JS
Registered: 10/06/09
Posts: 18
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Reply with quote | #1 |
We have said in lecture that the bond strength of sigma bonds are greater than pi bonds (pg. 29 of lecture supplement). Such is the reason why catalytic hydrogenation reactions are generally exothermic.
In reference to problem # 11, "product J" has two more sigma bonds than "product I". I understand that "product I" is conjugated. But would not the effect of having two extra sigma bonds overcome the extra stability given by conjugation (approx 3 kcal/mol), giving "product J" a lower heat of formation, and therefore a stabler and more likely product of the reaction than "product I"?
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edchang90
Registered: 10/05/09
Posts: 22
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Reply with quote | #2 |
I'm also confused about lecture supplement page 29 when it says "bond strength generalization: sigma > pi." However I don't think that means that sigma bonds are stronger than pi bonds because we know that it requires more energy to break pi bonds than sigma bonds. Therefore I think conjugation is the most important contributor to stability for product I and J. if you look at the second slide on page 29 of the lecture suppole, you see that the dH difference between 1,3 butadiene is more than the 3kcal/mole that you standed (-56.5 vs. -65.1) |
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PurplePpleEator
Registered: 10/04/09
Posts: 41
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Reply with quote | #3 |
It may also be helpful to look at lecture supplement page 38. The first slide says that the greater the stability in a reaction production (due to conjugation and aromaticity) is equal to a greater amount produced. Question 11 asks which product is more likely to be produced, not which bonds are stronger. Since product I has 5 conjugated atoms compared to product J having 0 conjugated atoms (no 3 adjacent, parallel, overallaping p orbitals present; only 2), the answer is product I. Page 34 in the lecture supplement also explains that the more conjugation present the lower the overall energy of the molecule since in conjugation the pi electrons have longer wavelenths (E=hc/wavelenth).
Hope this helps,
Michael
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JS
Registered: 10/06/09
Posts: 18
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Reply with quote | #4 |
edchang90: I believe you are incorrect in saying "because we know that it requires more energy to break pi bonds than sigma bonds." In fact, sigma bonds require more energy to break than pi bonds. (Dont confuse double bonds with pi bonds).
I am still unclear as to whether the two extra sigma bonds in "product J" overcomes the extra stability gained from conjugation in "product I".
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gloomfilter
Registered: 09/27/09
Posts: 25
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Reply with quote | #5 |
Hi, even though this is not answering your question, it provides another reason why product I is produced in greater amounts than J. H2O is acting like a base and then it wants to remove the H+ of COOH --> COO-. Product I is an expected acid-base reaction product, whereas product J is not. |
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