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TBR Organic Chemistry (2013) page 45 question #8

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shnarf9892

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I have attached photos of the passage, problem, and answer explanation in question. I was thinking it would be C, the unsubstituted alkene. I was okay with the explanation until it said that a lower heat of hydrogenation is found for choice D. Delta H for D is -26.4, while Delta H for option C is -32.6. These are all negative values, so the lowest heat of hydrogenation is found in option C, right? -32 is less than -26...Have I lost my mind?
 

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UTautomer

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Using general organic chemistry knowledge, Zaitsev's rule tells us that substituted alkenes are more stable than their unsubstituted forms. Your confusion stems from the difference in bond energy and heat of reaction. Table 2 values represent the energy given off as a result of the hydrogenation of each alkene. Delta H is always negative for exothermic reactions, as such the unsubstituted alkene gives off the most energy (32.6kcal/mol) and is the least stable, while the tetra substituted alkene gives off the least energy to a stable state alkane during hydrogenation.


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aldol16

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This passage is stupid because its sole intent is to trick you. At the beginning, they talk about BDEs, which is a measure of how strong a bond is. Now that you're in that mindset, they throw you a loop in "heat of hydrogenation" and start talking about that. You need to realize that BDE and heat of hydrogenation are opposite, but related, concepts. Heat of hydrogenation is the heat released from the breaking of a bond (by hydrogenating it). So if you look at Table 2, it's obvious that the hydrogenation of an unsubstituted alkene results in more heat released than the hydrogenation of a substituted alkene. What does this mean? This means that relative to the unsubstituted alkene, the hydrogenated product is much more stable, as compared to the relative energetics of the substituted alkene and its hydrogenated product. In other words, the substituted alkene is more relatively stable as compared to its product than the unsubstituted alkene compared to its product. More stable = harder to break. A more simplistic way to look at it might simply be that since the unsubstituted alkene releases more energy when the pi bond is broken, that bond must be easier to break.

However, I have one problem with this question in general. It's asking you which of the carbon-carbon bonds would be harder to break without specifying which bonds it's referring to to begin with! In sp3 carbons, we only have C-C sigma bonds. In sp2 carbons, we have C-C sigma bonds in addition to C-C pi bonds. Which bonds are they referring to? If they're referring to completely breaking any and all bonds between the carbons, their answer choice is correct. But if they're referring to breaking only one bond in each case, then the sp3 carbon-carbon bonds would be much harder to break than the sp2 carbon-carbon pi bonds.
 
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