Alkynes and Alkenes: Molecular Structure and Reactivity

Molecular Structure

• Alkynes:
  • Contain a carbon-carbon triple bond (C≡C)
  • Bond angle: 180° (linear shape)
  • Examples: ethyne (acetylene), propyne
• Alkenes:
  • Contain a carbon-carbon double bond (C=C)
  • Bond angle: 120° (planar, trigonal shape)
  • Examples: ethene (ethylene), propene

Reactivity

• Alkynes:
  • Highly reactive due to the high energy of the triple bond
  • React with electrophiles (e.g., HCl, Br2) to form addition products
  • Can undergo hydrogenation to form alkenes
  • Can undergo oxidative cleavage to form carboxylic acids
• Alkenes:
  • Less reactive than alkynes, but still reactive
  • React with electrophiles (e.g., HCl, Br2) to form addition products
  • Can undergo hydrogenation to form alkanes
  • Can undergo electrophilic addition reactions (e.g., Markovnikov's rule)

Note: These notes provide a brief overview of the molecular structure and reactivity of alkynes and alkenes. For a more in-depth understanding, additional study and review are recommended.

Molecular Structure

• Alkynes have a carbon-carbon triple bond (C≡C) and a linear shape with a 180° bond angle
• Examples of alkynes include ethyne (acetylene) and propyne
• Alkenes have a carbon-carbon double bond (C=C) and a planar, trigonal shape with a 120° bond angle
• Examples of alkenes include ethene (ethylene) and propene

Reactivity

• Alkynes are highly reactive due to the high energy of the triple bond
• Alkynes react with electrophiles (e.g., HCl, Br2) to form addition products
• Alkynes can undergo hydrogenation to form alkenes
• Alkynes can undergo oxidative cleavage to form carboxylic acids
• Alkenes are less reactive than alkynes, but still reactive
• Alkenes react with electrophiles (e.g., HCl, Br2) to form addition products
• Alkenes can undergo hydrogenation to form alkanes
• Alkenes can undergo electrophilic addition reactions (e.g., Markovnikov's rule)

Factors Affecting Boiling Points

• Chain length increases boiling points due to increased intermolecular forces.
• Branching decreases boiling points due to reduced intermolecular forces.
• Boiling points increase with increasing molecular mass.

Boiling Point Trends

• Boiling points increase as the number of carbon atoms in the alkane chain increases.
• The boiling point increases by approximately 20-30°C for each additional CH2 group.

Boiling Points of Selected Alkanes

• Methane (CH4): -161.5°C
• Ethane (C2H6): -88.5°C
• Propane (C3H8): -42.2°C
• Butane (C4H10): -1°C
• Pentane (C5H12): 36.3°C
• Hexane (C6H14): 68.7°C

Comparison with Other Hydrocarbons

• Alkenes and alkynes have lower boiling points than alkanes due to weaker intermolecular forces.
• Cycloalkanes have higher boiling points than alkanes due to stronger intermolecular forces.