17.12
NMR Spectroscopy of Aromatic Compounds
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the…
1H and 13C NMR spectroscopy are very helpful in identifying aromatic compounds.
Typically, aromatic hydrogens are highly deshielded by the aromatic ring current.
As a result, they exhibit characteristic 1H NMR absorptions in the range of 6.5–8 ppm.
For instance, aromatic hydrogens of benzene absorb at 7.3 ppm.
However, aromatic hydrogens of large rings absorb farther upfield or downfield.
Consider [18]annulene. It has 12 hydrogens outside the ring, which are highly deshielded by the aromatic ring current. The 6 hydrogens inside the ring are highly shielded.
Consequently, the outside hydrogens absorb downfield at a high frequency. The inside hydrogens absorb upfield at a low frequency.
Aromatic carbons exhibit 13C NMR absorptions in the range of 110–150 ppm.
For instance, aromatic carbons of benzene absorb at 128 ppm.
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Q1: What chemical shift range do aromatic hydrogens typically appear in 1H NMR spectra?
Aromatic hydrogens are highly deshielded by the aromatic ring current and exhibit characteristic 1H NMR absorptions in the range of 6.5–8 ppm. For example, aromatic hydrogens of benzene absorb at 7.3 ppm. However, aromatic hydrogens of larger rings may absorb farther upfield or downfield than this typical range.
Q2: Why do aromatic hydrogens appear at different chemical shifts in [18]annulene?
In [18]annulene, the 12 outer hydrogens are highly deshielded by the aromatic ring current and absorb downfield at 9.3 ppm, while the 6 inner hydrogens are highly shielded and absorb upfield at −3.0 ppm. This dramatic difference occurs because the ring current affects hydrogens inside and outside the ring in opposite directions.
Q3: What chemical shift range characterizes aromatic carbons in 13C NMR?
Aromatic carbons exhibit 13C NMR absorptions in the range of 110–150 ppm. For instance, aromatic carbons of benzene absorb at 128 ppm. This distinct chemical shift range helps identify aromatic carbon environments in complex organic molecules and distinguishes them from aliphatic carbons.
Q4: How does the aromatic ring current affect hydrogen deshielding in NMR?
The aromatic ring current creates a magnetic field that deshields hydrogens bonded directly to aromatic rings, causing them to absorb at lower frequencies or higher ppm values. This deshielding effect is responsible for the characteristic downfield shifts observed in aromatic 1H NMR spectra, distinguishing aromatic hydrogens from aliphatic hydrogens.
Q5: How can NMR spectroscopy be used to identify aromatic compounds?
1H and 13C NMR spectroscopy are very helpful in identifying aromatic compounds because aromatic hydrogens and carbons exhibit characteristic chemical shift ranges. Aromatic hydrogens appear at 6.5–8 ppm in 1H NMR, while aromatic carbons appear at 110–150 ppm in 13C NMR, providing diagnostic signals for aromatic ring systems.
Q6: What is the relationship between ring size and aromatic hydrogen chemical shifts?
Aromatic hydrogens of larger rings absorb farther upfield or downfield than the typical 6.5–8 ppm range observed for benzene. The size and geometry of the aromatic ring system influence the magnitude of the ring current effect, causing variations in chemical shift positions for different conjugated systems.
Q7: How do inside and outside hydrogens differ in their NMR behavior?
Inside hydrogens of aromatic systems like [18]annulene are highly shielded by the aromatic ring current and absorb upfield at low frequencies, while outside hydrogens are highly deshielded and absorb downfield at high frequencies. This opposite shielding effect demonstrates how spatial position relative to the ring current dramatically influences NMR chemical shifts.