Page 18 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

PREFACE
xvi
they are still indicated in blue. Thus, for any given atom or group, its color is maintained
the same throughout the mechanism so that its fate can be easily followed throughout
the reaction pathway. Curly arrows have throughout been indicated in red; certain atoms
“deserving” special attention are also indicated in red. In some cases, where a third reac-
tant is involved, it is indicated in green. In general, the color of a newly formed bond is the
same as the color of the lone pair or other electrons from which it may be thought to have
originated for bookkeeping purposes.
In this book, curly arrows typically begin from the nucleophilic electron pair and end
on the electrophilic atom being attacked. In general, to prevent clutter, we have not shown
lone pairs unless they are specifically engaged in a nucleophilic attack.
We have made sparse use of multiple bonds involving higher valent p-block elements.
Thus, we have preferred to use the left-hand structures for POCl and SO Cl , as opposed
3
2
2
to the multiply bonded structures to the right:
−
O O
+
P P
Cl Cl
Cl Cl
Cl Cl
−
O O
2+
− S Cl S Cl
O O
Cl Cl

Despite the unrealistic formal charges, we believe that the structures on the left give a clearer
sense of the bonding, whereas the multiple bonds shown to the right are harder to appreciate.
It is not easy to explain to an undergraduate audience which specific orbitals constitute the
double bonds in the right-hand structures. To instructors who would prefer to stick to the
more conventional multiply bonded structures, we say: by all means do so; for the vast
majority of reactions, arrow pushing will work equally well for both types of structures.
The end of descriptive inorganic chemistry? An interesting question to consider is the
following: Does a mechanistic approach, making extensive use of arrow pushing, signal
of the end of descriptive inorganic chemistry? The answer, in our opinion, is both yes and
no. By emphasizing arrow pushing as a universal tool for rationalizing main-group reactiv-
ity, we have placed the field at exactly the same level as organic chemistry. Just as no one
speaks of “descriptive organic chemistry,” there is no point in treating main-group chem-
istry as a descriptive subject. That, of course, does not diminish the importance of facts and
having an appropriate respect for them. Facts come first, whether it’s organic or inorganic
chemistry, and mechanisms are primarily useful for understanding and rationalizing them.
In that sense, mechanisms can never supplant a descriptive exposition of chemical facts.

Abhik Ghosh and Steffen Berg
The Arctic University of Norway, Tromsø, Norway