Alkenes, Alkynes, and Aromatic Compounds: Structure, Properties, and Reactions

Chapter 13: Alkenes, Alkynes, and Aromatic Compounds

13.1 Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons characterized by carbon-carbon multiple bonds. Their chemical properties and reactivity differ from saturated hydrocarbons (alkanes).

• Saturated molecules: Hydrocarbons with only single bonds between carbon atoms (e.g., alkanes).

• Unsaturated molecules: Hydrocarbons containing one or more carbon-carbon double or triple bonds (e.g., alkenes and alkynes).

• Alkenes: Contain at least one C=C double bond. Example: ethene (ethylene).

• Cycloalkenes: Alkenes with a double bond in a ring system.

• Alkynes: Contain at least one C≡C triple bond. Example: ethyne (acetylene).

• Applications: Alkenes and alkynes are used in drugs, explosives, paints, plastics, and pesticides.

13.2 Naming Alkenes and Alkynes

The IUPAC system is used to name alkenes and alkynes, with specific rules to indicate the location and number of multiple bonds.

• Main chain selection: Choose the longest chain containing the multiple bond(s).

• Suffixes: Use -ene for alkenes and -yne for alkynes.

• Index numbers: Number the chain so the multiple bond(s) have the lowest possible numbers.

• Multiple bonds: Use numerical prefixes (di-, tri-, tetra-, etc.) for more than one double or triple bond.

• Substituents: Assign numbers to branching substituents and list them alphabetically.

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

• Examples:

  • 1,3-butadiene: Two double bonds at positions 1 and 3.

  • 2-methyl-1,3-butadiene: Methyl group at position 2.

  • 4-methylcyclohexene: Methyl group at position 4 in a cyclohexene ring.

  • Common names: ethylene (ethene), acetylene (ethyne), isoprene.

13.3 Structure of Alkenes: Cis-Trans Isomerism

Alkenes can exhibit cis-trans (geometric) isomerism due to restricted rotation around the double bond.

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

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

• Isomerism: Only possible when each carbon of the double bond has two different substituents.

• Example: Cis-2-butene vs. trans-2-butene.

• 3D structure: Isomers have the same connectivity but differ in spatial arrangement.

13.4 Properties of Alkenes and Alkynes

Alkenes and alkynes have distinct physical and chemical properties due to their unsaturation.

• Nonpolar: Insoluble in water, soluble in organic solvents.

• Less dense than water.

• Flammable and often toxic.

• Chemically reactive: Multiple bonds are sites for reactions.

• Cis-trans isomerism: Can lead to different physical and biological properties.

13.5 Types of Organic Reactions

Organic compounds undergo several types of reactions, especially at sites of unsaturation.

• Addition reactions: Atoms are added to the carbon-carbon multiple bond, converting it to a single bond.

• Elimination reactions: Atoms are removed from a molecule, often forming a multiple bond.

• Substitution reactions: One atom or group replaces another.

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

13.6 Addition Reactions of Alkenes

Alkenes readily undergo addition reactions, converting the double bond to single bonds.

• Hydrogenation: Addition of H2 to a double bond to form an alkane. $\mathrm{C=C} + \mathrm{H}_2 \rightarrow \mathrm{C-C}$

• Halogenation: Addition of Cl2 or Br2 to form a dihalide. $\mathrm{C=C} + \mathrm{Cl}_2 \rightarrow \mathrm{Cl-C-C-Cl}$

• Hydrohalogenation: Addition of HCl or HBr to form an alkyl halide. $\mathrm{C=C} + \mathrm{HCl} \rightarrow \mathrm{C-C(Cl)}$

• Markovnikov's Rule: When adding HX to an alkene, the hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the carbon with fewer hydrogens.

• Hydration: Addition of H2O to form an alcohol, following Markovnikov's rule. $\mathrm{C=C} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C-C(OH)}$

• Common prefixes: hydroxy (OH), chloro (Cl), nitro (NO2), bromo (Br), phenyl (C6H5).

13.7 Alkene Polymers

Polymers are large molecules formed by the repetitive bonding of monomers, often alkenes.

• Polymer: Large molecule made from many smaller units (monomers).

• Monomer: Small molecule used to build a polymer.

• Example: Polyethylene is formed from ethylene monomers. $n\ \mathrm{CH}_2=CH_2 \rightarrow \mathrm{-(CH_2-CH_2)-}_n$

• Initiator: Substance that starts the polymerization process.

13.8 Aromatic Compounds and Benzene Structure

Aromatic compounds contain benzene-like rings and exhibit unique stability due to resonance.

• Aromatic compounds: Contain benzene rings; do not undergo addition reactions.

• Benzene: C6H6, a planar ring with alternating double bonds.

• Resonance: The true structure is an average of multiple Lewis structures differing only in the placement of double bonds. $\text{Benzene resonance:}$ $\begin{array}{c} \text{Structure 1: alternating double bonds} \\ \text{Structure 2: double bonds shifted} \end{array}$

• No addition reactions: Benzene is resistant to addition due to resonance stabilization.

13.9 Naming Aromatic Compounds

Aromatic compounds are named using benzene as the parent name, with prefixes indicating substituent positions.

• Disubstituted benzenes: Positions are indicated by prefixes:

  • Ortho (o-): 1,2- positions

  • Meta (m-): 1,3- positions

  • Para (p-): 1,4- positions

• Common aromatics: Toluene, phenol, benzoic acid, aniline, benzaldehyde, para-xylene.

• Phenyl group: C6H5–, used as a substituent.

13.10 Reactions of Aromatic Compounds

Aromatic rings undergo substitution reactions, not addition, due to their resonance stability.

• Nitration: Substitution of a NO2 group for a hydrogen. $\mathrm{C}_6\mathrm{H}_6 + \mathrm{HNO}_3 \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{NO}_2 + \mathrm{H}_2\mathrm{O}$

• Halogenation: Substitution of Cl or Br for a hydrogen. $\mathrm{C}_6\mathrm{H}_6 + \mathrm{Cl}_2 \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{Cl} + \mathrm{HCl}$

• Sulfonation: Substitution of SO3H for a hydrogen. $\mathrm{C}_6\mathrm{H}_6 + \mathrm{SO}_3 \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{SO}_3\mathrm{H}$

• Substitution reactions only: Benzene rings do not undergo addition reactions.

Summary Table: Types of Organic Reactions

Reaction Type	Description	Example
Addition	Atoms added to a multiple bond	Hydrogenation of ethene
Elimination	Atoms removed, forming a multiple bond	Dehydration of alcohol
Substitution	One atom/group replaces another	Halogenation of benzene
Rearrangement	Structure reorganized to form isomer	Isomerization of alkenes

Additional info:

• When an alkene undergoes hydration, it forms an alcohol (OH group added).

• Resonance in benzene provides extra stability, explaining its resistance to addition reactions.