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7 C–C Bond Formation via Carbocations in the Methane …
7.2.3 Outline of the Formation of Carbenium Ions
Carbenium ions, which are one of the key reaction intermediates for C–C bond
formation, can be generated via the interaction of hydrocarbons with catalysts as
follows:
(1) Addition of a proton to an alkene:
+
R CH CH 2
R CH 2 CH 3
+ H +
(7.4)
As shown in reaction (7.4), carbenium ions are generated by the addition of
a proton (H
+ ) to an alkene. Because of their relative instability, the formation of
primary carbenium ions hardly occurs.
(2) Reaction of an alkane with a proton:
This reaction can be further classified into two reaction routes. The first
alkane/proton reaction occurs via the formation of both a carbenium ion and hydrogen
by direct hydride abstraction from the alkane (R–H), as shown in reaction (7.5).
+
R H
R
+ H +
+ H 2
(7.5)
(3) Formation of a carbenium ion from a penta-coordinated carbonium ion
In the second alkane/proton reaction route, a proton reacts with a C–H bond to
form a penta-coordinated carbonium ion as shown in reaction (7.6). Subsequently,
the formation of a carbenium ion and H 2 occurs, as shown in reaction (7.7).
+ H +
+
R
H
H
R
H
(7.6)
R + + H 2
R
H
H
+
(7.7)