Alkenes & Alkynes: Structure, Nomenclature, and Isomerism

Alkenes & Alkynes

Introduction

Alkenes and alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon multiple bonds. Their structure, nomenclature, and isomerism are foundational topics in organic chemistry, relevant to both general and biological chemistry courses.

Definitions and Functional Groups

Key Terms

• Alkene: A hydrocarbon containing at least one carbon-carbon double bond (C=C).

• Alkyne: A hydrocarbon containing at least one carbon-carbon triple bond (C≡C).

• Saturated Hydrocarbon: A molecule whose carbon atoms are bonded to the maximum number of hydrogen atoms (alkanes).

• Unsaturated Hydrocarbon: A molecule that contains carbon-carbon multiple bonds, allowing for the addition of more hydrogen atoms.

Examples

• Alkane: CH3CH2CH3 (propane)

• Alkene: CH2CH=CH2 (propene)

• Alkyne: CH3C≡CH (propyne)

Nomenclature of Alkenes and Alkynes

Steps for Naming

1. Identify the Main Chain: Find the longest continuous carbon chain containing the double or triple bond.

2. Name the Parent Chain: Use the appropriate suffix: -ene for alkenes, -yne for alkynes. Example: CH3CH2CH2CH=CH2 is named as pentene (five-carbon chain with a double bond).

3. Number the Carbon Atoms: Begin numbering from the end nearest the multiple bond. If the multiple bond is equidistant from both ends, start from the end nearer the first branch point.

4. Assign Numbers to Substituents: Each substituent (branch) is given the lowest possible number.

5. List Substituents Alphabetically: When writing the full name, list substituents in alphabetical order, each with its position number.

Examples

• CH3CH2CH=CH2: 1-pentene

• CH3CH2C≡CCH3: 2-hexyne

• CH3CH2CH2CH=CHCH3: 2-hexene

Special Cases: Cyclic Compounds

• For cyclic alkenes, number the ring so that the double bond gets positions 1 and 2, and substituents get the lowest possible numbers.

• Example: Cyclohexene

Isomerism in Alkenes and Alkynes

Types of Isomers

• Constitutional Isomers: Compounds with the same molecular formula but different connectivity of atoms.

• Cis-Trans (Geometric) Isomers: Alkenes with the same connectivity but different spatial arrangement of groups around the double bond.

Cis-Trans Isomerism

• Cis Isomer: Similar groups are on the same side of the double bond.

• Trans Isomer: Similar groups are on opposite sides of the double bond.

Example

• Cis-2-butene: CH3CH=CHCH3 (CH3 groups on the same side)

• Trans-2-butene: CH3CH=CHCH3 (CH3 groups on opposite sides)

Molecular Geometry of Alkenes and Alkynes

Bond Angles and Shapes

• Methane (CH4): Tetrahedral geometry, bond angles of 109.5°.

• Ethylene (C2H4): Planar geometry, bond angles of 120° due to sp2 hybridization.

• Acetylene (C2H2): Linear geometry, bond angles of 180° due to sp hybridization.

Summary Table: Key Features of Alkanes, Alkenes, and Alkynes

Type	Bond Type	General Formula	Geometry
Alkane	Single (C–C)	CnH2n+2	Tetrahedral (109.5°)
Alkene	Double (C=C)	CnH2n	Trigonal planar (120°)
Alkyne	Triple (C≡C)	CnH2n-2	Linear (180°)

Important Equations

• General formula for alkanes: $C_nH_{2n+2}$

• General formula for alkenes: $C_nH_{2n}$

• General formula for alkynes: $C_nH_{2n-2}$

Additional info:

• Cis-trans isomerization can occur with light (photochemical reactions), converting cis alkenes to trans alkenes.

• Aromatic rings (e.g., benzene) are a distinct class of unsaturated hydrocarbons with unique stability and properties.