IUPAC Nomenclature of Organic Compounds — Naming systematically

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Rapid summary for last-minute revision before your exam.

The IUPAC (International Union of Pure and Applied Chemistry) system provides a universal naming convention for organic compounds. For SLMC, master the systematic naming algorithm: longest chain → numbering → substituent identification → alphabetical ordering of prefixes. The suffix tells you the functional group; prefixes tell you substituents.

High-Yield Facts for SLMC:

• Longest chain determines the parent name; number from the end giving substituents lowest numbers
• Alphabetize substituent prefixes (chloro-, methyl-, ethyl-) — ignore “di-”, “tri-”, “sec-”, “tert-” for alphabetizing
• Suffix changes with functional group: –ane (alkane), –ene (alkene), –yne (alkyne), –ol (alcohol), –al (aldehyde), –one (ketone), –oic acid (carboxylic acid)
• ⚡ Exam tip: “3-methylpentane” is wrong — the correct name is “2-methylpentane” because you must number to give the substituent the lowest possible locant

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IUPAC Nomenclature — SLMC Medical (Sri Lanka) Study Guide

The IUPAC Naming Algorithm — Step by Step

Step 1: Identify all functional groups → determine the principal functional group (highest priority) Step 2: Choose the longest carbon chain containing the principal functional group Step 3: Number the chain from the end that gives the principal functional group the lowest number Step 4: Name substituents (alkyl groups, halogens) in alphabetical order with position numbers Step 5: Assemble the name — position numbers + substituent names + parent chain name + suffix

Functional Group Priority (Highest to Lowest for Naming)

When a molecule has multiple functional groups, the highest-priority group determines the suffix and numbering:

Priority	Functional Group	Suffix	Example
1 (highest)	Carboxylic acid	–oic acid	Butanoic acid
2	Ester	–oate	Methyl ethanoate
3	Amide	–amide	Propanamide
4	Aldehyde	–al	Propanal
5	Ketone	–one	Butan-2-one
6	Alcohol	–ol	Butan-1-ol
7	Amine	–amine	Butan-1-amine
8	Alkene	–ene	But-1-ene
9	Alkyne	–yne	But-1-yne
10 (lowest)	Halide / alkyl substituent	halo- / alkyl	Chloromethane

Straight-Chain Alkanes (Parent Chains)

C atoms	Name	Formula
1	Methane	CH₄
2	Ethane	C₂H₆
3	Propane	C₃H₈
4	Butane	C₄H₁₀
5	Pentane	C₅H₁₂
6	Hexane	C₆H₁₄
7	Heptane	C₇H₁₆
8	Octane	C₈H₁₈
9	Nonane	C₉H₂₀
10	Decane	C₁₀H₂₂

Alkyl Substituents

Removing one hydrogen from an alkane gives an alkyl group (prefix used in naming):

Parent Alkane	Alkyl Group	Structure
Methane	Methyl	–CH₃
Ethane	Ethyl	–CH₂CH₃
Propane	Propyl	–CH₂CH₂CH₃
Propane	Isopropyl	–CH(CH₃)₂
Butane	Butyl	–CH₂CH₂CH₂CH₃
Butane	sec-Butyl	–CH(CH₃)CH₂CH₃
Butane	Isobutyl	–CH₂CH(CH₃)₂
Butane	tert-Butyl	–C(CH₃)₃

Alphabetizing rules:

• Prefixes di-, tri-, sec-, tert- are IGNORED for alphabetizing
• iso-, neo- ARE considered
• Correct order: 3-ethyl-2-methylpentane (e before m = ethyl before methyl)

Naming Alkenes and Alkynes

Alkenes (–ene):

1. Find the longest chain containing the double bond
2. Number to give the double bond the lowest possible number
3. The position of the double bond is given by the lower-numbered carbon of the double bond

Examples:

• CH₂=CH–CH₂–CH₃ → but-1-ene (double bond between C1 and C2)
• CH₃–CH=CH–CH₃ → but-2-ene (double bond between C2 and C3)
• CH₃–C(CH₃)=CH–CH₃ → 2-methylbut-2-ene

Geometric isomers of but-2-ene:

• CH₃–CH=CH–CH₃ with both CH₃ on same side → cis-but-2-ene (or (Z)-but-2-ene)
• CH₃–CH=CH–CH₃ with CH₃ on opposite sides → trans-but-2-ene (or (E)-but-2-ene)

Alkynes (–yne): Same rules as alkenes, suffix is –yne

• HC≡C–CH₂–CH₃ → but-1-yne
• CH₃–C≡C–CH₃ → but-2-yne

Naming Alcohols (–ol)

Examples:

• CH₃–CH₂–CH₂–OH → propan-1-ol
• CH₃–CH(OH)–CH₃ → propan-2-ol
• HO–CH₂–CH(OH)–CH₃ → propane-1,2-diol

Key rule: When naming compounds with multiple –OH groups (polyols), use “ethane-1,2-diol” not “ethylene glycol.”

