Supplement to Chemical Bond Notes

Supplement to Chemical Bond Notes - Supplement to Chemical...

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Supplement to Chemical Bond Notes : Exceeding the Octet, Hyperconjugation and Oxidation Numbers
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10/5/2010 Supplement to Chem Bond #1 2 Oxy-acids , such as HClO 4 can also be usefully described by Lewis structures. These are acids in which the hydrogen atoms are attached to oxygen atoms, themselves attached to large atoms coordinated with oxygen: Effective (Formal) Charge on Central Atom in Lewis Octet Picture HClO 4 HOClO 3 +3 HClO 3 HOClO 2 +2 H 2 SO 3 (HO) 2 SO +1 H 2 SO 4 (HO) 2 SO 2 +2 HNO 3 HONO 2 +1 Cl O O O O H S O O O O H H
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10/5/2010 Supplement to Chem Bond #1 3 One caveat is that the Lewis approach predicts large effective charges for the central atoms . E.g., HClO 4 has 32 valence electrons. This could be satisfied by having three single-bonded oxygen atoms each carrying a single negative charge, leaving a formal charge of +3 on chlorine . This does not make much sense in view of the high electronegativity of chlorine. The alternative within this restricted picture is to put double bonds between oxygen and chlorine, apparently violating the Octet Rule. This is actually closer to the real picture. Because of this experience, it is understood that third-row elements (Si, P, S, Cl) will not always conform to the Octet Rule. Note that the octet rule, which works well for the n=2 elements, was based on the availability of just 2s and 2p orbitals for bonding. However, at n=3, d orbitals can also play a role, so that while the octet rule is still a good guide, many exceptions are known.
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10/5/2010 Supplement to Chem Bond #1 4 A closer look at the Lewis picture for third-row elements First, review the case for sulfate ion, SO 4 2- , as shown above. VSEPR counts 32 valence electrons and gives a tetrahedral structure as shown above. What about the Lewis picture? There is a single octet structure: http://www.stolaf.edu/depts/chemistry/courses/toolkits/123/js/lewis/ It is also possible to reduce the formal charges by using double bonds:
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Supplement to Chem Bond #1 5 We have a conflict between reducing the formal charges and maintaining the Octet structure. Which one is the correct structure? Answer: Probably some average (resonance?) Some conclusions: 1. For second-row elements (C, N, O, F), we never go above 8 electrons per atom, so that the octet rule is considered to be rigorous. 2. For third-row elements (Si, P, S, Cl) and some heavier ones, sometimes, the
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Supplement to Chemical Bond Notes - Supplement to Chemical...

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