanation. When the –OH proton leaves, the resulting carboxylate anion (–COO−) delocalizes its negative charge evenly over both oxygen atoms through resonance. This stabilization makes proton loss far easier than in an alcohol, where the charge would sit on a single oxygen.

The group is planar, polar, and excellent at hydrogen bonding — both donating (through O–H) and accepting (through C=O). These traits shape everything from boiling points to biological roles.

Naming carboxylic acids

Carboxylic acids take the suffix -oic acid. Count the carboxyl carbon as C1 and name the longest chain containing it:

• HCOOH → methanoic acid (common name: formic acid)
• CH3COOH → ethanoic acid (common name: acetic acid)
• CH3CH2COOH → propanoic acid
• CH3CH2CH2COOH → butanoic acid

The common names — formic (from ants, which produce it), acetic (from vinegar) — are deeply entrenched for the first two members and remain standard in industry and biochemistry. Learn both systems; you will meet both.

Substituents are numbered from the carboxyl carbon, which is always position 1, so no locant is needed for the –COOH itself.

Weak acids

Carboxylic acids are weak acids: in water they ionize only partially.

CH3COOH ⇌ CH3COO− + H+

Ethanoic acid has Ka = 1.7×10^−5 (pKa 4.76), meaning roughly 1% of molecules are ionized in a typical dilute solution. Compare alcohols (Ka ~10^−16) — carboxylic acids are about ten billion times more acidic, entirely due to the resonance-stabilized anion.

They are still far weaker than strong mineral acids like HCl, which ionize completely. This middle ground — acidic enough to react with bases and carbonates, gentle enough for food — is exactly what makes carboxylic acids so useful.

Reactions with bases

With bases, carboxylic acids undergo straightforward neutralization to carboxylate salts plus water:

CH3COOH + NaOH → CH3COONa + H2O (sodium ethanoate)

The salts are named by replacing -oic acid with -oate: ethanoic acid gives ethanoates, propanoic acid gives propanoates. Sodium ethanoate and potassium citrate are common food additives.

With carbonates and hydrogencarbonates, the reaction also releases carbon dioxide:

2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2

This fizzing is the chemical basis of baking-powder leavening and the classic vinegar-and-baking-soda reaction — and a handy test distinguishing acids from neutral compounds.

Esterification

A carboxylic acid and an alcohol react to form an ester plus water, in a reversible, acid-catalyzed reaction called esterification:

CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O (ethyl ethanoate)

Because the reaction is reversible, it reaches equilibrium rather than completion; removing water or using excess reactant drives it toward the ester (Le Chatelier's principle).

Many esters have distinctive fruity odors — ethyl ethanoate smells of pears, pentyl ethanoate of bananas — making esterification central to the flavor and fragrance industries. On an industrial scale, repeated esterification builds polyesters such as PET.

Vinegar and everyday acids

Vinegar is dilute ethanoic acid — typically about 5% — produced by bacterial fermentation of ethanol. Its sour taste, preservative action in pickling, and mild cleaning power all trace to the carboxyl group.

Other everyday carboxylic acids include citric acid in citrus fruits, lactic acid in sour milk and exercised muscle, ascorbic acid (vitamin C), and formic acid, which ants spray as a defense.

The pattern is worth noting: nature uses carboxylic acids pervasively — in metabolism, in signaling, in defense — because their gentle acidity and hydrogen-bonding ability suit aqueous biological chemistry perfectly.

Physical properties

Carboxylic acids hydrogen-bond strongly, both as donors and acceptors, and they even pair up into dimers — two molecules joined by two hydrogen bonds — in the pure liquid and vapor.

Consequently their boiling points are high: ethanoic acid (molar mass 60) boils at 118°C, above propan-1-ol (97°C) of similar mass. The dimer effectively doubles the molecular unit that must be separated.

Short-chain acids (up to butanoic) mix with water; longer ones become increasingly oily. And they announce themselves: butanoic acid is responsible for the smell of rancid butter — a reminder that functional groups carry sensory signatures too.

Common mistakes

Real-world applications

• Vinegar (dilute ethanoic acid) is used in cooking, pickling, and cleaning.

• Esters made from carboxylic acids provide fruit flavors and perfume fragrances.

• Polyester fabrics and PET bottles are built from repeated ester linkages.

• Aspirin (acetylsalicylic acid) and many medicines contain the carboxyl group, which affects solubility and absorption.