Addition reactions(typically anti) in the oxymercuration step argues against an open carbonium ion as inter-
mediate. Instead, it has been proposed, there is formed a cyclic mercurinium ion, analogous
to the bromonium and chloronium ions involved in the addition of halogens. In 1971, Olah
reported spectroscopic evidence for the preparation of stable solutions of such mercurinium
ions. The mercurinium ion is attacked by the nucleophilic solvent water, in the present case
to yield the addition product. This attack is back-side (unless prevented by some structural
feature) and the net result is anti addition, as in the addition of halogens. Attack is thus of
the SN2 type; yet the orientation of addition shows that the nucleophile becomes attached
to the more highly substituted carbon as though there were a free carbonium ion interme-
diate. As we shall see, the transition state in reactions of such unstable threemembered
rings has much SN1 character. Reduction is generally not stereospecific and can, in certain
special cases, be accompanied by rearrangement. Despite the stereospecificity of the first
stage, then, the overall process is not,in general, stereospecific. Rearrangements can occur,
but are not common. The reaction of 3,3-dimethyl-1-butene illustrates the absence of the
rearrangements that are typical of intermediate carbonium ions.
55.3.4 Diels-Alder Reaction
The Diels–Alder reaction is a reaction (specifically, a cycloaddition) between a conjugated
diene and a substituted alkene, commonly termed the dienophile, to form a substituted
cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed
ring are not carbon. Some of the Diels–Alder reactions are reversible; the decomposition
reaction of the cyclic system is then called the retro-Diels–Alder.
Figure 157 Diels-alder for 1,3-butadiene-Ethylene
The Diels–Alder reaction is generally considered one of the more useful reactions in organic
chemistry since it requires very little energy to create a cyclohexene ring, which is useful
in many other organic reactions A concerted, single-step mechanism is almost certainly
involved; both new carbon-carbon bonds are partly formed in the same transition state,
although not necessarily to the same extent. The Diels-Alder reaction is the most important
example of cycloaddition. Since reaction involves a system of 4 π electrons (the diene) and
a system of 2 π it electrons (the dienophile), it is known as a [4 + 2] cycloaddition.
55.3.5 Catalytic addition of hydrogen
Catalytic hydrogenation of alkenes produce the corresponding alkanes. The reaction is car-
ried out under pressure in the presence of a metallic catalyst. Common industrial catalysts
are based on platinum, nickel or palladium, but for laboratory syntheses, Raney nickel^7
(formed from an alloy of nickel and aluminium) is often employed.
The catalytic hydrogenation of ethylene to yield ethane proceeds thusly:
CH 2 =CH 2 + H 2 + catalyst → CH 3 -CH 3