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I understand that hydrogens can be deshielded by electron-withdrawing groups, and the more powerful the group, the greater the chemical shift.

However, I ran into a problem when qualitatively compare the chemical shifts of the hydrogens in the methyl group of methanol and ethanoic acid: the carboxylic acid group is stronger than the alcohol group, the oxygens of the carboxylic acid group isn't directly attached to the methyl carbon, unlike the alcohol oxygen. How would I qualitatively compare the chemical shifts of the methyl groups of methanol and ethanoic acid?

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  • $\begingroup$ That carbon is part of the carboxylic group ($\ce{-C(O)OH}$) $\endgroup$ – Buck Thorn Mar 12 at 8:59
  • $\begingroup$ @NightWriter thanks, I've edited to better reflect what I mean $\endgroup$ – George Tian Mar 12 at 9:02
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It's a good idea to start with some data. The table below shows relevant $\ce{^1H}$ shifts in $\pu{ppm}$ for three compounds in various deuterated solvents (shift values are from Cambridge Isotope Labs): methanol (MetOH), formic acid (FormAc) and ethanoic or acetic acid (AcAc).

enter image description here

$$ \begin{array}{|c|c|c|c|c|c|} \hline \text{Compound} & \ce{CDCl3} & \ce{(CD3)2SO} & \ce{C5D5N}\;\text{or}\;\ce{C5H5N} & \ce{C6D6}\;\text{or}\;\ce{C6H6} & \ce{D2O} \\ \hline \text{Acetic acid} & 2.13 & 1.95 & 2.13 & 1.63 & 2.16\\ \hline \text{Formic acid} & 8.02 & 8.18 & 8.54 & 7.24 & 8.22\\ \hline \text{Methanol} & 3.48 & 3.20 & 3.57 & 3.09 & 3.35\\ \hline \end{array} $$

The protons in methanol are more deshielded than those in ethanoic acid because in methanol an electron withdrawing oxygen atom is directly bonded to the carbon, while in ethanoic acid there is an intervening carbon.

If you remove that intervening carbon things get interesting, as the case of formic acid illustrates. The carbon atom in a carboxylic group is $\mathrm{sp^2}$ hybridized, which makes it more electrophilic than the $\mathrm{sp^3}$ hybridized carbon in methanol. Of course the carbon in a carboxylic group is also bound to two electron withdrawing oxygen atoms, rather than one, which results in greater deshielding. The formic acid proton is also subject to deshielding magnetic anisotropy associated with the delocalized electrons of the carboxylic group. The anisotropy effect is less pronounced in ethanoic acid because the methyl group is further away from the carboxylic group. The ultimate result is that the formic acid proton is very strongly deshielded compared to protons in the other compounds.

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In NMR spectroscopy, the chemical shift value of active nucleus is telling us the strength of the magnetic field that particular nucleus feels. Nuclei in a molecule are not separated from charged particles such as electrons; therefore, the electrons will always have an impact on the nuclei. A nucleus feels a field that doesn’t exactly equal to applied external magnetic field. The difference between the external magnetic field and the field felt by the nucleus is very small and we call this small difference, shielding. For example, if the nucleus is a hydrogen atom (In vocabulary in NMR spectroscopy, it is called a proton) in the molecule, it feels different magnetic fields depending upon its location and the neighbor it directly attached to. To answer your question of this difference properly, there needs to be a few words in NMR vocabulary to define:

Shielding (Upfield shift): Shielding is a barrier made of inner-shell electrons and it decreases the nucleus’ pull on the outer electrons. In most books, shielded is defined as a nucleus whose chemical shift has been decreased due to addition of electron density, magnetic induction, or other effects. Hence, the nucleus feels weaker magnetic field than the applied field.

Deshielding (Downfield shift): Deshielding is the opposite of shielding. When we say that a nucleus is deshielded, what we mean is that nucleus’ chemical shift has been increased due to removal of electron density (e.g., a barrier made of inner-shell electrons), magnetic induction, or other effects. Hence, the nucleus feels stronger magnetic field than the applied field.

Magnetic Induction: In NMR spectroscopy, magnetic induction (e.g., magnetic anisotropy) is the phenomenon that an external magnetic field (applied field) causes $\pi $-electrons in an electron cloud to circulate. As these electrons move around, their own electric charge causes them to create their own magnetic field (induced field). Possibly the best example of magnetic induction occurs with the benzene molecule, in which its 6 $\pi $-electrons are delocalized and free to move around the aromatic ring (6 $\mathrm{sp}^2$ carbons), inducing a magnetic field at the center such a way that it is opposite to the applied field (right hand rule). Thus, outside the ring feels more than applied field thus benzene protons are deshielded (see picture below). On the other hand, protons inside the ring (molecule in the RHS of the picture) feel less magnetic field and hence, shielded.