Naming Aldehydes (–al)

The aldehyde carbon is always C-1 (it gets priority in numbering):

• HCHO → methanal (formaldehyde)
• CH₃CHO → ethanal (acetaldehyde)
• CH₃–CH₂–CHO → propanal
• OHC–CH₂–CHO → propanedioic acid (for two aldehyde groups, treated as carboxylic acid)

Naming Ketones (–one)

• CH₃–CO–CH₃ → propanone (acetone)
• CH₃–CO–CH₂–CH₃ → butan-2-one (methyl ethyl ketone)
• CH₃–CH₂–CO–CH₂–CH₃ → pentan-3-one

Naming Carboxylic Acids (–oic acid)

• HCOOH → methanoic acid (formic acid)
• CH₃COOH → ethanoic acid (acetic acid)
• CH₃–CH₂–COOH → propanoic acid
• HOOC–CH₂–COOH → propane-1,3-dioic acid (malonic acid)

Naming Halides (Halo–alkanes)

Alphabetically: fluoro-, chloro-, bromo-, iodo-

Examples:

• CH₃Cl → chloromethane
• CH₃–CH(Cl)–CH₃ → 2-chloropropane
• CH₃–CCl₃ → 1,1,1-trichloro-2-methylpropane or 2-methyl-1,1,1-trichloropropane
• BrCH₂CH₂Br → 1,2-dibromoethane

Naming Cyclic Compounds

Cycloalkanes:

• C₆H₁₂ with no substituents → cyclohexane
• C₆H₁₂ with one methyl → methylcyclohexane (number not needed if only one substituent)
• 1,2-dimethylcyclohexane: two methyl groups on adjacent ring carbons
  • cis-1,2-dimethylcyclohexane: both methyls on same face of the ring
  • trans-1,2-dimethylcyclohexane: methyls on opposite faces

Aromatic compounds:

• C₆H₅–CH₃ → methylbenzene (IUPAC) or toluene (preferred common name)
• 1,2-dichlorobenzene → ortho-dichlorobenzene or o-dichlorobenzene
• 1,3-dichlorobenzene → meta-dichlorobenzene or m-dichlorobenzene
• 1,4-dichlorobenzene → para-dichlorobenzene or p-dichlorobenzene

Numbering Rules — Minimizing the Lowest Set of Locants

When there is a choice, use the “lowest set of locants” rule:

1. Compare the first locant in each direction — the direction giving the lower first locant wins
2. If tied, compare the second locant, and so on
3. If sets are identical, use alphabetical order to break ties

Example: CH₃–CH(Cl)–CH₂–CH₃

• Number from left: C1=CH₃, C2=CH(Cl), C3=CH₂, C4=CH₃ → locants = 2
• Number from right: C1=CH₃, C2=CH₂, C3=CH(Cl), C4=CH₃ → locants = 3
• Correct: 2-chlorobutane (not 3-chlorobutane)

Example: CH₃–CH₂–CH(Br)–CH₃

• Number from left: bromine at C3
• Number from right: bromine at C2 → 2-bromobutane (lower number wins)

Special Cases and Common Names to Know

Structure	Common Name	IUPAC Name
CH₄	Methane	Methane
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
CH₃COCH₃	Acetone	Propanone
CH₃COOH	Acetic acid	Ethanoic acid
CH₃OH	Methyl alcohol	Methanol
CH₃CH₂OH	Ethyl alcohol	Ethanol
C₆H₅–CH₃	Toluene	Methylbenzene
C₆H₅–CHO	Benzaldehyde	Benzenecarbaldehyde
C₆H₅–OH	Phenol	Benzenol
C₆H₅–COOH	Benzoic acid	Benzenecarboxylic acid
(CH₃)₂CH–	Isopropyl	1-methylethyl

The –CO– (Oxo) Group vs –CHO (Aldehyde)

When a carbonyl group is within a chain (not at the end), it is a ketone and uses “–one”:

• CH₃–CO–CH₂–CH₂–CHO → 4-oxopentanal (aldehyde has priority; carbonyl gets “oxo” prefix)

E/Z Nomenclature for Alkenes

E (Entgegen = opposite): Higher priority groups on opposite sides of the double bond Z (Zusammen = together): Higher priority groups on the same side

CIP Priority rules:

1. Higher atomic number attached to the double bond carbon → higher priority
2. If the first atom is the same, compare the atoms attached to it (treat double bonds as duplicate)
3. Keep comparing until a difference is found

Example: (CH₃)₂C=CHCH₃

• C1: two CH₃ groups → identical, so no geometric isomers possible here
• If C1 has: Br and CH₃, and C2 has: Cl and H → compare Br (Z=35) vs Cl (Z=17): Br is higher priority on C1; compare CH₃ (C,H,H) vs H (H,H,H): CH₃ is higher priority on C2
• If Br and CH₃ are on same side → Z; if opposite → E

Common Traps on the SLMC Exam

1. Omitting the position number for double/triple bonds — “butene” is not a valid IUPAC name; it must be but-1-ene or but-2-ene
2. Alphabetical order ignores di-, tri-, sec-, tert- — “dimethyl” starts with ‘d’ which is NOT ignored for alphabetizing? Actually di- IS ignored — so 1,2-dimethyl is alphabetized under ‘m’ not ‘d’
3. Forgetting that the aldehyde carbon is always C-1 — propanal is CH₃CH₂CHO; the CHO carbon is C-1
4. Cyclic compounds require position numbers when substituted — even a single methyl on cyclohexane is named methylcyclohexane (but if it’s the only substituent, no number needed)
5. Wrong suffix — knowing that –al means aldehyde and –one means ketone is a common exam question

⚡ Exam tip: When given a structural formula and asked to name it: (1) find the longest chain, (2) find the highest-priority functional group, (3) number from the end that gives that group the lowest number, (4) name substituents alphabetically with numbers. Practice: CH₃–CH₂–CH(CH₃)–CH₂–CH₃ → 3-methylpentane (chain = pentane, methyl at C3)

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