54 Relative stability
Observing the reaction of the addition of hydrogen to 1-butene, (Z)-2-butene, and (E)-2-
butene, we can see that all of the products are butane. The difference between the reactions
is that each reaction has a different energy: -30.3 kcal/mol for 1-butene, -28.6 kcal/mol for
(Z)-2-butene and -27.6 kcal/mol for (E)-2-butene. This illustrates that there are differences
in the stabilities of the three species of butene isomers, due to the difference in how much
energy can be released by reducing them.
The relative stability of alkenes may be estimated based on the following concepts:
- An internal alkene (the double bondnoton the terminal carbon) is more stable than a
terminal alkene (the double bondison a terminal carbon).
Internal alkenes are more stable than terminal alkenes because they are connected to more
carbons on the chain. Since a terminal alkene is located at the end of the chain, the double
bond is only connected to one carbon, and is called primary (1°). Primary carbons are the
least stable. In the middle of a chain, a double bond could be connected to two carbons.
This is called secondary (2°). The most stable would be quaternary (4°).
- In general, the more and the bulkier the alkyl groups on a sp^2 -hybridized carbon in the
alkene, the more stable that alkene is. - A trans double bond is more stable than a cis double bond.