Page 555 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

536 intermediate is rate determining and nucleophilic capture is fast. The positive bromine
in a bromonium ion intermediate is a softer electrophile and also has unshared electron
CHAPTER 5 pairs that permit a total of four electrons to participate in the bonding. The bromonium
Polar Addition ion can be represented as having two covalent bonds to bromine and is electrophilic
and Elimination
Reactions but not electron deficient. This results in a more strongly bridged and more stable
species than is possible for the proton.
Where do mercuration reactions fit into this picture? A mercurinium ion has
both similarities and differences, as compared with the intermediates that have
been described for other electrophilic additions. The electrophile in oxymercuration
+
2+
reactions, HgX or Hg , is a soft Lewis acid and polarizes the -electrons of an
alkene to the extent that a three-center two-electron bond is formed between mercury
and the two carbons of the double bond. However, there is also back bonding from
2+
Hg d orbitals to the alkene orbital. There is weaker bridging in the mercurinium
∗
ion than in the three-center four-electron bonding of the bromonium ion.
Hg

Cremer and Kraka have provided another perspective on the nature of some the
cyclic structures represented in Figure 5.9 by focusing on the bond paths for the
three-membered ring. 239 (See p. 63 to review the discussion of bond paths.) Neutral
cyclopropane, aziridine, and oxirane rings are well described by the bent bond model,
but the C−O bonds are somewhat less bent than those in cyclopropane. On the
other hand, in protonated oxirane, the bonds begin to bend inward. For the bridged-
fluorine species, there is no longer a ring; instead the structure is that of a complex.
(Remember, however, from p. 495 that a bridged fluoronium ion is unstable relative
to the corresponding carbocation.) The differences in the nature of the bonds result
from the relative ability of the bridging atom to donate electron density to the ring-
+
forming orbitals, which is in the order CH > NH > O > O H > F . These ideas are
+
2
summarized in Figures 5.9.

5.9. Additions to Alkynes and Allenes

Reactions of alkynes with electrophiles are generally similar to those of alkenes.
Since the HOMO of alkynes is also a type orbital, it is not surprising that there is a
good deal of similarity to the reactivity of alkenes. 240 The fundamental questions about
additions to alkynes include the following: How reactive are alkynes in comparison
with alkenes? What is the stereochemistry of additions to alkynes? What is the regio-
chemistry of additions to alkynes? The important role of bridged ions in addition
reactions of alkenes raises the question of whether similar species are involved with
alkynes, where the ring includes a double bond and bridged intermediates and would
be expected to be substantially more strained.

239 D. Cremer and E. Kraka, J. Am. Chem. Soc., 107, 3800 (1985).
240
G. H. Schmid, The Chemistry of the Carbon-Carbon Triple Bond, Part. 1, S. Patai, ed., Wiley, New York,
1978, Chap. 3.