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IUPAC Naming
Alkenes, Alkynes, & Alkyl Halides

Master the systematic rules for naming unsaturated hydrocarbons and halogen-substituted alkanes using official IUPAC nomenclature.

Organic Chemistry 1

What Is IUPAC Nomenclature?

IUPAC (International Union of Pure and Applied Chemistry) provides a universal system for naming organic compounds so that every unique structure has exactly one unambiguous name.

Today we focus on three families: alkenes (\( \text{C=C} \) double bond), alkynes (\( \text{C}\equiv\text{C} \) triple bond), and alkyl halides (\( \text{C-X} \) halogen substituent).

Three Functional Group Families

Alkenes
Contain at least one \( \text{C=C} \) double bond. Suffix: -ene. Example: ethene, propene.
Alkynes
Contain at least one \( \text{C}\equiv\text{C} \) triple bond. Suffix: -yne. Example: ethyne, propyne.
Alkyl Halides
An alkane with a halogen substituent (F, Cl, Br, I). Prefix: fluoro-, chloro-, bromo-, iodo-.

Visualizing Double and Triple Bonds

Ethene (Alkene) C C H H H H
\( \text{C=C} \) double bond
 Suffix: -ene Ethyne (Alkyne) C C H H
\( \text{C}\equiv\text{C} \) triple bond
 Suffix: -yne

The Core Rules for Naming

1
Find the longest chain that contains the functional group (double bond, triple bond, or the carbon bearing the halogen).
2
Number the chain so the functional group gets the lowest possible locant.
3
Name the parent: use -ene for alkenes, -yne for alkynes, or -ane for alkyl halides.
4
Add substituents as prefixes (alphabetical order), each with a locant: methyl-, ethyl-, chloro-, bromo-, etc.
5
Assemble the name: locants + prefixes + parent root + locant + suffix.

Naming Alkenes: Suffix -ene

Replace the -ane ending of the parent alkane with -ene. Insert the locant of the first carbon of the double bond right before the suffix.

substituent-parent root-locant-ene

Naming Alkynes: Suffix -yne

The same process applies: replace -ane with -yne. Number the chain so the triple bond gets the lowest locant.

substituent-parent root-locant-yne

Naming Alkyl Halides: Halogen as Prefix

Halogens are treated as substituents, not part of the parent chain. Name the halogen with the corresponding prefix, and give it a locant.

F →
fluoro-
Cl →
chloro-
Br →
bromo-
I →
iodo-

Suffix vs Prefix: The Key Distinction

Alkenes / Alkynes Alkyl Halides
Bond type \( \text{C=C} \) or \( \text{C}\equiv\text{C} \) \( \text{C-X} \) (single bond)
How named Changes the suffix (-ene, -yne) Added as a prefix (chloro-, bromo-)
Numbering priority Double/triple bond gets lowest locant Halogen treated like any substituent
Parent chain selection Must include the multiple bond Longest chain overall
Example pent-2-ene 2-chloropentane

Step-by-Step Naming Checklist

1
Identify the functional group: Is there a \( \text{C=C} \), \( \text{C}\equiv\text{C} \), or halogen?
2
Find the parent chain: Longest continuous chain that includes the functional group.
3
Number the chain: Lowest locant to the functional group. Ties? Lowest set of locants to substituents.
4
Name substituents: Alphabetical order, each with a locant. Use di-, tri- for duplicates (ignore these in alphabetizing).
5
Combine: substituent locant-prefix + parent root + FG locant-suffix. Use hyphens between numbers and words; commas between numbers.

Example 1: Naming a Simple Alkene

C1 C2 C3 C4 C5
\( \text{C=C} \)

Example 2: A Substituted Alkyne

\( \text{CH}_3 \)
C1 C2 C3 C4 C5 C6
\( \text{C}\equiv\text{C} \)

Example 3: Naming an Alkyl Halide

Br Cl C1 C2 C3 C4 C5

Example 4: Combining Functional Groups

Cl C1 C2 C3 C4 C5

Common Mistakes to Avoid

Wrong numbering direction
Numbering so the double/triple bond gets a higher locant. Always give the functional group the lowest possible number.
Halogen in the suffix
Writing "pentachloride" instead of chloropentane. Halogens are always prefixes in IUPAC naming.
Ignoring alphabetical order
Writing "chloro-bromo" instead of bromo-chloro. Substituents must be listed alphabetically (ignoring di-, tri-).

Summary Comparison

Feature Alkene Alkyne Alkyl Halide
Bond \( \text{C=C} \) (double) \( \text{C}\equiv\text{C} \) (triple) \( \text{C-X} \) (single)
Suffix / Prefix -ene -yne halo- prefix
Locant priority Lowest to C=C Lowest to C≡C Lowest set overall
General formula \( C_nH_{2n} \) \( C_nH_{2n-2} \) \( C_nH_{2n+1}X \)
Example but-1-ene hex-2-yne 2-chloropropane

Question 1 of 5

What is the IUPAC name for a 5-carbon chain with a double bond starting at carbon 1?

Question 2 of 5

In IUPAC nomenclature, how are halogens treated when naming an alkyl halide?

Question 3 of 5

What is the correct IUPAC name for a 6-carbon chain with a triple bond at C-2 and a methyl group at C-5?

Question 4 of 5

A student names a compound "3-bromobut-3-ene." What mistake did they make?

Question 5 of 5

Which name correctly lists substituents in alphabetical order?