Organic Chemistry · Carboxylic Acids and Nitriles
Structure and Properties of Carboxylic Acids
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In 30 seconds
A carboxylic acid is built from a carbonyl group (C=O) and a hydroxyl group (-OH) joined on the same carbon, written -COOH. That single arrangement produces a remarkable set of properties: the carbonyl carbon is trigonal planar and electrophilic Electron-poor and attracted to electron-rich species (nucleophiles). Full entry →, the O-H hydrogen is weakly acidic (pKa -logKa, a measure of acid strength; smaller means stronger. Full entry → about 4–5), and the molecule hydrogen-bonds to itself strongly enough to raise boiling points far above those of comparable aldehydes, ketones, or alcohols. The key to understanding all of this is the carboxylate anion, RCOO-, whose negative charge is shared equally between two oxygens through resonance Electron delocalization over multiple atoms, drawn as equivalent contributing structures. Full entry →. This topic explains the carboxyl structure, why these acids are weak, and how hydrogen bonding controls boiling points, solubility, and spectra.
Why this matters
Carboxylic acids are the chemistry of everyday sourness and of biology itself. Acetic acid gives vinegar its tang and preservative power; citric acid sours citrus fruit; fatty acids build the membranes of every cell; amino acids are the monomers of proteins. In medicine, aspirin and ibuprofen are carboxylic acids, and a drug's acidity controls whether it is absorbed in the stomach or the intestine. Knowing how the carboxyl structure produces these properties lets you predict boiling points, solubility, and acidity from a drawing — a skill used in drug formulation, food science, and biochemistry.
The college version
Core Concepts
Anatomy of the carboxyl group
The carboxyl carbon is sp2-hybridized: it forms a double bond to the carbonyl oxygen, single bonds to the hydroxyl oxygen and to a carbon or hydrogen, and its three bonds lie in a plane with roughly 120° angles. Because oxygen is more electronegative than carbon, both C-O bonds are polar, and the carbonyl carbon carries a partial positive charge that makes it electrophilic — the reactive site for the nucleophilic acyl substitution chemistry of Chapter 21.
The carboxylate anion: resonance makes the acid special
When a carboxylic acid loses its acidic proton, the resulting carboxylate ion The conjugate base RCOO- formed when the acid loses H+. Full entry → RCOO- is stabilized by resonance: the negative charge is delocalized over two equivalent oxygens, and the two C-O bonds become equivalent "one-and-a-half" bonds. This delocalization is the fundamental reason carboxylic acids are far more acidic than alcohols: an alkoxide ion RO- must carry its full negative charge on a single oxygen, while a carboxylate spreads the same charge over two oxygens.
Weak acidity: pKa ≈ 4–5
Carboxylic acids are weak acids. The equilibrium
RCOOH ⇌ RCOO- + H+
has an acidity constant pKa of roughly 4–5 for simple acids (acetic acid, 4.76; formic acid, 3.75). Alcohols, by contrast, have pKa near 16: a carboxylic acid is about 1011–1012 times more acidic than an alcohol of similar size. The entire difference comes from resonance stabilization of the carboxylate ion, which the alkoxide lacks. At neutral pH a simple carboxylic acid is mostly ionized, which is why these acids dissolve in aqueous base.
Hydrogen bonding: dimers and high boiling points
The carboxyl group The -COOH unit: carbonyl + hydroxyl on one carbon. Full entry → has both a hydrogen-bond donor (the O-H) and an acceptor (the carbonyl oxygen), so two acid molecules can form a cyclic hydrogen-bonded dimer held by two hydrogen bonds at once. This strong self-association raises boiling points dramatically: butanoic acid boils at 163.7 °C, while butanal (74.8 °C) and even 1-butanol (117.7 °C) — which can form only single hydrogen bonds — boil much lower. The dimer persists even in the vapor phase.
Solubility: the balance of head and tail
Small carboxylic acids are very soluble in water because the carboxyl group hydrogen-bonds with water; formic and acetic acids are completely miscible. As the hydrocarbon chain grows, the nonpolar tail dominates and solubility drops: octanoic acid is nearly insoluble. Adding base converts the acid to its carboxylate salt, RCOO-Na+, an ionic species that dissolves readily — the chemistry that turns fats into soap.
Spectroscopic fingerprints
The carboxyl group has signature spectra: a very broad O-H stretch from about 2500–3300 cm⁻¹ (broadened by hydrogen bonding), a strong C=O stretch near 1710 cm⁻¹, and a C-O stretch near 1200–1300 cm⁻¹. In 1H NMR, the carboxyl proton appears far downfield at δ 10–12 ppm; in 13C NMR, the carboxyl carbon appears near δ 178–180, upfield of aldehyde and ketone carbonyl carbons.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Carboxyl group (-COOH) | Carbonyl group (C=O) | The carboxyl group contains a carbonyl plus an -OH; it is acidic and hydrogen-bonding. |
| Carboxylic acid acidity | Alcohol acidity | Acids (pKa 4–5) are ~1011× more acidic because the carboxylate ion is resonance-stabilized. |
| Hydrogen-bonded dimer | Single hydrogen bond | Two molecules, two hydrogen bonds — the doubling explains the unusually high boiling points. |
| Acid strength (pKa) | Acid concentration | pKa is intrinsic to the molecule; concentration is how much is present. |
| Carboxyl proton δ 10–12 | Aldehyde proton δ 9–10 | Both are downfield, but the acid shows a broad IR O-H band and 13C near 180. |
| Carboxylate salt solubility | Free-acid solubility | The ionic salt is water-soluble even when the neutral acid is not. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A carboxylic acid is like a bucket with a long handle: the handle is the carbon chain, and the bucket is the -COOH part that grabs water and other molecules. When it lets go of its hydrogen, the leftover part is like a coin with two sides — the negative charge can live on either oxygen, which makes it calm and stable. That stability is why acids like vinegar are sour but gentle, and why they stick together in pairs, making them boil hotter than similar molecules.
