A double bond in a six-membered ring is usually more stable in an...

Question

A double bond in a six-membered ring is usually more stable in an endocyclic position than in an exocyclic position. Hydrogenation data on two pairs of compounds follow. One pair suggests that the energy difference between endocyclic and exocyclic double bonds is about 9 kJ/mol. The other pair suggests an energy difference of about 5 kJ/mol. Which number do you trust as being more representative of the actual energy difference? Explain your answer. Endoclyclic
exocyclic
107
116
105
110
heats of hydrogenation (kJ/mol)

exocyclic

107

116

105

110

heats of hydrogenation (kJ/mol)

Accepted Answer

Hey everyone and welcome back in today's video, we are looking at the problem which reads a double bond is usually more stable in an endo cyclic position of a five member ring than in the exhaust cyclic position, comparing the estimated hydrogenation energies of the two pairs given below. The first pair shows an energy difference of about five kg per mole. While the second pair shows an energy difference of about nine kg per mole explain this discrepancy and suggests which numbers should be trusted under this effect. Now, first of all, we are given the fact that and the cyclic AL keens are generally more stable. So we can say that in the first pair, this difference of five kg per mole essentially favors and the cyclic is being more stable. So we can say that the difference that the the inequality sign would actually pointing in an opposite direction because this compound requires a greater hydrogenation energy since endo cyclic compound is more stable. And similarly Here we have an endo cyclic and an exhaust cyclic version of the compound. So we can say that since exocet click compound is less stable, it requires a greater hydrogenation energy. So the difference of nine kg per mole results in this structure requiring a greater amount of energy to undergo hydrogenation. Now let's recall that the stability of Calkins increases with substitution. Simply speaking. Whenever you have an L keen, we have a mono substituted al keen being less stable than a di substituted, then try substituted would be more stable than the di substituted one and tetra substituted would be the most stable AL keen. Whenever we only compare their substitution or the number of alcohol groups surrounding the double bond. Now, if you look at these structures, we also noticed that the difference between the hydrogenation energies of the first two compounds is just five joules per mole. In contrast to the second number, the difference here is nine kg per mole. Our question is why do we have such a discrepancy? And the reason is simply based on substitution. If you look at these two structures, the endo cyclic and the exhaust cyclic, the exhaust cyclic structure essentially has this additional methyl group and disposition and the methyl group, as you know, stabilizes an AL keen because with an increase in substitution, the stability of an al kini increases. So this exhaust cyclic version becomes more stable due to the presence of death metal group which stabilizes it and therefore it lowers the difference in hydrogenation energies. So it's stabilizing that second structure, which is eggs. A cyclic, is stabilizing it. So it lowers the hydrogenation energy and the difference is lowered. However, here, if we look at the second pair, we don't have any substitutions in this position, there are simply two hydrogen atoms. So this is exhaust cyclic which is not stabilized. There are no submissions that would stabilize it and and there are no substitutions that would as a result lower that energy difference. So this number represents the true difference between their hydrogenation energies and as a result, we have explained that that discrepancy with respect to the presence of that method group that was stabilizing the structure and that lowered the energy difference. We also may state that the number that we should trust is the difference of nine kg per most. So this is the number that we wish to use. And as a result, we may conclude that the correct answer to this multiple choice question is a if you have any questions, don't hesitate to leave them down below in the comment section and thank you for watching.

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