Alkenes: Physical Properties
Alkenes show similar properties as alkanes. The density is less than that of water (0.6-0.8 ). They are immiscible with water but easily dissolve in alcohols and ether. At room temperature, ethene, propene and butene are gaseous while higher, unbranched alkenes are liquids. Starting with 16 C atoms, alkenes are solids.
Free rotation about the π bond is restricted. At a torsion angle of 90° the two p orbitals do not overlap anymore, i.e. the π bond is completely broken. Experimentally, it is known that 722 are required to completely brake the double bond in ethene and that the energy required for cis-trans isomerization in ethene, more exactly dideuteroethene, is 277 , i.e. the following values can be assigned to bond energies:
|Bond||Energy [ ]|
These data indicate that breaking a σ bond obviously is more difficult than opening a π bond; accordingly, π bonds are more reactive.
In general, cis-trans isomerizations are observed only at temperatures above several hundred degrees. On the other hand, rotations around single bonds already take place at 40 . A comparison of the torsion energies required for the isomerization of 2-butene and n-butane is shown below.
However, cis-trans isomerization can be more easily achieved photochemically. For example, the isomerization of maleic acid ( cis-butenedioic acid) to fumaric acid (trans-butendioic acid) readily proceeds under light.
Stereoisomers that are stable at room temperature are called configurational isomers (for example, cis-trans isomers), while those that interconvert at these temperatures are conformational isomers (for example, anti-gauche isomers).