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Organic Chemistry · Carboxylic Acids and Nitriles

Naming Carboxylic Acids and Nitriles

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Science note: Naming rules follow standard IUPAC organic nomenclature as taught in undergraduate organic chemistry. No experimental data are fabricated.
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

A systematic name is a compact code that lets any chemist in the world draw the exact same structure from a string of words and numbers. Carboxylic acids are named with the suffix -oic acid: the is the longest chain that contains the carboxyl carbon, and that carbon is always numbered 1. Nitriles, which contain the C ≡ N triple bond, are named with the suffix -nitrile, and here the carbon itself counts as part of the parent chain. Because these two functional groups are intimately connected in synthesis — nitriles hydrolyze to carboxylic acids, and carboxylic acids can be dehydrated to nitriles — learning both naming systems together makes the chemistry of Chapter 20 much easier to follow. Common names such as acetic acid, formic acid, and succinic acid remain so widespread in biology, medicine, and industry that they must be learned alongside the systematic names.

Why this matters

Names are how chemists, pharmacists, and clinicians communicate precisely. Aspirin is acetylsalicylic acid (ethanoate ester of 2-hydroxybenzoic acid); ibuprofen is 2-(4-isobutylphenyl)propanoic acid; amino acids, fatty acids, and the citric acid cycle intermediates all carry carboxylic acid names. Nitriles matter because they are versatile synthetic intermediates: a nitrile adds two carbons' worth of chemistry in one step and can be converted into carboxylic acids, amides, or amines. If you cannot name these compounds, you cannot read a drug label, a synthesis scheme, or a biochemical pathway — and you will lose easy points on any exam that tests nomenclature.

The college version

Core Concepts

The carboxyl group is the naming anchor

The -COOH is the highest-priority functional group in the IUPAC priority list, so whenever it is present, the parent name must end in -oic acid and the carboxyl carbon is automatically carbon 1. This rule overrides chain-length considerations: the parent chain is always the longest chain that contains the carboxyl carbon, even if a longer chain exists that does not.

Steps for naming a simple acyclic acid

  1. Find the longest continuous carbon chain that includes the carboxyl carbon.
  2. Name the corresponding alkane, drop the final -e, and add -oic acid. Methane → methanoic acid; ethane → ethanoic acid.
  3. Number the chain starting at the carboxyl carbon (C1) and use the lowest possible numbers for substituents.
  4. Name substituents alphabetically with their position numbers (2-chloro-, 3-methyl-, and so on).

For example, CH3CH2CH2COOH is butanoic acid, and a methyl group on the third carbon gives 3-methylbutanoic acid — not 2-methylbutanoic acid, because numbering must start at the carboxyl carbon.

Common names you must recognize

Small acids are still routinely called by their common names, which are based on Latin/trivial roots: formic acid (methanoic, 1 C), acetic acid (ethanoic, 2 C), propionic acid (propanoic, 3 C), butyric acid (butanoic, 4 C), valeric acid (pentanoic, 5 C), and caproic acid (hexanoic, 6 C). The two-carbon acid is particularly important: "acetic acid" is the systematic vinegar acid, and its IUPAC name ethanoic acid is used interchangeably in modern texts.

Dicarboxylic acids

When a molecule has two carboxyl groups, the suffix becomes -dioic acid and both carboxyl carbons are numbered: HOOC-CH2-CH2-COOH is butanedioic acid. The common names of the straight-chain dicarboxylic acids are heavily used in biology and polymer chemistry: oxalic (2 C), malonic (3 C), succinic (4 C), glutaric (5 C), and adipic (6 C) acids. Adipic acid is a nylon-6,6 monomer; succinic acid is a citric acid cycle intermediate.

Aromatic and substituted acids

Benzoic acid, C6H5COOH, is the parent name for aromatic acids — do not try to force the ring into a chain name. Substituents on the ring are numbered from the carboxyl carbon: 2- (ortho, o-), 3- (meta, m-), and 4- (para, p-) positions. Salicylic acid is 2-hydroxybenzoic acid, and acetylsalicylic acid (aspirin) is its acetate ester.

Naming nitriles

A nitrile contains -C ≡ N, and the suffix is -nitrile. Crucially, the nitrile carbon is counted as part of the parent chain: CH3CH2CH2C ≡ N is butanenitrile (four carbons, including the nitrile carbon), named by dropping the -e of butane and adding -nitrile. When the nitrile group is attached to a ring, the suffix is -carbonitrile: benzonitrile (phenyl cyanide) and cyclohexanecarbonitrile. Older common names treat the nitrile as a cyanide salt: acetonitrile is methyl cyanide, and benzonitrile is phenyl cyanide. The nitrile carbon is numbered 1 in the parent chain, and substituents are numbered from it.

