Nomenclature

Step-by-Step Naming Method

1. Find the longest carbon chain containing the functional group.

2. Number the chain to give the functional group(s) the lowest numbers.

3. Name and number all substituents, arrange alphabetically.

Key Rules:

• Numbers are separated by commas (2,3-dimethyl).

• Numbers and letters are separated by dashes (2-methyl).

• When identical groups appear, use di-, tri-, tetra- prefixes.

• Alphabetise substituents (ignore di-, tri-, tetra- when alphabetising).

Key Rule: The highest priority group gets the suffix and determines chain numbering. All  others become prefixes.

Alkanes

• Suffix: -ane

• Only contain C-H bonds

• Example: CH₃-CH₂-CH₃ = propane

Complex Example: 2-methylpentane

• Longest chain = 5 carbons (pentane)

• Methyl substituent on carbon-2

Alkenes

• Suffix: -ene (needs position number for chains ≥4C)

• Contains C=C double bond

• Example: CH₃-CH₂-CH=CH₂ = but-1-ene

Key Point: Number from end giving C=C the lowest number

Complex Example: 4,5-dimethyloct-2-ene

• Longest chain containing C=C: 8 carbons (octene)

• Double bond between carbons 2 and 3 (so "2-ene")

• Two methyl substituents on carbons 4 and 5

• Number from end giving double bond lowest number

Halogenoalkanes

• Prefixes: fluoro-, chloro-, bromo-, iodo-

• Always prefixes, never suffixes

• Example: CH₃-CHBr-CH₃ = 2-bromopropane

Simple Examples:

• CH₃-CH₂-I = iodoethane (no number needed - only one position possible)

• CH₃-CHCl-CH₃ = 2-chloropropane

• CH₃-CH₂-CH₂-F = 1-fluoropropane

Multiple Halogens:

• CCl₂H-CH₃ = 1,1-dichloroethane (both chlorines on same carbon)

• CH₂F-CH₂Br = 1-bromo-2-fluoroethane (alphabetical order: bromo before fluoro)

• CH₃-CHBr-CHCl-CH₃ = 2-bromo-3-chlorobutane (alphabetical order: bromo

before chloro)

Complex Example: 4-bromo-3,6-dimethyloctane

• Longest chain: 8 carbons (octane)

• Substituents: bromo (carbon-4), methyl (carbon-3), methyl (carbon-6)

• Alphabetical order: bromo, methyl

• Number to minimise ALL substituent positions

Alcohols

• Suffix: -ol (needs position number)

• Prefix: hydroxy-

• Example: CH₃-CH₂-CH₂-OH = propan-1-ol

Complex Example: 3-ethylhexan-2-ol

• Longest chain containing -OH: 6 carbons (hexanol)

• Alcohol group on carbon-2 (so "2-ol")

• Ethyl substituent on carbon-3

• Number from end giving alcohol lowest number

Aldehydes

• Suffix: -al

• Prefix: formyl-

• Key rule: Aldehyde carbon is ALWAYS carbon-1

Challenge: How would you name a compound with both aldehyde and carboxylic acid groups? Answer: The carboxylic acid takes priority, aldehyde becomes "formyl-"

Complex Example: 4-chloro-2-methylheptanal

• Longest chain containing -CHO: 6 carbons (hexanal)

• Aldehyde automatically on carbon-1

• Methyl substituent on carbon-2

• Chloro substituent on carbon-4

• Alphabetical order: chloro, then methyl

Ketones

• Suffix: -one (needs position number for chains ≥5C)

• Prefix: oxo

Complex Example: 5-bromo-3-methylheptan-2-one

• Longest chain containing C=O: 7 carbons (heptanone)

• Ketone on carbon-2 (so "2-one")

• Methyl substituent on carbon-3

• Bromo substituent on carbon-5

• Number from end giving ketone lowest number

Carboxylic Acids

• Suffix: -oic acid

• Key rule: Carboxylic acid carbon is ALWAYS carbon-1

Complex Example: 3-chloro-4-methylpentanoic acid

• Longest chain containing -COOH: 5 carbons (pentanoic acid)

