Unsaturated Hydrocarbons and Alkenes: Structure, Nomenclature, and Properties

Unsaturated Hydrocarbons

Definition and Characteristics

Unsaturated hydrocarbons are organic compounds that contain one or more double or triple bonds between carbon atoms. These multiple bonds significantly influence the physical and chemical properties of the molecules, including their reactivity.

• Multiple bonds can be formed between carbon atoms or between carbon and other elements such as nitrogen (N) or oxygen (O).

• Hydrocarbons with double or triple bonds are classified as unsaturated hydrocarbons.

• Examples include alkenes (with double bonds) and alkynes (with triple bonds).

Example: Ethylene (C2H4) and acetylene (C2H2) are common unsaturated hydrocarbons.

Alkenes

General Properties

Alkenes are a class of unsaturated hydrocarbons that contain at least one carbon–carbon double bond. The presence of this double bond makes alkenes more reactive than alkanes.

• Alkenes are unsaturated because they have fewer than the maximum number of hydrogen atoms possible.

• The carbon atoms involved in the double bond are sp2 hybridized.

• Bond angles around the double bond are approximately 120° (as predicted by VSEPR theory).

Example: Ethene (CH2=CH2) is the simplest alkene.

Structure of Alkenes

Bonding and Rotation

The double bond in alkenes consists of one sigma (σ) bond and one pi (π) bond. The π bond is formed by the side-to-side overlap of p orbitals, which restricts rotation around the double bond.

• Unlike alkanes, alkenes cannot rotate freely about the double bond without breaking the π bond.

• This rigidity leads to the formation of geometric (cis-trans) isomers.

Example: 90° rotation around the double bond is not possible without breaking the π bond.

Geometric Isomerism in Alkenes

Cis-Trans (E-Z) Isomerism

Alkenes can exhibit geometric isomerism due to restricted rotation around the double bond. The spatial arrangement of substituents leads to different isomers:

• Cis (Z) isomer: Identical or higher priority groups are on the same side of the double bond.

• Trans (E) isomer: Identical or higher priority groups are on opposite sides of the double bond.

Example: Z-2-pentene (cis) and E-2-pentene (trans) are geometric isomers of pentene.

Naming Alkenes

IUPAC Rules for Alkene Nomenclature

Systematic naming of alkenes follows specific rules to ensure clarity and consistency.

• Find the longest unbranched carbon chain containing the double bond.

• Name the chain according to the number of carbon atoms and add -ene as a suffix.

• Number the carbon atoms in the main chain, starting from the end closest to the double bond.

• The location of the double bond is indicated by the lowest-numbered carbon in the double bond.

Example: For a seven-carbon chain with a double bond starting at carbon 2, the name is 2-heptene or hept-2-ene.

Naming Alkenes with Substituents

When alkenes have substituent groups, additional rules apply:

• Identify and name substituent groups (e.g., methyl, cyclopropyl).

• Specify the position of each group on the main chain.

• Add substituent names alphabetically before the parent alkene name.

Example: An alkene with a methyl group at carbon 5 and a cyclopropyl group at carbon 4 is named 4-cyclopropyl-5-methyl-2-heptene.

Physical Properties of Alkenes

Influence of Structure on Properties

The structure of alkenes, including the position of the double bond and the presence of isomers, affects their physical properties such as boiling point.

• Isomers of C4H8 (butene) have different boiling points:

Compound	Boiling Point (°C)
1-Propene	-47
1-Butene	-6
Cis-2-Butene	4
Trans-2-Butene	1

• Alkenes can have more than one double bond, such as 1,3-butadiene.

Example: The boiling point of cis-2-butene is higher than that of trans-2-butene due to differences in molecular interactions.

Additional info: The E/Z nomenclature is based on the Cahn-Ingold-Prelog priority rules, which assign priorities to substituents based on atomic number.