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I am confused with this $\mathrm{^1H}$-$\mathrm{NMR}$ spectroscopy result:

NMR of Ethanol

Shouldn't $\ce{H}$ in $\ce{OH}$ group the singlet be more towards the left of the $\mathrm{NMR}$ spectrum?

It is near the highly electronegative atom $\ce{O}$ rather than the two $\ce{H}$ in $\ce{CH2}$ group they have a weak eletronegative atom $\ce{C}$ so it shouldn't be that desheilded.

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  • $\begingroup$ What about that same $\ce{CH2}$ attached to very electronegative $\ce{O}$ atom? $\endgroup$ Jun 30 '20 at 22:27
  • $\begingroup$ But the two H's has to throw the C atom and they see the O atom but in H in OH group its directly next to it. $\endgroup$
    – Avon97
    Jul 1 '20 at 4:33
  • $\begingroup$ Exactly, it's directly next to the electron-rich O atom. $\endgroup$ Jul 1 '20 at 5:52
  • $\begingroup$ Electronegativity is not the only thing matter here. $\endgroup$ Jul 1 '20 at 6:01
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It is a myth that electronegativity is the only things that effect chemical shifts. However, electronegativity is only one of many things that effect chemical shift of a proton. Other things effecting chemical shift are magnetic anisotropy (e.g., large deshielding effects on aromatics and alkenes), steric compression (e.g., measurable $\Delta \delta$ values of alkene protons on cis- and trans-isomers), anisotropy of double bonds (e.g., measurable positive $\Delta \delta$ values of methyl group of methylcyclohexene and methylcyclohexane, $\delta_{\mathrm{sp^2}(\ce{CH3})}-\delta_{\mathrm{sp^3}(\ce{CH3})} \approx 1.60-0.92= 0.68$), solvent effects, etc.

All of above mentioned factors effect chemical shifts of proton attached to electronegative atom such as $\ce{OH}$ and $\ce{NH}$. In addition, solvent used and solute concentration are also fators changing chemical shifts of these protons. For example, let's consider alcohol $\ce{OH}$ protons. In dilute solution of alcohols in non-hydrogen-bonding solvents such as $\ce{CCl4, CDCl3,}$ and $\ce{C6D6,}$ the $\ce{OH}$ signal generally appears at $\delta \ 1\!- \!2$. For instance, residual peak in $\ce{CDCl3}$ is always appeared at around $\delta \ 1.6$. At higher concentrations in non-polar solvents, the $\ce{OH}$ signal moves downfield ($+\delta$) because when concentration increase, the $\ce{H}$-bonding increases as a result. For instance, the $\ce{OH}$ signal of ethanol comes at $\delta \ 1.0$ in a 0.5% solution in $\ce{CCl4}$, and it appears at $\delta \ 5.13$ in the pure ethanol (5-HMR-2 Chemical Shift):

NMR Spectra of ethanol at various concentrations

The solvent used to take the given spectrum is unknown. Based on the chemical shift of $\ce{OH}$ signal, one can assume solvent might be $\ce{CDCl3}$, which is the most common among NMR solvents, which does not promote $\ce{H}$-bonding. As you can judge by the image above, it might have taken in the concentration of 1-5% range.

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  • $\begingroup$ @Jan: What I meant to say is non H-bonding. I couldn't find the word at midnight before I finished. It's my mistake, but corrected it. $\endgroup$ Jul 1 '20 at 15:36
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    $\begingroup$ Ah, aprotic might be the word you’re looking for? ;) $\endgroup$
    – Jan
    Jul 2 '20 at 15:01
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    $\begingroup$ Won't work since DMSO and acetone would promote H-bonding. $\endgroup$ Jul 2 '20 at 15:40
  • $\begingroup$ What exactly is the effect of H-bonding that leads to deshielding? After all, the H-atom involved in the H-bond becomes a bridging atom and among other things you might expect local charge to either increase or be shifted so as to increase symmetry about the H. But that does not seem to be the case. $\endgroup$
    – Buck Thorn
    Jul 3 '20 at 8:07

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