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In my exam, my teacher asked me if the following is true: "The chair cyclohexane has more axial hydrogens" I did not have time to think, so I just put false. And apparently I was right. But, at home, I thought to myself: Does this question even make sense? Axial hydrogens are by definition only seen on cyclohexanes according to what I know.

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  • $\begingroup$ The two should have the same number of axial hydrogens. $\endgroup$ – Dissenter Sep 1 '14 at 18:50
  • $\begingroup$ Hmm, how is that? $\endgroup$ – yolo123 Sep 1 '14 at 18:50
  • $\begingroup$ Cyclohexane has 12 hydrogens and at any given time half are axial, and half are equatorial. $\endgroup$ – Dissenter Sep 1 '14 at 18:51
  • $\begingroup$ chem.ucla.edu/harding/IGOC/B/boat_conformation01.jpg Can you show me the axial and equatorial here? $\endgroup$ – yolo123 Sep 1 '14 at 18:53
  • $\begingroup$ The boat conformation only has pseudo axial and equatorial positions. What's the complete question? $\endgroup$ – Dissenter Sep 1 '14 at 18:54
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It says fully: TRUE OR FALSE: Boat cyclohexane is a slightly higher energy conformer than chair cyclohexane because: (a).... (a sub question) T OR F (b).... (a sub question) T OR F (c)chair cyclohexane has more axial hydrogens. T OR F

The answer to part (c) of that question is "False." It turns out that the chair and boat conformations of cyclohexane both have 6 axial hydrogens - so the chair does not have more axial hydrogens. Here's a picture of the chair (on the left) and boat (in the middle), you should be able to see the 6 axial hydrogens in both the chair and the boat.

enter image description here

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  • $\begingroup$ Ron, I still can't see all the axial hydrogens on the boat. I only see 4, the ones that point down in this picture. $\endgroup$ – yolo123 Sep 1 '14 at 19:10
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    $\begingroup$ @yolo123 Good start, there are two more axial hydrogens that have special significance and one of them is labeled with an "f" in the middle drawing. The "f" hydrogen is axial, but is often referred to as the "flagpole" hydrogen. It causes steric problems with the other axial, flagpole hydrogen on C-4 (for reference, the original flagpole hydrogen is on C-1). This flagpole-flagpole interaction is one of the factors that destabilizes the boat relative to the chair. $\endgroup$ – ron Sep 1 '14 at 19:15
  • $\begingroup$ So, universally speaking, does the term "axial" hydrogen on boat conformations make sense? I always thought the term was restricted to chair conformations. $\endgroup$ – yolo123 Sep 1 '14 at 19:18
  • $\begingroup$ The term "axial" is widely used to describe the axial hydrogens in the boat. Personally, I think it makes sense because if you look at a model, they look a lot like the axial hydrogens in the chair. $\endgroup$ – ron Sep 1 '14 at 19:20
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    $\begingroup$ Yes. I need to use up 15 spaces. $\endgroup$ – ron Sep 1 '14 at 19:24
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If you want to be very precise (perhaps pedantic), then I would conclude that the question itself is a little dubious without additional clarification. Consider the IUPAC recommendation on Basic Terminology of Stereochemistry. It contains the following definition of axial with respect to ring conformations (emphasis mine):

In the chair form of cyclohexane ring bonds to ring atoms (and molecular entities attached to such bonds) are termed axial or equatorial according to whether the bonds make a relatively large or small angle, respectively, with the plane containing or passing closest to a majority of the ring atoms.

The point here is that the definition is contextually limited to chair conformations. Granted, this is only a recommendation, but I think it's a logical and authoritative one. Seen in that light, the question itself is malformed inasmuch as it admits either answer in the absence of an agreement on exact terms. To wit, "true" might be a valid answer, since by the strictest definition a boat conformation has zero axial hydrogens; on the other hand, if a similar definition (based on bond angles relative to a plane containing a majority of ring atoms) is applied to the boat conformation, one can identify six axial hydrogens roughly analogous to those in the chair conformation and the correct answer would be "false." Really, the question is murky unless you (and more importantly, your professor) are clear on the formal definition being used.

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    $\begingroup$ wrote, "The point here is that the definition is contextually limited to chair conformations." But further down in the IUPAC link, the definition goes on to state, "The terms axial and equatorial have similarly been used in relation to the puckered conformation of cyclobutane, crown conformer of cyclooctane, etc." In my understanding of these statements there is no limitation on the use of "axial" to chair conformations. $\endgroup$ – ron Sep 1 '14 at 21:15
  • $\begingroup$ If we use apical not axial then all these problems can be circumvented. $\endgroup$ – Dissenter Sep 1 '14 at 21:17
  • $\begingroup$ @ron, in my opinion, the use of the term "axial" w/r/t other conformers and molecules is usually valid. This question, however, is itself proof of the potential for ambiguity with that usage. I can only assume there's a reason that the chair conformation was explicitly mentioned at the outset. While I'm definitely not claiming that IUPAC disapproves of related uses for the term "axial" in other conformers and molecules, that sentence you cite looks to me like a usage note, not an endorsement. I would argue that some explicit definition be given if the term is used other than for chairs. $\endgroup$ – Greg E. Sep 1 '14 at 21:46
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In my exam, my teacher asked me if the following is true: "The chair cyclohexane has more axial hydrogens" I did not have time to think, so I just put false. And apparently I was right.

What's the complete question? The chair form has more axial hydrogens than what other form? If the test were referring to the other chair conformation, then you are right.

Cyclohexane has 12 hydrogens, and in its two chair conformations, six substituents are axial while the other are equatorial.

enter image description here

Basically, the question is does boat have more axial H's than cyclohexane?

The boat conformation doesn't have any true axial positions. I'm not sure what your professor considers to be axial or not, but if we take the flagpole hydrogens to be "axial," then both the chair and the boat should still have the same number of axial substituents.

TRUE OR FALSE: Boat cyclohexane is a slightly higher energy conformer than chair cyclohexane because: (a).... (a sub question) T OR F (b).... (a sub question) T OR F (c)chair cyclohexane has more axial hydrogens. T OR F

Regarding the question now in context the instability of the boat conformation likely has more to do with flagpole-eclipsing interactions rather than the number of hydrogens.

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  • $\begingroup$ Dissenter, I may not have been clear. The teacher asked to compare boat and chair conformation. Basically, the question is does boat have more axial H's than cyclohexane? $\endgroup$ – yolo123 Sep 1 '14 at 18:54
  • $\begingroup$ Do you refer to flagpole H's as the ones attached to C4 and C1? In that case, there is only 4H's that are "AXIAL" according to the "hypothetical definition" of my teacher. Am I lost? $\endgroup$ – yolo123 Sep 1 '14 at 19:00
  • $\begingroup$ @Dissenter, your chair conformation next to the boat one, is incorrect. $\endgroup$ – RBW Sep 1 '14 at 19:02
  • $\begingroup$ C4 and C1 have flagpole hydrogens; the Hs on the other carbons may be considered to be axial. $\endgroup$ – Dissenter Sep 1 '14 at 19:03
  • $\begingroup$ Dissenter, so sorry for not having been clear from the beginning. I'll edit my question and I'll scan my exam. Please comment again for any thing you may want to add. $\endgroup$ – yolo123 Sep 1 '14 at 19:06

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