Answer
¹H NMR Spectrum Analysis of Benzil (C₁₄H₁₀O₂)
Molecular Overview:
Benzil is a symmetrical aromatic diketone, consisting of two phenyl rings linked via a 1,2-dione (CO-CO) group. Due to its symmetry, the proton environments on each ring are equivalent. The presence of electron-withdrawing carbonyl groups influences the chemical shifts of nearby aromatic protons.
Observed NMR Data:
- Peak at ~8.02 ppm (7.95–8.36) – Integration: 4H – Multiplet
- Peak at 7.43–7.90 ppm – Integration: 6H – Multiplet
- ~1.7 ppm – Minor trace impurity
Interpretation:
The molecule’s symmetry reduces the number of unique proton signals. Each phenyl ring contributes 5 protons:
- Ortho protons (H₁, H₅, H₆, H₁₀) are closest to the carbonyls, showing the most deshielding → appear downfield (~8.02 ppm).
- Meta protons (H₂, H₄, H₇, H₉) are moderately deshielded.
- Para protons (H₃, H₈) are least affected.
- Meta and para protons overlap into a broad multiplet (~7.43–7.90 ppm) with a combined integration of 6H.
Tabulated Assignment:
| Chemical Shift (ppm) | Integration | Splitting | Assignment |
|---|---|---|---|
| ~8.02 (7.95–8.36) | 4H | Multiplet | Ortho protons (H₁, H₅, H₆, H₁₀) |
| 7.43–7.90 | 6H | Multiplet | Meta (H₂, H₄, H₇, H₉) and Para (H₃, H₈) |
Note: All assignments are consistent with the expected chemical shift range for aromatic protons and influenced by deshielding effects from adjacent carbonyl groups in a symmetrical aromatic diketone.