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Unformatted text preview: he more highly substituted the vinylic cation, the greater its
stability. This preference determines the regioselectivity of the addition. As a consequence, the
bromide is located on the more highly substituted carbon (i.e, the internal carbon). It turns out that
the vinyl cation is much less stable (i.e., much more reactive) than the corresponding alkyl
cation. One reason is because sp hybrid orbitals, consisting of 50% s character, are less well-suited
to stabilize positive charge compared to sp2 hybrid orbitals that have only 33% s character (the s
orbital resides closer to the positively charged nucleus).
primary alkyl cation primary vinylic cation RCH
H RCH C–H 3-EPDs
(sp hybridized) Relative Stability of Various Carbocations
The order of stability for various carbocations is provided in the
chart below. It can be seen that the secondary vinyl cation is
not even as stable as the primary alkyl cation. Successive Addition
Since the addition product to an alkyne is a vinyl bromide – a type of alkene – it
should not be surprising that subsequent reaction involving further addition is
possible. In fact, s...
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