Supplementary 2 · Organic Chemistry

Nomenclature Reference

IUPAC nomenclature is the universal language of organic chemistry. Every name encodes the carbon skeleton, functional groups, substituents, and stereochemistry in a precise, unambiguous way. Master the rules once and you can name — or decode — any organic compound.

IUPAC rules Suffix & prefix system Priority order (all groups) Locant assignment Common names Worked examples

S2.1

IUPAC Naming — The Core Logic

Every IUPAC name is built from three components: a parent chain (the longest carbon chain containing the principal functional group), suffixes (indicating the principal functional group), and prefixes (indicating substituents and secondary functional groups). The process is always the same: identify the principal functional group, find the longest chain containing it, number the chain to give the principal group the lowest locant, then name all substituents alphabetically as prefixes.

The Five-Step IUPAC Method

1. Identify the principal functional group — use the priority order (S2.9) to determine which group gets the suffix.
2. Find the parent chain — the longest carbon chain containing the principal functional group. (For rings, the ring IS the parent.)
3. Number the chain — from the end nearer to the principal group. If a tie, from the end nearer to the first substituent.
4. Name substituents — as prefixes, alphabetically (ignore multiplying prefixes di-, tri- when alphabetising). Use hyphens between letters and numbers; commas between numbers.
5. Assemble the name — [locants-prefixes]-[parent chain]-[suffix]

Multiplying Prefixes (Not Alphabetised)

di- (×2) · tri- (×3) · tetra- (×4) · penta- (×5) · hexa- (×6) · hepta- (×7) · octa- (×8) · nona- (×9) · deca- (×10)
Use bis-, tris-, tetrakis- when the substituent name itself begins with di-, tri-, etc. (e.g., bis(2-methylpropyl))

Test yourself — S2.1

• What gets the lowest locant when numbering the chain? → The principal functional group (highest priority suffix group).
• How are substituents listed? → Alphabetically as prefixes; multiplying prefixes (di-, tri-) ignored when alphabetising.
• What is the format of an IUPAC name? → [locants-prefixes]-[parent chain]-[suffix].

S2.2

Alkanes & Cycloalkanes

Chain length	Parent name	Cyclo- prefix	Formula
1 C	meth-	cyclopropane (C3)	CH₄ / C₃H₆
2 C	eth-	cyclobutane (C4)	C₂H₆ / C₄H₈
3 C	prop-	cyclopentane (C5)	C₃H₈ / C₅H₁₀
4 C	but-	cyclohexane (C6)	C₄H₁₀ / C₆H₁₂
5 C	pent-	cycloheptane (C7)	C₅H₁₂ / C₇H₁₄
6 C	hex-	cyclooctane (C8)	C₆H₁₄ / C₈H₁₆
7 C	hept-		C₇H₁₆
8 C	oct-		C₈H₁₈
9 C	non-		C₉H₂₀
10 C	dec-		C₁₀H₂₂

Alkane Suffix & Common Substituents

Suffix: -ane
Common alkyl groups:
• –CH₃ = methyl · –C₂H₅ = ethyl · –C₃H₇ = propyl or isopropyl [–CH(CH₃)₂]
• –C₄H₉ = n-butyl / sec-butyl [–CH(CH₃)CH₂CH₃] / isobutyl [–CH₂CH(CH₃)₂] / tert-butyl [–C(CH₃)₃]
Halo substituents (prefixes): fluoro- · chloro- · bromo- · iodo- (alphabetical priority as normal)

S2.3

Alkenes & Alkynes

The parent chain must include both carbons of the double or triple bond. The chain is numbered to give the double/triple bond the lowest possible locant. With IUPAC 2013 rules the position number immediately precedes the suffix: but-1-ene (not 1-butene). E/Z (or cis/trans) stereodescriptors are added as a prefix for alkenes with two different groups on each carbon.

