Alkynes are a group of hydrocarbons that contain at least one carbon–carbon triple bond (C≡C). Their physical properties are influenced by this triple bond as well as by molecular size. Here’s a detailed overview:
1. State and Appearance
- Small alkynes (up to about C4, e.g., ethyne, propyne, butyne) are gases at room temperature.
- Medium alkynes (C5–C16) are typically liquids.
- Higher alkynes (C17 and above) are usually solids.
- They are generally colorless and odorless, though some lower alkynes may have a faint acetylene-like smell.
2. Boiling and Melting Points
- Alkynes have higher boiling points than alkenes but lower than alcohols of similar molecular weight.
- Boiling points increase with molecular weight due to increasing van der Waals (London dispersion) forces.
- Triple bonds make the molecule linear around the triple-bonded carbons, which slightly reduces steric interactions, so boiling points are slightly lower than alkanes of similar molecular weight.
3. Solubility
- Alkynes are nonpolar molecules.
- Insoluble in water, because water is polar.
- Soluble in nonpolar organic solvents like benzene, ether, and chloroform.
4. Density
- Alkynes are less dense than water.
- As molecular weight increases, density also increases slightly.
5. Reactivity and Stability
- Alkynes are more reactive than alkanes and alkenes due to the high electron density in the triple bond.
- Terminal alkynes (R–C≡C–H) have a slightly acidic hydrogen that can react with strong bases.
- They undergo typical addition reactions, such as hydrogenation, halogenation, and hydrohalogenation.
6. Polarity
- Alkynes are generally nonpolar, except for some substituted alkynes with electronegative groups.
- The triple bond is linear, which minimizes dipole effects.