• Carboxylic acid automatically on carbon-1

• Methyl substituent on carbon-4

• Chloro substituent on carbon-3

• Alphabetical order: chloro, then methyl

Nitriles

• Suffix: -nitrile

• Critical point: The cyano carbon (C≡N) counts as carbon-1

Complex Example: 3-fluoro-4-methylhexanenitrile

• Longest chain containing C≡N: 6 carbons (hexanenitrile)

• Nitrile carbon automatically carbon-1

• Methyl substituent on carbon-4 (from nitrile end)

• Fluoro substituent on carbon-3

• Alphabetical order: fluoro, then methyl

Amines - The Dual System Challenge

Two naming systems exist:

System 1 (Common): -alkylamine

System 2 (IUPAC): -alkan-N-amine

Complex Example: N-ethyl-1,3-dimethylpentan-1-amine

• Longest chain containing NH: 5 carbons (pentanamine)

• Amine on carbon-1 (so "1-amine")

• Methyl substituent on carbon-1 and carbon-3

• Ethyl substituent on nitrogen (so "N-ethyl")

• Order: N-ethyl comes first, then 1,3-dimethyl

Carboxylic Acid Derivatives

Esters - Two-Part Names

Pattern: [alcohol part]-yl [acid part]-oate

• CH₃-COO-CH₂-CH₃ = ethyl ethanoate

Acyl Chlorides

Pattern: [stem]-oyl chloride

Amides

Primary: [stem]-amide

• CH₃-CONH₂ = ethanamide

Secondary/Tertiary: Use N- prefix

• CH₃-CONH-CH₃ = N-methylethanamide

Isomerism - Understanding Molecular Relationships

Structural Isomerism Types

Chain Isomerism

Definition: Different carbon skeleton arrangements

Position Isomerism

Definition: Same functional groups, different positions Simple explanation: It's like having the same furniture in a room, but arranged differently

Easy way to remember: The number in the name tells you WHERE the functional group is

Functional Group Isomerism

Definition: Same molecular formula, different functional groups

Key Examples:

• C₃H₆O: propanal vs propanone.

• C₄H₈O₂: butanoic acid vs methyl propanoate vs ethyl ethanoate.

• C₃H₉N: propylamine vs trimethylamine.

Simple Alkane Naming

Example 1: CH₃-CH₂-CH(CH₃)-CH₂-CH₃

1. Longest chain = 5 carbons (pentane)

2. Methyl substituent on carbon-3 (number from either end gives same result) 3.

Example 2: CH₃-CH(CH₃)-CH(CH₃)-CH₂-CH₃

1. Longest chain = 5 carbons (pentane)

2. Two methyl groups on carbons 2 and 3

Functional Group Priority Examples

Example: CH₃-CH(OH)-CH₂-CHO

1. Functional groups: alcohol (-OH), aldehyde (-CHO)

2. Aldehyde has higher priority → gets suffix (-al)

3. Aldehyde carbon = carbon-1

4. Alcohol on carbon-3 → becomes "3-hydroxy"

Complex Multi-functional Molecules

Challenge Example: NH₂-CH₂-CH₂-COOH

1. Functional groups: amine (NH₂), carboxylic acid (COOH)

2. Carboxylic acid has higher priority

3. Carboxylic acid carbon = carbon-1

4. Amine on carbon-3 → becomes "3-amino"

Common Mistakes and How to Avoid Them

Numbering Errors

Mistake: Not giving the priority functional group the lowest possible number.

Example: Naming CH₃-CH₂-CH(OH)-CH₃ as "butan-3-ol" Correct: "butan-2-ol" (number from the other end).

Wrong Parent Chain

Mistake: Not choosing the longest chain containing the functional group.

Solution: Always find the longest possible chain that includes the priority group.

Ester Naming Confusion

Mistake: Naming the parts backwards Example: CH₃-COO-CH₂-CH₃ incorrectly as "ethanoyl ethyl" Correct: "ethyl ethanoate" (alcohol part first, then acid part)

Extension Topics

Cyclic Compounds

• Prefix: cyclo-

• Examples: cyclohexane, cyclopentanol, cyclohexanone.

• Numbering: Start from functional group, minimize other substituent numbers.

Remember: IUPAC nomenclature is a logical coding system - once you understand the rules, you can decode any molecule's structure from its name, or construct the correct name from any structure.