Alkene & Alkyne Suffixes

Group	Suffix	Example	Note
C=C (one)	`-ene`	prop-1-ene, but-2-ene	Locant = lower-numbered C of C=C
C=C (two)	`-diene`	buta-1,3-diene	Both locants cited
C≡C	`-yne`	prop-1-yne, but-2-yne	Locant = lower C
C=C + C≡C	`-en-yne`	pent-1-en-4-yne	-en before -yne

E/Z rule (CIP at each C of the double bond): if higher-priority groups are on the same side = Z (zusammen = together); opposite sides = E (entgegen = opposite).

S2.4

Aromatic Compounds (Benzene Derivatives)

When benzene is the principal structure it is the parent ring. Substituents on the ring are named as prefixes with locants 1–6. When two substituents are present, the relative positions are given as numbers (1,2- = ortho; 1,3- = meta; 1,4- = para — the o/m/p notation is still widely used in common names but locants are strictly correct in IUPAC). When a functional group higher in the priority order is present, benzene becomes a substituent named phenyl- (or benzyl- for –CH₂–C₆H₅).

Common Aromatic Parent Names (Retained in IUPAC)

Structure	IUPAC retained name	Systematic base
C₆H₅–OH	phenol	hydroxybenzene
C₆H₅–NH₂	aniline	aminobenzene (benzenamine)
C₆H₅–CHO	benzaldehyde	benzenecarbaldehyde
C₆H₅–COOH	benzoic acid	benzenecarboxylic acid
C₆H₅–CH₃	toluene (methylbenzene)	methylbenzene
C₆H₅–COCH₃	acetophenone	1-phenylethan-1-one

Disubstituted benzenes: 1,2- (ortho/o-) · 1,3- (meta/m-) · 1,4- (para/p-)

S2.5

Alcohols, Phenols & Ethers

Suffixes & Prefixes

Group	Suffix (principal)	Prefix (substituent)	Example
–OH (aliphatic)	`-ol`	hydroxy-	propan-1-ol, butan-2-ol
–OH (multiple)	`-diol, -triol`	hydroxy-	ethane-1,2-diol (ethylene glycol)
–OH (aromatic)	phenol (retained)	phenoxy-	4-methylphenol (p-cresol)
–O– (ether)	— (not a principal group)	alkoxy-	methoxybenzene (anisole); 1-ethoxypropane

Ethers: Named as alkoxy- substituents on the larger alkane chain. Or use "X ether" common name (diethyl ether, methyl tert-butyl ether = MTBE).
Numbering: –OH gets the lowest possible locant.

S2.6

Aldehydes & Ketones

Suffixes & Rules

Group	Suffix	Prefix	Notes & Examples
R–CHO (aldehyde)	`-al`	formyl- / oxo-	C-1 always the aldehyde carbon; no locant needed. Ethanal, propanal, butanal
–CHO at ring	`-carbaldehyde`	formyl-	Cyclohexanecarbaldehyde; benzaldehyde (retained)
R–CO–R' (ketone)	`-one`	oxo-	Locant on carbonyl C. Propan-2-one (acetone), butan-2-one (methyl ethyl ketone)

When both –CHO and C=O: use -oxo- prefix for the ketone and -al suffix for the aldehyde (aldehyde has higher priority; see S2.9).

S2.7

Carboxylic Acids & Derivatives

Suffixes for All Carboxylic Acid Derivatives

Compound class	Suffix (chain COOH)	Suffix (ring –COOH)	Example
Carboxylic acid	`-oic acid`	`-carboxylic acid`	ethanoic acid; cyclohexanecarboxylic acid
Acyl chloride	`-oyl chloride`	`-carbonyl chloride`	ethanoyl chloride (acetyl chloride)
Anhydride	`-oic anhydride`		ethanoic anhydride; mixed: ethanoic propanoic anhydride
Ester	`-[alkyl] -oate`	`-[alkyl] -carboxylate`	ethyl ethanoate (ethyl acetate)
Amide	`-amide`	`-carboxamide`	ethanamide (acetamide); N-methylpropanamide
Nitrile	`-nitrile`	`-carbonitrile`	ethanenitrile (acetonitrile = –C≡N attached)

Ester naming: alkyl part first (from alcohol), then acyl part (from acid) with -ate suffix. E.g., methyl propanoate = CH₃CH₂COOCH₃.
Amide N-substituents: named with N- locant. E.g., N,N-dimethylethanamide (= DMF minus the formyl: actually N,N-dimethylformamide).

