Page 118 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 14 ELEMENTS
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compounds are stable and nonreducing. On the other hand, many tetravalent lead com-
pounds are strongly oxidizing; lead tetraacetate (LTA), for example, is an important
oxidizing agent in organic chemistry.
• Silyl anions are comparatively stable and may be generated under surprisingly mild
conditions, relative to unstabilized carbanions. This may seem surprising, given that
silicon is more electropositive than carbon. Once again, the reason appears to be the
greater valence s–p energy gap for Si, relative to carbon; thus the Si lone pair of a silyl
anion is thought to have greater s character and less p character than the lone pair of
an analogous carbanion.
• Unlike in organic chemistry, multiple bonds are rare (indeed essentially unknown,
except for a few designer molecules) for silicon as well as for other p-block elements
below the second period. Taking Si as an example, a Si–Si bond is simply too long for
effective sideways overlap of p orbitals that would result in a stable -bond. This also
explains why CO is a gas under ordinary conditions, whereas SiO is a high-melting
2 2
polymeric solid, essentially pure sand!
With these general considerations in mind, we should now be in a position to explore
group 14 chemistry.

REVIEW PROBLEM 4.1

Dissolved carbon dioxide consists largely of hydrogen-bonded CO molecules,
2
existing in equilibrium with a small quantity of bicarbonate and carbonate anions.
Indeed, until recently, true carbonic acid, H CO , was thought not to exist. It does
2 3
exist, however, and has been recently isolated as the pure, unhydrated solid at low
temperature (Loerting, T., et al. Angew. Chem. Int. Ed. 2000, 39, 892–894). Despite
this remarkable proof of kinetic stability, carbonic acid is thermodynamically
unstable with respect to dehydration, and a water-catalyzed pathway involving a
six-membered transition state is thought to provide one of the lower-energy paths
for the process.
H CO + H O → CO + 2H O
2
2
2
3
2
Use arrow pushing to suggest a mechanism for the process.

REVIEW PROBLEM 4.2
If you are a science fiction fan, you may have heard about the prospect of silicon-based
life. What is your opinion on this issue, especially for an aerobic planetary home?

4.1 SILYL PROTECTING GROUPS

Substituted silyl groups (R Si–, where R is alkyl or aryl) are widely used as protecting
3
groups for alcohols. A variety of reagents are available for this purpose, from simple and