Alkynes and Alkenes: Molecular Structure and Reactivity

Study Notes

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.