Dicarboxylic Acids (Common Names Retained)

IUPAC name	Common name	Memory
ethanedioic acid	oxalic acid	OX = 2C
propanedioic acid	malonic acid	MAL = 3C
butanedioic acid	succinic acid	SUC = 4C
pentanedioic acid	glutaric acid	GLU = 5C
hexanedioic acid	adipic acid	ADI = 6C; nylon precursor
(Z)-butenedioic acid	maleic acid	cis C4 diacid
(E)-butenedioic acid	fumaric acid	trans C4 diacid

S2.8

Amines & Nitrogen Compounds

Amines: Suffix & Prefix

Class	Suffix	Prefix	Examples
Primary amine (R–NH₂)	`-amine`	amino-	methylamine, ethanamine, propan-1-amine
Secondary (R₂NH)	`-amine` + N-	amino-	N-methylethanamine (= dimethylamine if both Me)
Tertiary (R₃N)	`-amine` + N,N-	amino-	N,N-dimethylethanamine (= TEA if ethyl base)
Aromatic amine	aniline (retained)	amino-	aniline; 4-methylaniline (p-toluidine)
Diazonium	`-diazonium salt`		benzenediazonium chloride

N-substituents: Always cite N- as locant. "N-methyl" = methyl on the nitrogen, not the carbon chain.

S2.9

Functional Group Priority Order (for Suffix Selection)

When a molecule has two or more functional groups, only the one highest in the priority order gets the suffix; all others become prefixes. This table is the backbone of all polyfunctional naming — memorise the top 8.

Priority Order — Highest to Lowest (Suffix First)

Rank	Group	Suffix	Prefix
1 (highest)	Carboxylic acid (–COOH)	-oic acid	carboxy-
2	Anhydride (–CO–O–CO–)	-oic anhydride	—
3	Acyl halide (–COX)	-oyl halide	halocarbonyl-
4	Amide (–CONH₂)	-amide	carbamoyl-
5	Nitrile (–CN)	-nitrile	cyano-
6	Aldehyde (–CHO)	-al	formyl- / oxo-
7	Ketone (C=O)	-one	oxo-
8	Alcohol / Phenol (–OH)	-ol	hydroxy-
9	Amine (–NH₂)	-amine	amino-
10	Ether (–O–)	— (never suffix)	alkoxy-
11 (lowest)	Alkene / Alkyne	-ene / -yne	alkenyl- / alkynyl-

Mnemonic — Priority Order Top 7

"Can A Clever Alderman Know Alcohol?"
Carboxylic acid · Anhydride · Cacyl halide · Amide · nitrile · aldehyde · ketone · alcohol · amine
(Alternatively: "A Awful Chemist Angrily Named Alkyl Ketones And Alcohols Amid Ethers")

Test yourself — S2.9

• A compound has –OH and –COOH. Which gets the suffix? → –COOH (carboxylic acid, rank 1); –OH becomes hydroxy- prefix.
• A compound has –CHO and C=O. Which gets the suffix? → –CHO (aldehyde, rank 6 > ketone rank 7); C=O becomes oxo- prefix.
• Does –O– (ether) ever become a suffix? → No — always a prefix (alkoxy-).

S2.10

Common Names You Must Know

Examiners, textbooks, and clinical pharmacology all use common names alongside IUPAC. Knowing the common name tells you the molecule; knowing IUPAC tells you the structure. You need both.