Worked examples
Compare three four-carbon molecules: butanoic acid (CH3CH2CH2COOH), butanal (CH3CH2CH2CHO), and 1-butanol (CH3CH2CH2CH2OH). Their boiling points are 163.7 °C, 74.8 °C, and 117.7 °C respectively. All three have similar molar masses, so the differences come almost entirely from hydrogen bonding. 1-Butanol forms one hydrogen bond per molecule pair. Butanal cannot hydrogen-bond to itself at all (it has no O-H or N-H donor), so it boils lowest. Butanoic acid forms two hydrogen bonds per dimer, tying pairs of molecules together so tightly that more thermal energy is needed to separate them. The pattern for a given carbon count: acid > alcohol > aldehyde.
Octanoic acid, CH3(CH2)6COOH, has an eight-carbon chain and is nearly insoluble in water: the nonpolar tail cannot be solvated by water, and one carboxyl group cannot compensate. Now add sodium hydroxide:
CH3(CH2)6COOH + NaOH → CH3(CH2)6COO-Na+ + H2O
The product, sodium octanoate, is an ionic salt whose charged head interacts strongly with water, so it dissolves. This acid→salt conversion is the principle behind soap-making and explains why sodium salts of preservative acids (sodium benzoate, sodium propionate) are the water-soluble forms used in food. Whenever a carboxylic acid "won't dissolve," the fix is base: convert it to its carboxylate salt.
Ethanol, CH3CH2OH, has pKa ≈ 16; acetic acid, CH3COOH, has pKa = 4.76. Each pKa unit is a factor of 10 in Ka, so the difference of about 11 units means acetic acid is about 1011 times more acidic. The reason is the conjugate base: ethoxide CH3CH2O- holds its negative charge on one oxygen, while acetate CH3COO- spreads the same charge over two equivalent oxygens by resonance. Charge spread over a larger volume is more stable, so the acid that produces it dissociates more readily. This insight — "stabilize the conjugate base, strengthen the acid" — powers the next topic on substituent effects.
Key takeaways
- Carboxyl group = carbonyl + hydroxyl on one carbon: -COOH; the carbon is sp2 and planar.
- The carboxylate ion RCOO- is resonance-stabilized with two equivalent C-O bonds — the key to acidity.
- Simple carboxylic acids have pKa ≈ 4–5 (acetic 4.76, formic 3.75); alcohols are about 1011–1012 times less acidic.
- Carboxylic acids form hydrogen-bonded dimers, giving unusually high boiling points (butanoic acid 163.7 °C vs butanal 74.8 °C).
- Small acids are water-soluble; longer chains are not; carboxylate salts are always water-soluble.
- IR: broad O-H 2500–3300 cm⁻¹, C=O ~1710 cm⁻¹; 1H NMR: δ 10–12; 13C NMR: δ ~178–180.
- At physiological pH (~7.4), simple carboxylic acids exist mostly as carboxylate ions.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why is the carboxylate ion more stable than an alkoxide ion?
Show answer
The negative charge is delocalized by resonance over two equivalent oxygens in the carboxylate, whereas an alkoxide holds the charge on a single oxygen; more delocalization means more stability.
Which is more acidic, acetic acid or ethanol, and by roughly how many orders of magnitude?
Show answer
Acetic acid is more acidic (pKa 4.76 vs ~16), a difference of about 11 pKa units, i.e., about 1011 times.
Why does butanoic acid boil at a higher temperature than 1-butanol of the same molar mass?
Show answer
Butanoic acid forms hydrogen-bonded dimers held by two hydrogen bonds per pair of molecules, while 1-butanol can form only single hydrogen bonds.
How can you make an insoluble long-chain carboxylic acid dissolve in water?
Show answer
Add base to convert the acid to its carboxylate salt (e.g., sodium octanoate), which is ionic and water-soluble.
Where do the carboxyl proton and carboxyl carbon appear in 1H and 13C NMR?
Show answer
Carboxyl proton at δ 10–12 in 1H NMR; carboxyl carbon near δ 178–180 in 13C NMR.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- carboxyl group
- The -COOH unit: carbonyl + hydroxyl on one carbon.
- carboxylate ion
- The conjugate base RCOO- formed when the acid loses H+.
- resonance
- Electron delocalization over multiple atoms, drawn as equivalent contributing structures.
- hydrogen-bonded dimer
- Two acid molecules held together by two O-H⋯O=C hydrogen bonds.
- pKa
- -logKa, a measure of acid strength; smaller means stronger.
- electrophilic
- Electron-poor and attracted to electron-rich species (nucleophiles).
Sources & references
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