53 Properties
Alkenes are molecules with carbons bonded to hydrogens which contain at least two sp^2
hybridized carbon atoms. That is, to say, at least one carbon-to-carbon double bond, where
the carbon atoms, in addition to an electron pair shared in asigma(σ) bond, share one
pair of electrons in api(π) bond between them.
The general formula for an aliphatic alkene is: CnH2n--e.g.C 2 H 4 orC 3 H 6
53.1 Diastereomerism
53.1.1 Restricted rotation
Because of the characteristics of pi-bonds, alkenes have very limited rotation around the
double bonds between two atoms. In order for the alkene structure to rotate the pi-bond
would first have to be broken - which would require about 60 or 70 kcal of energy. For
this reason alkenes have different chemical properties based on which side of the bond each
atom is located.
For example, but-2-ene exists as two diastereomers^1 :
(Z)-but-2-ene (E)-But-2-ene
Figure 145cis-but-2-ene Figure 146trans-but-2-ene1 Chapter 36 on page 151