Essential Common Names → IUPAC

Common name	IUPAC name	Structure
Formic acid	Methanoic acid	HCOOH (C1)
Acetic acid	Ethanoic acid	CH₃COOH (C2)
Propionic acid	Propanoic acid	CH₃CH₂COOH (C3)
Butyric acid	Butanoic acid	C3H7COOH (C4)
Valeric acid	Pentanoic acid	C4H9COOH (C5)
Acetone	Propan-2-one	CH₃COCH₃
Formaldehyde	Methanal	HCHO
Acetaldehyde	Ethanal	CH₃CHO
Chloroform	Trichloromethane	CHCl₃
Ether (diethyl ether)	Ethoxyethane	C₂H₅OC₂H₅
Glycerol	Propane-1,2,3-triol	HOCH₂CH(OH)CH₂OH
Ethylene glycol	Ethane-1,2-diol	HOCH₂CH₂OH
Lactic acid	2-hydroxypropanoic acid	CH₃CH(OH)COOH
Pyruvic acid	2-oxopropanoic acid	CH₃COCOOH
Aspirin	2-acetoxybenzoic acid	Acetyl ester of salicylic acid
Salicylic acid	2-hydroxybenzoic acid	C₆H₄(OH)(COOH) ortho
Paracetamol	4-acetamidophenol	Amide + phenol

Test yourself — S2.10

• IUPAC name for acetone? → Propan-2-one.
• Common name for ethanoic acid? → Acetic acid.
• What is the IUPAC name for aspirin? → 2-acetoxybenzoic acid (acetylated ester at C-2 of benzoic acid).
• Paracetamol = ? → 4-acetamidophenol — has an amide bond (N-acetyl) + para-hydroxyphenol.

🎓

Worked Naming Examples

1. Name: CH₃–CH(OH)–CH₂–CHO

Principal group: –CHO (aldehyde, rank 6) → suffix -al. Parent chain: 4 carbons including the CHO = butanal. Number from CHO end: C1=CHO, C2=CH₂, C3=CH(OH), C4=CH₃. The –OH is a substituent: 3-hydroxy. Name: 3-hydroxybutanal (also an intermediate in the aldol reaction!).

2. Name: CH₃–CH₂–C(=O)–CH₂–CH₂–COOH

Two functional groups: –COOH (rank 1) and C=O ketone (rank 7). Principal group = –COOH → suffix -oic acid. Parent chain with both groups: 6 carbons → hexanoic acid base. Number from COOH end: C1=COOH, C2=CH₂, C3=CH₂, C4=C=O, C5=CH₂, C6=CH₃. Ketone at C4 → prefix 4-oxo-. Name: 4-oxohexanoic acid (levulinic acid in common name).

3. Name: (CH₃)₂CH–CH₂–CH₂–NH₂

Principal group: –NH₂ (amine, rank 9) → suffix -amine. Longest chain containing N: 4 carbons = butyl chain but N is on C1 → butan-1-amine. The (CH₃)₂CH– branch: C1 of butane has an isobutyl substituent? Actually re-examine: the full chain is (CH₃)₂CH–CH₂–CH₂–NH₂. Longest chain through all carbons: C1(NH₂)–C2–C3–C4(H)–two methyls at C4 → pentane (5C), NH₂ on C1, methyl on C4? No — it's 3-methylbutan-1-amine: 4C chain with a methyl at C3. Count: NH₂–CH₂–CH₂–CH(CH₃)₂. From the NH₂ end: C1=CH₂NH₂, C2=CH₂, C3=CH, C4=CH₃, branch CH₃ at C3. Name: 3-methylbutan-1-amine.

4. Name the ester formed from propanoic acid + 2-butanol (esterification).

Ester = acyl part (propanoate) + alkyl part (from 2-butanol = butan-2-ol → the alkyl group is butan-2-yl). Name: butan-2-yl propanoate. (Common name: sec-butyl propionate.)

5. Name: 4-aminobenzoic acid (PABA) — identify every functional group.

Parent: benzoic acid (benzenecarboxylic acid). Principal group: –COOH at C1 → -oic acid suffix. Substituent: –NH₂ at C4 → amino- prefix. IUPAC name: 4-aminobenzoic acid. Groups: carboxylic acid + primary aromatic amine. Clinical note: PABA is a folate precursor — sulfonamides compete with PABA for dihydropteroate synthase.

Quick Suffix Cheat-Sheet

-ane alkane · -ene alkene · -yne alkyne
-ol alcohol · -al aldehyde · -one ketone
-oic acid carboxylic acid · -oate ester · -amide amide
-oyl chloride acyl chloride · -nitrile nitrile · -amine amine