Catalysis : an Integrated Approach to Homogeneous ...

4 -BONDING AND ELEMENTARY STEPS IN CATALYSIS 153
H H
H
\/
I
r:
8: hpdrgen formeti00
R - C ~ R ' + R-$-R' t H~
y2 H2
R"-C-C-R" R'ltt CH3-CHz-Rtn
'H'
t
b: crack&
Fig. 4.70. Carbenium ion formation from carbonium ion.
Carbenium ions have been shown to isomerize readily (Fig. 4.71). As long as
no primary carbenium ions are involved, isomerization of carbenium ions occurs
with low activation energy and at low temperatures. Isoalkane formation occurs
by reaction with another alkane and transfer of a hydride ion (Fig. 4.72).
Because the activation energy of the hydride transfer reaction is lower than that
of carbonium ion formation, after a short initiation period the hydride transfer
reaction will maintain the carbenium ion concentration at a steady-state level.
However, secondary reactions occur that give rise to short catalyst lifetimes.
CH3
Fig. 4.71. Isomerization of carbenium ions via a triangular intermediate.
+ t
H
H~C-C-C-C-CC-CH~
t
H~C-C-CC-CC-CH~
I H2 H2 ' H2 H2 H2
c H3 c H3
Fig. 4.72. The hydride transfer reaction.