SolutionsPhysical Properties
1. Melting points of the but-2-enes
Why does trans- or E-but-2-ene melt at a higher temperature than its cis or Z isomer?
| Alkene | Formula | b.p. (°C) | m.p. (°C) |
|---|---|---|---|
| cis-But-2-ene | CH3CH=CHCH3 (Z) | 4 | −139 |
| trans-But-2-ene | CH3CH=CHCH3 (E) | 1 | −105 |
A higher melting point means a stronger intermolecular attraction in the solid, due to better molecular packing. The zigzag structure of the trans or E isomer packs better than the tweezer-shaped cis isomer. Check it with similar everyday objects…
The boiling point, in contrast, depends more on polarity: the cis isomer, with a small dipole moment, boils at a slightly higher temperature than the trans isomer, which has none.
2. Order by boiling point
Order from lowest to highest boiling point: pent-1-ene, hex-1-ene, 2-methylbut-1-ene and hept-1-ene.
Pent-1-ene (30 °C) ≈ 2-methylbut-1-ene (31 °C) < hex-1-ene (63 °C) < hept-1-ene (94 °C).
As in alkanes, the boiling point rises by about 20-30 °C per CH2 group and usually falls with branching, although for the two C5 alkenes the difference is negligible.
3. Solubility
Is hex-1-ene soluble in water? And in hexane? Does it float or sink in water?
It is practically insoluble in water, because it cannot form hydrogen bonds, and it dissolves well in hexane (like dissolves like). Its density (0.67 g/mL) is lower than that of water, so it floats.