How many equivalent hydrogens are there in octane?

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Discussion Overview

The discussion revolves around the number of equivalent hydrogens in octane, specifically focusing on n-octane and the concept of non-equivalent hydrogens. Participants explore the implications for NMR shifts and the substitution test for determining hydrogen equivalence.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant states that Kaplan's test indicates there are 3 equivalent hydrogens, while they believe there are 4.
  • Another participant asks for clarification on which structural isomer of octane is being discussed and whether the focus is on non-equivalent hydrogens.
  • It is suggested that for n-octane, there are 4 types of non-equivalent hydrogens based on a substitution test, but only 3 types may be relevant for NMR shifts due to nearest-neighbor effects.
  • A later reply challenges the idea that only 3 types matter for NMR, suggesting that while the shifts may be small, distinctions still exist between 3- and 4-halooctanes.
  • Another participant agrees that there are 4 non-equivalent types of hydrogens in n-octane but questions whether current technology can distinguish the shifts between them.
  • Clarification is provided that the discussion is focused on n-octane without heteroatoms, emphasizing the substitution test as a thought experiment for determining non-equivalence.
  • The original poster confirms they were indeed referring to n-octane and non-equivalent hydrogens.

Areas of Agreement / Disagreement

Participants express differing views on the number of non-equivalent hydrogens in n-octane, with some asserting there are 4 while others argue that only 3 are relevant for NMR analysis. The discussion remains unresolved regarding the implications of these distinctions.

Contextual Notes

There is uncertainty regarding the ability of current NMR technology to distinguish between the chemical shifts of hydrogens on different carbons in n-octane, and the discussion does not clarify the assumptions behind the substitution test or the conditions under which the equivalence is assessed.

MechRocket
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The answer to my Kaplan test (studying for the DAT, just FYI) says 3, but I think it's 4?

Am I wrong?
 
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To which structural isomer of octane are you referring? And do you mean non-equivalent hydrogens?

Assuming you mean n-octane, I think you are correct in principle that there are 4 kinds .. in other words, if you use the substitution test, you can create 4 distinct molecules by substituting a single H-atom with an F-atom. However, for the purpose of determining NMR shifts in alkanes (which I assume is what you are asking about), I am fairly sure that only the nearest-neighbor groups matter. Assuming that is true, can you now see why n-octane has only 3 non-equivalent types of hydrogens?
 
SpectraCat said:
To which structural isomer of octane are you referring? And do you mean non-equivalent hydrogens?

Assuming you mean n-octane, I think you are correct in principle that there are 4 kinds .. in other words, if you use the substitution test, you can create 4 distinct molecules by substituting a single H-atom with an F-atom. However, for the purpose of determining NMR shifts in alkanes (which I assume is what you are asking about), I am fairly sure that only the nearest-neighbor groups matter. Assuming that is true, can you now see why n-octane has only 3 non-equivalent types of hydrogens?

I'd be inclined to disagree there, even for NMR, though for e.g. distinguishing between 3- and 4-halooctanes there may not be that much of a difference in shift, in principle this still exists.
 
sjb-2812 said:
I'd be inclined to disagree there, even for NMR, though for e.g. distinguishing between 3- and 4-halooctanes there may not be that much of a difference in shift, in principle this still exists.

I agree .. in practice it's all a question of resolution. The fact is the the OP is right, and strictly speaking there are 4 non-equivalent types of hydrogens in n-octane. However I am not sure if there is a strong enough magnet to distinguish the difference in chemical shifts of H-atoms on the 3rd and 4th carbons.

However given the phrasing of your post, I think it's important to point out that we are not talking about halo-octanes at all, but just normal n-octane (at least I think so, the OP has still not confirmed that is the correct structural isomer). So, there is no heteroatom in the molecule we are considering .. the substitution test I mentioned is just a thought-experiment to determine chemical non-equivalence. I do agree that if there were a heteroatom in the molecule, the dependence of the chemical shifts of H-atoms on the distance from the substitution site would be more pronounced.
 
Yes, I was talking about n-octane and yes, I meant non-equiv hydrogens.

Thanks guys, I guess Kaplan is wrong here.
 

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