Common Confusions

Do Not ConfuseWithDifference
-oic acid ending-al (aldehyde) or -one (ketone) endingsThe acid ending marks the highest-priority group; only the carboxyl group earns -oic acid.
Acetic acidEthanoic acidSame molecule; acetic is the common name, ethanoic the systematic name.
Nitrile carbon countingNitrile carbon not countingThe nitrile carbon IS part of the parent chain: CH3CH2CN is propanenitrile (3 C), not ethanenitrile.
Succinic acidAdipic acidSuccinic is 4 carbons (butanedioic); adipic is 6 carbons (hexanedioic).
Benzoic acid"Benzene carboxylic acid"The accepted parent name is simply benzoic acid; the ring is not treated as a substituent.
Methyl cyanideAcetonitrileSame compound; "cyanide" names come from treating the nitrile as a cyanide salt.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

A systematic name is like a mailing address for a molecule: the "street" is the longest chain of carbons, the "house number" tells you where a group sits, and the ending — like -oic acid or -nitrile — tells you what kind of molecule it is. If everyone uses the same address system, chemists anywhere in the world can build the exact same molecule from the name.

Worked examples

Name the compound CH3CH(CH3)CH2COOH. Step 1: find the longest chain containing the carboxyl carbon — four carbons, so the parent is butanoic acid. Step 2: number from the carboxyl carbon, making the carboxyl carbon C1, so the chain runs C1(COOH)–C2–C3–C4. Step 3: the methyl substituent sits on C3. Step 4: alphabetize and assemble: 3-methylbutanoic acid. Notice the name would change entirely if you numbered from the other end, which is why the "carboxyl carbon is C1" rule must come first.

Name HOOC-CH2-CH2-COOH. The chain is four carbons with a carboxyl at each end, so the parent is butanedioic acid (common name: succinic acid). Now name 2-hydroxybenzoic acid: the parent is benzoic acid, the hydroxyl is at C2 (ortho), so the systematic name is 2-hydroxybenzoic acid — commonly called salicylic acid, the precursor to aspirin. Acetylation of the hydroxyl converts it to acetylsalicylic acid, aspirin, showing how the naming system tracks real drug chemistry.

Name CH3CH2CH2C ≡ N. The parent chain includes the nitrile carbon: four carbons total, so the name is butanenitrile (methyl cyanide in older usage). If a methyl branch sits on the second carbon, the name becomes 2-methylbutanenitrile — the nitrile carbon is C1. If the same nitrile group attaches to a benzene ring instead, the compound is benzonitrile (phenyl cyanide), and on a cyclohexane ring it is cyclohexanecarbonitrile. The "-carbonitrile" form is required because "cyclohexanenitrile" would incorrectly imply the nitrile carbon is part of the ring.

Key takeaways

  • Carboxylic acids end in -oic acid; the carboxyl carbon is always C1, and the parent chain must contain it.
  • Numbering starts at the carboxyl carbon, so substituents get the lowest possible numbers from that end.
  • Common names to memorize: formic (1C), acetic (2C), propionic (3C), butyric (4C); oxalic (2C diacid), malonic (3C), succinic (4C), glutaric (5C), adipic (6C).
  • Benzoic acid is the parent aromatic acid; use ortho/meta/para (2-/3-/4-) for ring substituents.
  • Nitriles end in -nitrile, and the nitrile carbon counts in the parent chain: CH3CH2CN is propanenitrile.
  • Nitriles on rings use -carbonitrile: benzonitrile, cyclohexanecarbonitrile.
  • Common "cyanide" names: methyl cyanide = acetonitrile; phenyl cyanide = benzonitrile.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Why is the carboxyl carbon always numbered 1 in a carboxylic acid name?

    Show answer

    The carboxyl carbon is the highest-priority functional-group carbon, and IUPAC rules fix it at position 1 so numbering is unambiguous.

  2. What is the systematic name of CH3CH2COOH, and what is its common name?

    Show answer

    Propanoic acid; common name propionic acid.

  3. Name the compound HOOC-CH2-CH2-COOH both systematically and by common name.

    Show answer

    Butanedioic acid; common name succinic acid.

  4. How many carbons are in the parent chain of CH3CH2CH2C ≡ N, and what is its name?

    Show answer

    Four carbons total including the nitrile carbon; the name is butanenitrile.

  5. What suffix do you use when a nitrile group is attached directly to a benzene ring?

    Show answer

    -carbonitrile: benzonitrile (phenyl cyanide).

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

carboxyl group
The -COOH group: a carbonyl (C=O) plus a hydroxyl (-OH) on the same carbon.
parent chain
The longest carbon chain that contains the principal functional group.
-oic acid suffix
Ending used when a carboxyl group is the principal group.
-dioic acid suffix
Ending for a molecule with two carboxyl groups.
nitrile
A compound containing the -C ≡ N group.
-carbonitrile suffix
Ending used when a nitrile group is attached to a ring.
ortho / meta / para
Ring positions 2-, 3-, and 4- relative to the principal group on a benzene ring.

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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