Benzene

Magnetic anisotropy is in effect when delocated $\pi$-electrons are in nearby such as in alkenes, alkynes and carbonyl groups (See picture below). As Night Writer pointed out, electronegativity difference between neighboring nuclei plays a bigger role in methanol and other compounds such as $\ce{CH3X}$ (see the Table below as well).

Magnetic Anisotropy

$$ \begin{array}{|c|c|c|c|c|c|} \hline \ce{CH3X} & \ce{CH3F} & \ce{(CH3OH} & \ce{CH3Cl} & \ce{CH3Br} & \ce{CH3I} & \ce{CH3H} & \ce{(CH3)4Si} \\ \hline \ce{X} & \ce{F} & \ce{O} & \ce{Cl} & \ce{Br} & \ce{I} & \ce{H} & \ce{Si} \\ \hline \text{Electronegativity of}\; \ce{X} & 4.0 & 3.5 & 3.1 & 2.8 & 2.5 & 2.1 & 1.8 \\ \hline \delta \; \text{in ppm} & 4.26 & 3.4 & 3.05 & 2.68 & 2.16 & 0.23 & 0 \\ \hline \end{array} $$ The above picture and the data of the table are directly from University of Calgary page.

Keep in mind that this chemical shifts values can vary by the solvents used (see Night Writer's Table), operating temperature, pH, etc. (Ref.1). Some values listed in Ref.1 are displayed in following Table for your convenience: $$ \begin{array}{|c|c|c|c|c|c|} \hline \text{Compound} & \ce{C-H} & \ce{CDCl3} & \ce{(CD3)2CO} & \ce{(CD3)2SO} & \ce{C6D6} & \ce{CD3CN} & \ce{CD3OD} & \ce{D2O} \\ \hline \text{acetic acid} & \ce{CH3} & 2.10 & 1.96 & 1.91 & 1.55 & 1.96 & 1.99 & 2.08 \\ \hline \text{acetone} & \ce{CH3} & 2.17 & 2.09 & 2.09 & 1.55 & 2.08 & 2.15 & 2.22 \\ \hline \text{acetonitrile} & \ce{CH3} & 2.10 & 2.05 & 2.07 & 1.55 & 1.96 & 2.03 & 2.06 \\ \hline \text{DMSO} & \ce{CH3} & 2.62 & 2.52 & 2.54 & 1.68 & 2.50 & 2.65 & 2.71 \\ \hline \text{nitromethane} & \ce{CH3} & 4.33 & 4.43 & 4.42 & 2.94 & 4.31 & 4.34 & 4.40 \\ \hline \text{ethyl acetate} & \ce{CH3CO} & 2.05 & 1.97 & 1.99 & 1.65 & 1.97 & 2.01 & 2.07 \\ \hline \text{ ethyl acetate} & \ce{OCH2} & 4.12 & 4.05 & 4.03 & 3.89 & 4.06 & 4.09 & 4.14 \\ \hline \text{ methanol} & \ce{OCH2} & 3.49 & 3.31 & 3.16 & 3.07 & 3.28 & 3.34 & 3.34 \\ \hline \text{ ethanol} & \ce{OCH2} & 3.72 & 3.57 & 3.44 & 3.34 & 3.54 & 3.60 & 3.65 \\ \hline \text{2-propanol} & \ce{OCH} & 4.04 & 3.90 & 3.78 & 3.67 & 3.87 & 3.92 & 4.02 \\ \hline \text{toluene} & \ce{CH3} & 2.36 & 2.32 & 2.30 & 2.11 & 2.33 & 2.32 & - \\ \hline \end{array} $$

Keep in mind that chemical shift of regular alkane $\ce{-CH3}$ is around $\pu{0.9 ppm}$, yet the downfield shifts shown in entries 1-3 and 6 are due to magnetic anisotropy induced by carbonyl group ($\ce{-C=O}$). Similarly, relatively large shifts in entries 4 and 5 are due to $\ce{-S=O}$ and $\ce{-N(O)=O}$ groups, respectively. The similar effect on last entry is duo to nearby double bond ($\ce{-CH=C-CH3}$).

References:

  1. H. E. Gottlieb, V. Kotlyar, A. Nudelman, “NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities,” J. Org. Chem. 1997, 62(21), 7512–7515 (DOI: 10.1021/jo971176v).
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In brief:

Indeed is not exactly O (of the carboxyl group) that withdraws electrons from the methyl group. but the carboxyl itself. Not surprisingly we call this -M effect. So it does not matter much that there is a carbon in between.

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