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I am new to organic chemistry and was asked to rank the following materials according to energetic stability. I think I have understood that the way to do this is to count the amount of α-hydrogens: the more α-hydrogens, the more stable the molecule. However, the definition of an α-hydrogen is "a hydrogen that is attached to an α-carbon", where the α-carbon is the first carbon next to a functional group. In this case, it is not clear to me where the α-carbon is located: the only candidate for a functional group is the CH2/CH3 (depending on the case) at the very end of the chain, but then would the α-carbon be the very first one or the one next to it?

Any further general insight into the ranking of molecules according to energetic stability would also be very much appreciated… Many thanks!

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  • $\begingroup$ sigh your question is (most probably) about enthalpy of hydrogenation, which in this case approximately depends on number of groups neighbouring the double bond $\endgroup$ – Mithoron Aug 6 at 19:10
  • $\begingroup$ @Mithoron So what yardstick would you use to rank the energetic stability of three molecules like these? We never studied enthalpy of hydrogenation, so I guess it must be something else... $\endgroup$ – Pregunto Aug 6 at 19:38
  • $\begingroup$ @mithoron Btw, it says explicitly in my textbook that α-hydrogens stabilize molecules. I am aware I might be looking at the wrong thing, but then please help me find a more correct parameter... $\endgroup$ – Pregunto Aug 6 at 19:53
  • $\begingroup$ Wow, f*ck alpha-hydrogens. This questions contains five-rings with 0, 1 and 2 sp2 carbon atoms. If your teacher tells you this can be answered by only looking at alpha-hydrogens, that might be correct, but how do you know that ring sterics are similar for all three? How do you know in which alpha-hydrogens actually stabilise the molecule, given that two of them have a rather fixed geometry? $\endgroup$ – Karl Aug 6 at 20:43
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    $\begingroup$ I guess this stupid book of yours was talking about hyperconjugation. One can rationalize that things like change of enthalpy of hydrogenation (which are actual measurables not some "Energetic stability") are influenced by hyperconjugation depending on amount of α-hydrogens. $\endgroup$ – Mithoron Aug 6 at 20:58
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Apparently the intent of the question is to assess the stability of the three alkenes. Evaluation of the heats of formations provides useful information. The NIST site provides heats of formation and hydrogenation all of which are shown in black in the diagram. Computed values appear in red. Because hydrogenation of A, B and C all provide ethylcyclopentane, the greater the heat of hydrogenation the less stable the alkene. Alternatively, the less negative the heat of formation, the less stable. Thus, the order of stability is A < C < B.

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You said your problem is finding the $\alpha$-hydrogen. Thus, first find the $\alpha$-carbon (so, hydrogen on it is $\alpha$-hydrogen). To do so you need to know what is the functional group. Here, the functional group of all three examples is the double bond. Following diagram shows you how to find $\alpha$-carbon when the functional group is a double bond or a carbonyl group:

AlphaCarbons

Consequently, structure A has only one $\alpha$-hydrogen (on one $\alpha$-carbon), B has seven $\alpha$-hydrogens (on three $\alpha$-carbons), and C has four $\alpha$-hydrogens (on two $\alpha$-carbons).

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  • $\begingroup$ How do I know the hydrogens of the methyl group in B count as as three, not 1? Or perhaps, say, 1.4? $\endgroup$ – Karl Aug 6 at 20:49

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