CHEM 357 (Qatar) Recitation 2 Answers (Fall 2008)

CHEM 357 (Qatar) Recitation 2 Answers (Fall 2008) - Organic...

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Organic Recitation #2 August 24, 2008 Chemistry 357 - Qatar Organic Chemistry for the Life Sciences Recitation Exercises – ANSWER KEY 1. ( 24 Marks ) Using curved-arrow notation, show how electrons in the left structure move to produce the second resonance structure. For each example, provide the resonance hybrid that reflects the overall bonding picture including partial charges where necessary. In each example in, the second resonance structure is a MORE IMPORTANT CONTRIBUTOR to the resonance hybrid structure and thus, the properties of the species. Explain the reason(s) why? a) H C C H O C H H resonance hybrid H C C H O C H H H C C H O C H H ! + ! - Explanation: There are two reasons that explain why the right resonance structure is more important. First and most importantly, the structure on the left contains a carbon without a filled octet. This is very destabilizing. Second, recall that “No molecule or atom in its right mind wants to be charged”…. Dr. P . The first resonance structure (left) has the destabilizing separation of charge . Moving the electrons to generate the second resonance structure results in a molecule without any formal charges. Therefore, the structure on the right will be a stronger contributor to the chemistry of the hybrid. b) resonance hybrid C N N H H H C N N H H H C N N H H H ! + ! + Explanation: Again, the structure on the left contains an atom that does not have a filled octet (i.e. the terminal nitrogen). However, the structure on the right has all atoms meeting their octet requirement. A much less important factor is that in the structure on the right, the nitrogen with the positive charge is closer to the electron-donating methyl group. c) !" resonance hybrid H C C H O H H C C H O H : : H C C H O H : : !" Explanation: The first resonance structure (left) has a carbon atom bearing a negative charge. In the second resonance structure, the negative charge resides on an oxygen atom. Since oxygen is the more electronegative atom, it is more capable of bearing this negative charge than the carbon atom. Therefore, the second resonance structure is a more important structure.
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d) resonance hybrid C H H O H C H H O H ! + : C H H O H : ! + Explanation:
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This note was uploaded on 10/26/2008 for the course CHEM 3570 taught by Professor Ganem, b during the Fall '06 term at Cornell University (Engineering School).

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CHEM 357 (Qatar) Recitation 2 Answers (Fall 2008) - Organic...

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