Organic Chemistry 2 · Aromaticity and cyclic conjugation
Benzene 1H NMR at 7.3 ppm: the ring current
Conditions: ¹H NMR: benzene δ 7.3; cyclohexene vinyl H δ 5.7
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The question
Why do benzene's protons appear near 7.3 ppm, well downfield of alkene protons?
Reveal the answer and explanation
Answer and explanation
Correct answer A π-electron ring current deshields protons on the outside of the ring
Correct: the induced field reinforces B₀ at the ring edge.
Not this one The sp² carbons are more electronegative than in alkenes
Hybridization is the same; this cannot explain the extra 1.5 ppm.
Not this one The ring current shields the outer protons
Shielding occurs inside and above the ring, not at its edge.
Not this one Hydrogen bonding between benzene molecules
Benzene C–H bonds do not hydrogen bond meaningfully.
Worked solution
In the applied field, benzene's delocalized π electrons circulate around the ring and create an induced magnetic field. Outside the ring, where the hydrogens sit, this field adds to the applied field, so the protons are deshielded and resonate downfield near 7–8 ppm. A downfield shift of this kind is NMR evidence of aromaticity.
What you'll practice
Explain the downfield chemical shift of aromatic protons with the ring current.
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