Aamc 3 #123 (bs)

[balambfishie]

Which of the following alkanes has the lowest heat of combustion per CH2 group?

a. cyclopropane
b. cyclobutane
c. cyclohexane
d. cycloheptane

So, just to confirm that my line of thinking is correct since I'm not really sure I understood the aamc explanation: since cyclohexane is the most stable compound there, the energy required to combust the molecule would be the most, so that means that the energy difference between the products and reactants in a combustion reaction would be less? Can someone please clarify the concept for me a little more?

Also, if it's correct, then does the CH2 part of the question matter then? Is there a trend with heats of combustion based on the size of the molecule?

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[Split Infinity]

Hmm, not quite. I remember that question too. Think of it this way: clearly, if you have more carbons, you will be able to achieve more combustion, so they're trying to normalize the comparison by saying "per CH2 group." So, the question comes down to a consideration of which molecule is most stable, because the most stable molecule will release the least energy per methylene group upon combustion. I hope that clears it up (if not, let me know!).

 

[LoLCareerGoals]

Yeah this is a memorization question.
a/b can be eliminated because they have a high ring strain due to their tight angles.
Cyclohexane if I remember correctly is stabilized by its armchair configuration to a point that it is more stable than cycloheptane.

 

[Eastfolding]

Yeah this is a memorization question.
a/b can be eliminated because they have a high ring strain due to their tight angles.
Cyclohexane if I remember correctly is stabilized by its armchair configuration to a point that it is more stable than cycloheptane.

No, it's more stabilized because the interior angles the molecule of cyclohexane is closest to the interior angles of a tetrahedral group.

This means that the bond angles do not need to be strained as much in order to form a cyclohexane molecule.

 

[balambfishie]

Hmm, not quite. I remember that question too. Think of it this way: clearly, if you have more carbons, you will be able to achieve more combustion, so they're trying to normalize the comparison by saying "per CH2 group." So, the question comes down to a consideration of which molecule is most stable, because the most stable molecule will release the least energy per methylene group upon combustion. I hope that clears it up (if not, let me know!).

Ok, so essentially, the higher the heat of combustion is per CH2 group, the less stable the compound is (that we were initially comparing, anyways). Then the question's just trying to ask which compound is the most stable, because that compound will release the least amount of energy. We know that cyclohexane is the most stable due to both it's angles (which are closest to the ideal angle of 109) and its ability to form the chair conformation.

Sorry if I'm just repeating information, but does that sound right?