Aldehydes, Ketones and Carboxylic Acids · Lesson 9 of 15
Carboxylic Acid Structure, Naming and Physical Properties
“Recognise carboxyl connectivity and relate naming, resonance and hydrogen bonding to observable properties.”
• Name mono-, di- and ring-attached carboxylic acids. • Distinguish neutral acid resonance from carboxylate resonance. • Explain acid dimers, boiling points and solubility trends. • Connect common acid names to structural formulas.
A carboxyl group is one connected functional group
The carboxyl group is –C(=O)OH, often written –COOH or –CO₂H. Its OH oxygen is bonded directly to the carbonyl carbon. HOCH₂CHO has an alcohol and an aldehyde, but it is not a carboxylic acid because its OH is on another carbon. Aliphatic acids are RCOOH and aromatic acids can have ArCOOH with direct ring attachment. In phenylacetic acid, PhCH₂COOH, a CH₂ spacer separates the ring from the acid group.
Fatty acids are carboxylic acids with substantial hydrocarbon chains. Many C₁₂–C₁₈ members occur as glycerol esters in fats rather than as free acids. A functional group can therefore remain identifiable in a derivative even when its O–H proton has been replaced. Esters, amides, anhydrides and acyl chlorides are chemically related through the acyl carbon, but their physical and reaction properties are not interchangeable.
Count the acid carbon as carbon 1
For a simple open-chain acid, choose the appropriate chain including the carboxyl carbon and replace the alkane “-e” with “-oic acid”. CH₃CH₂COOH is propanoic acid, with carboxyl C1. Substituent and unsaturation locants follow that numbering. Thus (CH₃)₂C=CHCOOH is 3-methylbut-2-enoic acid. A ketone or alcohol in an acid is expressed by an oxo or hydroxy prefix because COOH supplies the principal suffix in these examples.
When both acid carbons belong to an acyclic parent, use an “-dioic acid” name: HOOCCH₂COOH is propanedioic acid and HOOC(CH₂)₄COOH is hexanedioic acid. When carboxyl groups are outside a ring or an explicitly selected parent, use “carboxylic acid”, “dicarboxylic acid” or “tricarboxylic acid” with attachment positions. C₆H₁₁COOH is cyclohexanecarboxylic acid; HOOCCH₂CH(COOH)CH₂COOH is propane-1,2,3-tricarboxylic acid. The external COOH carbons are not silently inserted into that parent ring or propane skeleton.
| Condensed structure | Common name | Systematic 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 |
| HOOCCH₂COOH | Malonic acid | Propanedioic acid |
| HOOC(CH₂)₂COOH | Succinic acid | Butanedioic acid |
| HOOC(CH₂)₃COOH | Glutaric acid | Pentanedioic acid |
| HOOC(CH₂)₄COOH | Adipic acid | Hexanedioic acid |
| C₆H₅COOH | Benzoic acid | Benzenecarboxylic acid; retained name benzoic acid |
| C₆H₅CH₂COOH | Phenylacetic acid | 2-Phenylethanoic acid |
| o-C₆H₄(COOH)₂ | Phthalic acid | Benzene-1,2-dicarboxylic acid |
Problem
Construct 3-bromo-4-phenylpentanoic acid and hex-2-en-4-ynoic acid.
- 1.Number the first from COOH: HOOC–CH₂–CHBr–CH(Ph)–CH₃. Bromine is on C3 and phenyl on C4; the acid carbon is included among the five parent carbons.
- 2.For the second, C1 is COOH, C2=C3 and C4≡C5 in a six-carbon chain: HOOC–CH=CH–C≡C–CH₃. The acid suffix controls numbering before multiple-bond locants.
Resonance couples the two oxygens
Bonds at the carboxyl carbon lie approximately in a plane with angles near 120°. In the neutral acid, a lone pair on the hydroxyl oxygen can donate toward C=O. A charge-separated contributor has positive charge on that OH oxygen and negative charge on the other oxygen. These contributors are not equivalent: neutral and charge-separated structures do not have equal weight, and the acid has distinguishable C–O and C=O bonds. Donation lowers the acyl carbon electrophilicity relative to an otherwise comparable aldehyde or ketone.
Deprotonation produces RCOO⁻, whose two principal resonance contributors are equivalent: either oxygen can be drawn bearing the formal negative charge. The real ion has delocalised charge and equivalent C–O bonds in a symmetric environment. Do not draw the neutral acid as if it already has two identical negatively charged oxygens. The distinction will explain acid strength and lower nucleophilic addition reactivity.
Two hydrogen bonds can pair acid molecules
A neutral carboxylic acid can donate a hydrogen bond from OH and accept one at C=O. Two molecules can associate into a cyclic dimer with two O–H···O contacts, especially in suitable nonpolar media and in some vapour conditions. These contacts are intermolecular attractions, not new covalent O–H bonds. In water, acid–water hydrogen bonding and ionisation also matter, so the fraction present as dimers depends on medium, temperature and concentration.
Strong association commonly gives acids higher boiling points than aldehydes, ketones or alcohols of comparable mass. Small aliphatic acids, approximately C1–C4, mix well with water through hydrogen bonding and partial ionisation. Increasing hydrocarbon chain length lowers water solubility; benzoic acid is only sparingly soluble in cold water. Its salts can be much more water-soluble because charged ions interact strongly with water. Melting and physical state also depend on packing: simple statements that every acid below a particular carbon count is liquid have exceptions.
Problem
Explain why ethanoic acid and methyl methanoate differ despite both being C₂H₄O₂.
- 1.Ethanoic acid is CH₃COOH and has an O–H donor plus a carbonyl acceptor, enabling strong association and acid–base reaction. Methyl methanoate is HCOOCH₃ and has no COOH proton.
- 2.The ester cannot form the same acid dimer or ordinary bicarbonate effervescence. Matching a molecular formula does not imply matching boiling point, acidity or hydrolysis behaviour.
Quick Quiz
Quiz
Which formula contains a carboxyl group?
Explanation: The OH must attach directly to the carbonyl carbon.
What is HOOCCH₂CH₂COOH called systematically?
Explanation: Both carboxyl carbons belong to the four-carbon acid chain.
What distinguishes neutral acid and carboxylate resonance?
Explanation: Deprotonation makes the two main carboxylate contributors equivalent.
Why do suitable carboxylic acids associate as dimers?
Explanation: Two molecules can form a pair of O–H···O contacts.
What is the structure of phenylacetic acid?
Explanation: A methylene group separates phenyl from the acid group.
Practice Problems
- Name PhCH₂CH₂COOH, (CH₃)₂C=CHCOOH and (CH₃)₃CCH₂COOH.
- Explain why carboxylic acids often boil higher than comparable ketones.
- Why does forming sodium benzoate increase solubility relative to benzoic acid in cold water?
- Draw propane-1,2,3-tricarboxylic acid and explain the carbon count.
- Explain why neutral acid resonance does not mean the two O–H locations are identical resonance forms.
The first has a propanoic acid chain with phenyl on C3: 3-phenylpropanoic acid. The second is 3-methylbut-2-enoic acid. The third has a butanoic acid parent and two additional methyl groups on C3: 3,3-dimethylbutanoic acid. Count from the acid carbon each time.
Key Takeaways
• COOH connectivity distinguishes acids from separate OH and CHO groups. • The acid carbon is carbon 1 in an open-chain acid parent. • Use dioic acid when both acid carbons are included in the parent. • Ring-attached external acid groups use carboxylic acid naming. • Neutral acid and carboxylate resonance are different. • Acid hydrogen bonding explains association and high boiling points. • Water solubility depends on size, charge and medium as well as oxygen atoms.