IUPAC Nomenclature

IUPAC Nomenclature

🟢 Lite — Quick Review (1h–1d)

Definition: IUPAC Nomenclature is the standardized system by the International Union of Pure and Applied Chemistry for naming organic compounds, ensuring each structure has one unique name.

Core rules (memorise this sequence):

1. Parent chain — longest continuous carbon chain; root name: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C)
2. Functional group priority — highest-ranking group determines suffix: carboxylic acid > ester > aldehyde > ketone > alcohol > amine
3. Number from the end giving lowest locants to the principal group
4. Alphabetise substituent names; ignore di-/tri- prefixes for ordering
5. Unsaturation — -en- (double bond), -yn- (triple bond) with position locant

3 high-yield SLMC pointers:

• Name CH₃-CH₂-CHO as propanal (3-carbon chain, aldehyde suffix)
• C₆H₅-COOH → benzenecarboxylic acid; aspirin = 2-acetyloxybenzenecarboxylic acid
• Halogen substituents: fluoro-, chloro-, bromo-, iodo- (alphabetical order)

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🟡 Standard — Regular Study (2d–2mo)

Selecting the Parent Chain

The parent chain is the longest continuous carbon chain containing the principal functional group. Chain length determines the root:

Carbons	Root	Carbons	Root
1	meth-	5	pent-
2	eth-	6	hex-
3	prop-	7	hept-
4	but-	8	oct-

Branches on the chain are named as alkyl substituents: methyl (–CH₃), ethyl (–C₂H₅), propyl (–C₃H₇). Cyclic alkanes take the prefix cyclo- (cyclopentane, cyclohexane).

Functional Group Priority

The principal functional group (highest priority) determines the suffix:

• Carboxylic acid: –COOH → -oic acid (e.g., ethanoic acid)
• Ester: –COOR → -oate (e.g., ethyl ethanoate)
• Aldehyde: –CHO → -al (e.g., propanal)
• Ketone: –CO– → -one (e.g., propanone)
• Alcohol: –OH → -ol (e.g., propan-2-ol)
• Amine: –NH₂ → -amine (e.g., ethanamine)

Numbering and Alphabetising

Number from the end nearest the principal group. The locant (position number) is placed before the suffix. For multiple substituents, the chain is numbered to give the lowest set of locants at the first point of difference.

Substituents are listed alphabetically in the name, ignoring prefixes such as di-, tri-, tetra-. Example: 4-ethyl-3-methyloctane (e is before m, so ethyl precedes methyl despite ethyl appearing first when alphabetically sorted).

Unsaturation

A double bond uses the infix -en- with a locant (propen-1-ol); a triple bond uses -yn- (prop-1-yn-amine). Multiple bonds require a locant for each.

Worked Example

CH₃-CH(Br)-CH₂-CH(CH₃)-CHO

1. Longest chain = 5 carbons → pent
2. Principal group = aldehyde (–CHO) → suffix -al
3. Number from aldehyde end: C1 = CHO, so locants: bromine at C2, methyl at C4
4. Alphabetical substituents: bromo- (b), methyl- (m)
5. Name: 2-bromo-4-methylpentanal

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🔴 Extended — Deep Study (3mo+)

Complex Functional Group Scenarios

When a molecule contains multiple functional groups, the highest-priority group takes the suffix; the others become substituents. For instance, HOOC–CH₂–CH₂–NH₂ is named 3-aminopropanoic acid (carboxylic acid outranks amine). When two or more identical substituents exist, multiplicative prefixes (di-, tri-) are used, but alphabetical ordering ignores these prefixes: 1,3-dimethylcyclohexane has methyl at C1 and C3, numbered to give the lowest locant set.

Alkyl Group Branching and Nomenclature

Branched alkyl substituents require correct alphabetical placement. Iso- and tert- count for alphabetisation, unlike di-/tri-. The compound CH₃-CH(CH₃)-CH₂-CH₂-CH₃ is 2-methylpentane; the longest chain is six carbons for hexane, not five.

Cyclic and Aromatic Compounds

For cycloalkanes, the prefix cyclo- precedes the stem. Numbering starts at a substituent and proceeds in the direction giving the lowest locants. Benzene derivatives use locants (1,2-dimethylbenzene) or positional descriptors: ortho- (adjacent), meta- (one carbon apart), para- (opposite). Aspirin is systematically 2-(acetyloxy)benzenecarboxylic acid; paracetamol is N-(4-hydroxyphenyl)acetamide.

Common Examiner Traps

1. Selecting a shorter chain that contains a functional group instead of the objectively longest chain
2. Alphabetising substituents incorrectly (di- is ignored; iso- is not)
3. Misapplying functional group priority — e.g., treating an ester as a ketone or alcohol
4. Failing to minimise locants at the first point of difference when numbering from either direction gives the same set
5. Omitting locants on symmetric molecules where the structure could still be interpreted unambiguously

Exam-Relevant Connections

IUPAC naming is essential in pharmacology (identifying drug structures like penicillin derivatives), biochemistry (carbohydrates and nucleotides use specialized IUPAC conventions), and clinical chemistry (metabolite identification from systematic names). SLMC questions frequently present a structural formula and require the correct systematic name, especially for compounds with medical relevance.

Practice Prompts

Prompt 1: Name the compound (CH₃)₂CH–CH₂–CH(Cl)–CH₃. Identify the parent chain length, principal functional group, and alphabetical substituent order. [Answer: 2-chloro-3-methylpentane — 5-carbon chain, no principal group, chloro before methyl alphabetically.]

Prompt 2: Write the IUPAC name for aspirin (C₆H₄(CH₃COO)COOH). Explain why the carboxylic acid takes priority over the ester group. [Answer: 2-(acetyloxy)benzenecarboxylic acid — carboxylic acid ranks highest in IUPAC functional group priority, so –COOH determines the suffix; the acetyloxy group is a substituent.]

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