Page 59 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.23 INORGANIC ARROW PUSHING: THINKING LIKE A LONE PAIR 39
often tacitly assumed that the S 2-Si mechanism is operative. Although there is no
N
question that it is pervasive, whether it applies near-universally to all heavier p-block
scenarios has yet to be settled.
3. Compared with organic chemistry, charged centers are much more common in
2−
2−
main-group element chemistry. Thus, monatomic anions such as O ,S , and
N 3− are all stable species when stabilized by cations in a solid lattice. Ammonium,
phosphonium, and sulfonium ions are an important part of the chemistry of the
elements in question. Oxoanions are an important part of the chemistry of the great
majority of p-block elements. Thus, rule no. 3 (concerning charge-neutral reactants
and products) applies rather more weakly in inorganic chemistry than in organic
chemistry; there are many exceptions.
−
−
4. We mentioned that strong bases such as OH and RO generally make poor leaving
groups. These, however, are “poor leaving groups” only from an organic perspective,
that is, where the reaction center is carbon. Non-carbon p-block centers vary hugely
in their electronegativity from near-metallic or metalloidal B, Al, Sn, and so on, to
highly electronegative elements such as N, O, and F. Indeed, for N, O, or halogen
−
−
reaction centers, OH and RO are rather good leaving groups. The reason for this is
that bonds between two highly electronegative atoms are weak and are readily broken,
both via S 2 displacements as well as homolytically. Rule no. 4 too thus has limited
N
application in inorganic chemistry.
Organic paradigms accordingly are not very helpful in providing rules of thumb for
arrow pushing in main-group inorganic chemistry, especially for elements below period 2.
Yet we are far from helpless. Simple pattern recognition skills, along with some basic ideas
about nucleophiles, electrophiles, and leaving groups, go a long way in helping us arrive at
reasonable mechanisms for reactions involving main-group elements. Our approach may
be summarized as follows:

1. Look at the product structure(s) carefully and determine what bonds have been broken
in the course of the reaction and what new bonds have been formed.
2. Identify the nucleophile and the electrophilic site of attack.
3. Apply steps (1) and (2) iteratively until you arrive at the product structures (assuming
they are known).

Step (1) consists of pattern recognition, somewhat similar to the logic involved in putting
together a puzzle. Note that in the quote at the beginning of the chapter, Sherlock Holmes
describes this ability to “reason backward” as easy! Based on many years of experience,
we would echo the same assessment. Step (2) is where your chemistry knowledge comes
in handy: Apply your knowledge of nucleophilicity, electrophilicity, leaving groups, bond
strengths (e.g., bonds between two electronegative atoms are easily cleaved), and so on.
Beyond that, we do not advocate an overly algorithmic approach. Several of the reac-
tions discussed in this book are too complex for that. To us, a semi-intuitive approach is
what makes inorganic arrow pushing both challenging and fun. Follow your nose! Or, to
use our favorite metaphor: think like a lone pair! Where would you attack if you were a
lone pair?

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