Page 31 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.6 THERMODYNAMIC CONTROL: BOND DISSOCIATION ENERGIES (BDEs) 11
Concentrated HBr is therefore a suitable reagent for converting simple alcohols to the
corresponding alkyl bromides (assuming there are no other acid-sensitive groups in the
molecule).
We’d be remiss if we didn’t make some amends for presenting a rather “organic-centered”
view of leaving groups: a variety of other factors are at play when the electrophilic center
is not carbon. Perhaps the most important of these is the fact that single bonds between
electronegative elements are typically weak and are easily cleaved. Unlike in organic
chemistry, a hydroxide anion is a fair leaving group for a substrate of the form ROOH.
−
Similarly, although thiolates (RS ) are hopelessly poor leaving groups in organic chem-
istry, rings of divalent sulfur atoms are readily broken down by nucleophiles, with S −
leaving groups as intermediates (as discussed in Section 6.4).

1.5 REDOX POTENTIALS

Broadly speaking, nucleophilicity correlates with reduction potential. Thus, stronger reduc-
ing agents tend to make better nucleophiles, which makes sense because both properties are
related to electron donation. The correlation, however, is best limited to a set of structurally
closely related nucleophiles. For a broader correlation, Edwards and Ritchie proposed an
“oxibase” scale, which afforded a linear correlation of the reactivity of a nucleophile with
the reduction potential of its oxidized form and the pK of its conjugate acid. Although
a
space doesn’t permit a more detailed discussion of this scale, redox potentials are broadly
important for the subject matter of this book.
Table 1.5 lists reduction potentials of representative inorganic substances of potential
interest in this book. Observe that the half-reactions are all written as reductions, following
current convention, as well as to avoid ambiguity. In this convention, the more positive the
reduction potential, the more the reduction is favored thermodynamically. Strong oxidants
thus exhibit high (i.e., more positive) reduction potentials. A couple of examples should
illustrate the utility of this table.
A table such as Table 1.5 provides an indication of whether an oxidant or reductant is
suitable for a given redox role. Thus, with a high reduction potential (1.19 V in Table 1.5),
ClO is clearly a strong oxidant, which underlies its wide use as a disinfectant.
2
−
Observe that several of the reductions involve an oxidant (e.g., O ,H O ,NO ) in acid
2
2
2
3
solution. This makes sense because protonation is expected to make an oxidant even more
powerful, that is, an even more avid acceptor of electrons.
Not much more needs to be said about redox potentials at this point. We will refer back
to Table 1.5 once in a while when we make arguments based on redox potentials.
1.6 THERMODYNAMIC CONTROL: BOND DISSOCIATION ENERGIES
(BDEs)

Many of the reactions discussed in this book occur under thermodynamic control. In other
words, the activation energies for the various pathways are low enough and the temperature
is high enough so that the products formed are thermodynamically the most stable ones
possible. In contrast, under kinetic control, reaction rates determine the products observed
and certain thermodynamically favored products may not predominate because their forma-
tion is too slow under the reaction conditions (particularly temperature). Reactions under