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How many of the following ethanol derivatives of the form $\ce{GCH2CH2OH}$ have a more stable gauche conformer compared to their anti conformer?

$G = \ce{F}, \ce{Cl}, \ce{OH}, \ce{NH2}, \ce{OMe}, \ce{NHMe}, \ce{NMe2}, \ce{NO2}$

I was sure that $\ce{F}$, $\ce{OH}$, $\ce{NH2}$ and $\ce{NO2}$ display the gauche effect. I'm not so sure regarding all the other groups: all of them seem bulky and won't form hydrogen bonds with the $\ce{OH}$ group on $\ce{C}_1$. How many of the other substituents are capable of displaying this effect? Also, is there a quantitative way to determine if the gauche conformer would be more stable than the anti conformer?

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7
  • $\begingroup$ Nice question, I am pretty sure that ethylene chlorohydrin also has gauche form more stable. Also I think for knowing most stable form we need to optimize the molecular geometry on different levels of quantum chemistry, starting from a conformer halfway between staggered and eclipsed. Levels are like HF/def2-TZVP, PBE-D3/def2-TZVP... Don't know direct way to find that. Further, I think G=NHMe would also form more stable gauche form (not sure). What's the answer given? $\endgroup$
    – Zenix
    Commented May 19, 2020 at 12:14
  • $\begingroup$ The answer is 7. I'm ignoring it for now until I have an explanation. $\endgroup$
    – Aniruddha Deb
    Commented May 19, 2020 at 12:16
  • $\begingroup$ Then, I guess, all except NMe2. $\endgroup$
    – Zenix
    Commented May 19, 2020 at 12:47
  • $\begingroup$ @Zenix More probably NO2 - it would have H-bond but with 6-membered ring. BTW even N(CH3)2 isn't all that bulky. $\endgroup$
    – Mithoron
    Commented May 19, 2020 at 23:45
  • $\begingroup$ See here $\endgroup$
    – Mathew Mahindaratne
    Commented May 20, 2020 at 1:38

1 Answer 1

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As per this source, there is evidence for the gauche effect for most of these groups on basis of intramolecular hydrogen bonding:

enter image description here

I tried looking for a similar evidence for 2-nitroethanol, and found this paper on the same:

enter image description here Figure 4. OCCN gauche structures with an intramolecular H-bond for 2-aminoethanol (7) and 2-nitroethanol (9); Conformations 8 and 10 indicate disrupted H-bonds after rotations by approximately 120º about the O-C axes

So it seems like all the groups shown here can show the gauche effect on the grounds of intramolecular hydrogen bonding,and so your answer should be 8

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