1) The group of higher priority on one carbon atom is compared with the group of
higher priority on the other carbon atom:
i) (Z)-alkene: If the two groups of higher priority are on the same side of the
double bond (German: zusammen, meaning together).
ii) (E)-alkene: If the two groups of higher priority are on opposite side of the
double bond (German: entgegen, meaning opposite).
CH 3 > H
H 3 C CH 3
CH 3
H 3 C
(Z)-2-Butene
(cis-2-butene)
(E)-2-Butene
(trans-2-butene)
CC
H H
CC
H
H
Cl > H
Br >
Br
Br
Cl
(E)-1-Bromo-1,2-dichloroethene (Z)-1-Bromo-1,2-dichloroethene
CC
Cl
H
Cl
CC
H Cl
Cl
7.3 RELATIVE STABILITIES OF ALKENES
7.3A HEATS OF HYDROGENATION
- The reaction of an alkene with hydrogen is an exothermic reaction; the enthalpy
change involved is called the heat of hydrogenation.
- Most alkenes have heat of hydrogenation near –120 kJ mol–1.
HH
H H
CC+ Pt CC
∆H° ≈ – 120 kJ mol–1
2) Individual alkenes have heats of hydrogenation may differ from this value by
more than 8 kJ mol–1.
3) The differences permit the measurement of the relative stabilities of alkene
