Page 16 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

PREFACE
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The approach. Many students are deeply impressed by the logic of organic chemistry.
Mechanistic rationales are available for essentially every reaction in the undergraduate
(and even graduate) organic curriculum and students learn to write reaction mechanisms
right from the beginning of their courses. A survey of current texts shows that a mecha-
nistic approach is universally adopted in introductory organic courses. The situation with
inorganic chemistry could not be more different; not one major introductory text adopts a
mechanistic approach in presenting descriptive main-group chemistry! In a telling exercise,
we went through several textbooks that do an otherwise excellent job of presenting descrip-
tive inorganic chemistry, without finding the words “nucleophile” and “electrophile.” Not
surprisingly, these texts do not present a single instance of arrow pushing either.
Arrow pushing above all provides a logical way of thinking about reactions, including
those as complex as the following:

P + 3NaOH + 3H O → 3NaH PO + PH
4 2 2 2 3
24 SCl + 64 NH → 4S N + S + 48 NH Cl
2 3 4 4 8 4
− − 4−
2 HXeO 4 + 2OH → XeO 6 + Xe + O + 2H O
2
2
These reactions represent important facets of the elements involved but are typically
presented as no more than facts. (Why does boiling white phosphorus in alkali lead to
hypophosphite and not phosphate?—Current texts make no attempt to address such ques-
tions.) Arrow pushing demystifies them and places them on a larger logical scaffolding.
The transformative impact of this approach cannot be overstated. Almost to a person,
students who have gone through our introductory course say that they cannot imagine how
someone today could remain satisfied with a purely descriptive, nonmechanistic exposition
of inorganic main-group chemistry.
A mechanistic approach has done wonders for the overall tenor of our classroom—now
very much a “flipped classroom,” where arrow pushing, instead of videos, have afforded
the “flip.” Well-designed traditional lectures are still important to us and our students, but
they now account for only 50% of total contact hours, with the rest devoted to various types
of active learning. Some students solve mechanism problems on their own, others do so in
groups, and still others solve them on the blackboard in front of the class. Importantly, such
a classroom affords continual feedback from the students so we always have a good idea of
their level of understanding and can assist accordingly.
Potential concerns. Given the plethora of advantages of a mechanistic approach, it’s
worth reflecting why it has never been adopted for introductory inorganic chemistry. A
plausible reason is that, in contrast to common organic functional groups, simple p-block
compounds such as hydrides, oxides, halides, and so forth, tend to be much more reactive
and their vigorous and even violent reactions have been much less thoroughly studied. As
good scientists, inorganic chemists may have felt a certain inhibition about emphasizing an
approach that has little grounding in experimental fact. This is a legitimate objection, but
hardly a dealbreaker, in our opinion, for the following reasons.
Our ideas on main-group element reactivity are not taken out of the blue but are based on
parallels with well-studied processes in organic and organoelement chemistry. Second, it
no longer necessarily takes a prohibitive amount of resources to test a mechanistic proposal,
at least in a preliminary way. Quantum chemical calculations, particularly based on density
functional theory (DFT), very often provide an efficient and economical way of evaluating
reaction mechanisms. Third, and perhaps most important, it’s vastly better to be able to