For the 1H NMR spectrum of 9-fluorenone, assign the peaks to the structure

For the 1H NMR spectrum of 9-fluorenone, assign the peaks to the structure. Note that the peaks are 7.45 are from overlapping signals. The doublet at 7.65 is at a high chemical shift due to a resonance effect. Draw a resonance structure that accounts for this high shift.

The Correct Answer and Explanation is :

I will describe the assignment of the NMR peaks and the resonance structure of 9-fluorenone based on general chemical principles.

Assignment of the ( ^1H ) NMR Peaks for 9-Fluorenone

9-Fluorenone consists of a fluorenyl core with a ketone at the 9-position. Its ( ^1H ) NMR spectrum typically displays aromatic proton signals, which can be analyzed based on their electronic environment.

1. Overlapping signals at 7.45 ppm

• These correspond to multiple protons on the aromatic rings that experience similar electronic environments.
• The C–H bonds in positions 2, 3, 6, and 7 in the fluorenone structure contribute to this region. These protons experience a combination of shielding and deshielding effects due to π-electron delocalization.

1. Doublet at 7.65 ppm

• This peak arises due to a proton in a deshielded environment.
• The resonance effect of the carbonyl group (-C=O) at position 9 influences the electronic distribution.
• This effect causes a significant downfield shift due to the withdrawal of electron density from adjacent π-systems, making the attached proton more deshielded.

Resonance Explanation for the High Chemical Shift at 7.65 ppm

• The ketone at position 9 is an electron-withdrawing group, which pulls electron density away from the conjugated system.
• A resonance structure that explains the deshielding effect involves movement of electron density from the aromatic rings toward the oxygen of the ketone: Resonance structure:
• A lone pair on the oxygen of the carbonyl group withdraws electron density through resonance, creating partial positive charge on the adjacent carbon.
• This results in greater deshielding of the neighboring aromatic protons, particularly the proton appearing at 7.65 ppm.

Conclusion

The peaks in the ( ^1H ) NMR spectrum of 9-fluorenone are influenced by both the conjugated aromatic system and the electron-withdrawing resonance effect of the carbonyl group. The downfield shift observed at 7.65 ppm is due to the resonance-induced deshielding effect. This is a common feature in ketone-containing aromatic systems where π-electron withdrawal alters the chemical shift of nearby protons.