Physical Properties of Alkynes
Physical State, Boiling Points, Solubility, and Dipole Nature.
Alkynes follow physical trends similar to alkanes and alkenes but possess distinct characteristics due to the high electronegativity of the $sp$ hybridized carbon and linear geometry.
1. Physical State & Smell
Alkynes are colorless. Pure Ethyne is odorless, but commercially prepared ethyne has a characteristic garlic smell due to impurities (Phosphine $PH_3$, $H_2S$).
- $C_2 - C_4$ (Ethyne to Butyne): Gases at room temperature.
- $C_5 - C_{12}$: Liquids.
- Higher Alkynes: Solids.
2. Boiling & Melting Points
Trends
Boiling points and melting points increase with an increase in molecular mass.
A. Comparison with Alkanes/Alkenes: Alkynes generally have higher boiling points than alkanes and alkenes of comparable molecular mass. This is due to the linear structure which allows closer packing and stronger intermolecular forces.
B. Branching: Like alkanes, branching decreases the boiling point due to reduced surface area.
C. Melting Point: Because of their linear symmetrical structure, alkynes pack well in the crystal lattice, leading to relatively high melting points.
3. Solubility & Density
Solubility
Alkynes are weakly polar but are still considered hydrophobic.
- Insoluble in water.
- Soluble in organic solvents (ether, benzene, CCl4).
Density
Alkynes are lighter than water. However, their density is generally higher than corresponding alkanes and alkenes.
4. Polarity & Dipole Moment
The carbon atoms of the triple bond are sp hybridized (50% s-character), making them more electronegative than $sp^2$ or $sp^3$ carbons.
Why are they weakly polar?
In terminal alkynes ($R-C \equiv C-H$), the drift of electrons towards the sp-carbon creates a dipole moment.
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