Alkenes and Alkynes: Structure, Nomenclature, Properties, and Reactions

Families of Organic Molecules: Hydrocarbons

Classification of Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are divided into several families based on the types of bonds between carbon atoms:

• Alkanes: Only single bonds (saturated hydrocarbons)

• Alkenes: At least one carbon–carbon double bond

• Alkynes: At least one carbon–carbon triple bond

• Aromatic Compounds: Six-membered rings with alternating double bonds

Alkanes are saturated, while alkenes and alkynes are unsaturated due to the presence of multiple bonds.

Nomenclature of Alkenes and Alkynes

General Principles of Naming

The IUPAC system for naming alkenes and alkynes follows a logical structure:

• Prefix: Indicates the location of substituents

• Parent: Specifies the number of carbon atoms in the longest chain containing the multiple bond

• Suffix: Identifies the family (-ene for alkenes, -yne for alkynes)

Step 1: Identify the Parent Chain

Find the longest continuous chain that includes the double or triple bond. The parent name is based on this chain, and the suffix is added accordingly.

• If there are multiple double or triple bonds, use prefixes such as di-, tri-, etc.

Step 2: Number the Carbon Chain

Number the chain so that the multiple bond gets the lowest possible index. If the multiple bond is equidistant from both ends, start numbering from the end nearest the first branch point.

Step 3: Naming Cycloalkenes

For cycloalkenes, assign the double-bonded carbons as positions 1 and 2, and number the ring to give substituents the lowest possible numbers.

Step 4: Write the Full Name

• Assign numbers to substituents and list them alphabetically.

• Indicate the position of the multiple bond by the number of the first carbon involved.

• Use commas to separate numbers and hyphens to separate words from numbers.

Structure and Bonding in Alkenes and Alkynes

Hybridization and Geometry

The type of bonding in hydrocarbons affects their geometry and bond angles:

• sp3 hybridization: Four single bonds, tetrahedral geometry, bond angle of 109.5°

• sp2 hybridization: Double bond, trigonal planar geometry, bond angle of 120°

• sp hybridization: Triple bond, linear geometry, bond angle of 180°

Cis–Trans (Geometric) Isomerism in Alkenes

Definition and Requirements

Alkenes can exhibit cis–trans isomerism (also called geometric isomerism) due to restricted rotation around the double bond. This occurs when each carbon of the double bond has two different substituents.

• Cis isomer: Substituents are on the same side of the double bond

• Trans isomer: Substituents are on opposite sides of the double bond

Physical Properties of Alkenes and Alkynes

General Properties

Alkenes and alkynes share many physical properties with alkanes, but their multiple bonds impart some differences:

• Nonpolar molecules; physical properties dominated by London dispersion forces

• Low boiling and melting points, which increase with molecular size

• Alkenes and alkynes with 2–4 carbons are gases at room temperature

• Insoluble in water, soluble in nonpolar solvents

• Less dense than water

• Flammable and generally nontoxic

Chemical Reactions of Alkenes and Alkynes

Types of Organic Reactions

Organic reactions are classified into four main types:

• Addition reactions: Atoms are added to the multiple bond, converting it to single bonds

• Elimination reactions: Atoms are removed, forming a multiple bond

• Substitution reactions: One atom or group is replaced by another

• Rearrangement reactions: The structure of the molecule is reorganized to form an isomer

Addition Reactions

Most reactions of alkenes and alkynes are addition reactions. A typical example is the addition of hydrogen (hydrogenation):

• Alkenes/alkynes + H2 (with metal catalyst) → Alkane

General equation:

$\mathrm{RCH=CHR' + H_2 \xrightarrow{Pd} RCH_2CH_2R'}$

Halogenation

Alkenes and alkynes react with halogens (Cl2, Br2) to form dihaloalkanes:

$\mathrm{RCH=CHR' + X_2 \rightarrow RCHX-CHXR'}$

Hydrohalogenation and Markovnikov's Rule

When alkenes react with hydrogen halides (HCl, HBr), the addition follows Markovnikov's rule:

• The hydrogen atom attaches to the carbon with more hydrogen atoms already attached.

• The halide attaches to the carbon with fewer hydrogen atoms.

Example equation:

$\mathrm{CH_2=CHCH_3 + HBr \rightarrow CH_3CHBrCH_3}$ (major product)

Summary Table: Comparison of Hydrocarbon Families

Family	General Formula	Bond Type	Geometry	Hybridization
Alkane	CnH2n+2	Single	Tetrahedral (109.5°)	sp3
Alkene	CnH2n	Double	Trigonal planar (120°)	sp2
Alkyne	CnH2n-2	Triple	Linear (180°)	sp

Additional info: The notes also briefly mention functional groups containing oxygen and nitrogen, which are covered in later chapters. The focus here is on hydrocarbons and their reactions.