HW17Key - conjugate base has more contributing resonance...

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Graded Homework Problem #17 – Answer Key Deadline : 3:00 p.m., Monday, 4/14/08 LATE WORK WILL NOT BE ACCEPTED OR GRADED!!! This problem is worth a total of 20 raw points. 1,3-Cyclopentadiene has a pK a of 16. 1,3,5- Cycloheptatriene has a pK a of 39. The most acidic proton in each of these compounds is circled in the structures given below: H H 1,3-Cyclopentadiene pKa = 16 H H 1,3,5-Cycloheptatriene pKa = 39 H H H H H H H H H H (a) (5 points) Draw all important contributing resonance structures for the conjugate base of 1,3- cyclopentadiene. You should be able to draw 5 resonance structures for this anion. (b) (7 points) Draw all important contributing resonance structures for the conjugate base of 1,3,5- cycloheptatriene. You should be able to draw 7 resonance structures for this anion. (c) (8 points) Why is 1,3,5-cycloheptatriene a weaker acid than 1,3-cyclopentadiene, even though its
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Unformatted text preview: conjugate base has more contributing resonance structures? Use the back of this page if you need more space for your answer. (Hint : Think about the concept of aromaticity) The conjugate base of 1,3-cyclopentadiene is considerably more stable, because it is aromatic . All of the Huckel criteria for aromaticity are met for this anion (cyclic, flat, every atom in the ring has a p orbital, total of 6 electrons). The conjugate base of 1,3,5-cycloheptatriene, on the other hand, is not nearly as stable, even though there are more resonance structures, because if this anion were flat, it would be antiaromatic (total of 8 electrons). H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H...
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This note was uploaded on 04/30/2008 for the course PHY 317L taught by Professor Ritchie/lang during the Fall '08 term at University of Texas at Austin.

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