Page 68 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
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• Oxidative additions and reductive eliminations are much more important in
inorganic chemistry than in organic chemistry. Many higher-valent period-6
compounds are prone to reductive elimination because of the inert pair effect;
several such compounds are employed as oxidants in organic synthesis.
• Ligand exchange or metathesis reactions are common in inorganic chemistry.
They are generally dictated by the HSAB principle. Mechanistically, they
generally involve bridged intermediates.
3. Many A reactions and oxidative additions lead to so-called hypervalent products.
S 2-Si intermediates are also generally hypervalent; that is, they contain main-group
N
centers with more than eight valence electrons around them in a Lewis structure. As
we saw from a case study (PF ), the bonding in such systems can be readily under-
5
stood in terms of simple molecular orbital considerations. No special considerations
are involved in applying arrow pushing to hypervalent compounds.
4. A two-pronged approach is suggested for coming up with reasonable mechanistic
proposals:
• Pattern recognition goes a long way toward identifying a reasonable mechanistic
pathway: simply look at the reactant and product structures and determine what
bonds have been broken and what new bonds have formed.
• Draw on your chemistry knowledge (nucleophilicity, pK values, HSAB, bond
a
energies, etc.) to identify plausible nucleophiles and electrophiles.
Apply the two strategies iteratively and, with a bit of practice, nice, well-crafted mecha-
nisms should emerge.

FURTHER READING
Much of the material presented in this chapter is well covered in the organic and inorganic texts listed
in Appendices 1 and 2. Among the references listed below, the first four are excellent resources for
arrow pushing in organic chemistry.

1. Weeks, D. P. Pushing Electrons: A Guide for Students of Organic Chemistry, 4th ed.; Brooks/Cole:
Belmont, CA, 2013; 224 pp. This is arguably the simplest and gentlest book on the subject. It’s a
good place to start if you feel if you need some extra help.
2. Levy, D. E. Arrow Pushing in Organic Chemistry: An Easy Approach to Understanding Reaction
Mechanisms; John Wiley & Sons, Inc.: Hoboken, NJ, 2008; 320 pp. A companion text not unlike
this one but meant for organic chemistry. The choice of subject matter is more advanced in this
book, since we have assumed previous exposure to organic chemistry.
3. Grossman, R. B. The Art of Writing Reasonable Organic Reaction Mechanisms, 2nd ed.; Springer:
New York, 2010; 360 pp. Another excellent companion to a standard organic text.
4. Scudder, P. H. Electron Flow in Organic Chemistry: A Decision-Based Guide to Organic Mech-
anisms; John Wiley & Sons, Inc.: Hoboken, NJ, 2013; 448 pp. This book presents a highly algo-
rithmic approach to organic arrow pushing. Applying such an approach to the more diverse world
of inorganic mechanisms, however, seems unwieldy from a teaching perspective.