Physical Properties of Alkenes
Physical State, Boiling Points, Melting Points, and Dipole Moments.
Alkenes resemble alkanes in many physical properties but show distinct differences due to the presence of the double bond ($\pi$-electrons), which introduces weak polarity.
1. Physical State & Nature
Alkenes are colorless and generally odorless (except Ethene, which has a faint sweet smell).
- $C_2 - C_4$ (Ethene, Propene, Butene): Gases at room temperature.
- $C_5 - C_{17}$: Liquids.
- $C_{18}$ and higher: Waxy Solids.
2. Boiling Points
Trends in Boiling Point
The boiling points of alkenes are very similar to alkanes of comparable molecular mass.
A. Effect of Molecular Mass: Boiling point increases with an increase in the number of carbon atoms (due to increased Van der Waals forces).
B. Effect of Branching: Among isomers, branching decreases the surface area, reducing Van der Waals forces. Thus, straight-chain alkenes have higher boiling points than branched isomers.
C. Cis vs Trans: Cis-alkenes generally have higher boiling points than trans-alkenes because cis-isomers are more polar (higher dipole moment), leading to stronger dipole-dipole interactions.
3. Melting Points
Effect of Symmetry
Melting point depends on how well molecules pack in the crystal lattice.
4. Solubility & Density
Solubility
Alkenes are non-polar (or weakly polar) molecules.
- Insoluble in water (Hydrophobic).
- Soluble in organic solvents (like benzene, petroleum ether, CCl4).
Density
Alkenes are lighter than water. Their density increases with molecular mass but generally remains around 0.6 - 0.7 g/cm³.
5. Polarity & Dipole Moment
The $sp^2$ hybridized carbon of the double bond is more electronegative than the $sp^3$ hybridized carbon of the alkyl group. This creates a weak dipole.
Dipoles add up.
$\mu \approx 0.33 D$ (Polar)
Dipoles cancel out.
$\mu = 0$ (Non-polar)
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