Problem 12 - Advanced Organic Chemistry. Part A. Structure and Mechanisms

Question

The \({ }^{1} \mathrm{H}-\mathrm{NMR}\) and \({ }^{13} \mathrm{C}-\mathrm{NMR}\) spectra of both the anion and cation derived from phenalene have been observed. The HMO pattern for phenalene is given below. The chemical shifts observed for the cation and anion are given below. $$ \begin{array}{lllllll} \hline & {{ }^{1} \mathrm{H}-\mathrm{NMR}} & &{{ }^{13} \mathrm{C}-\mathrm{NMR}} \\ \hline & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(1) & \mathrm{C}(2) & \mathrm{C}(9 \mathrm{~b}) & \mathrm{C}(3 \mathrm{a}) \\ {[\text { Cation }]} & 9.38 & 8.55 & 155.5 & 133.7 & 123.7 & 133.7 \\ {[\text { Anion }]} & 5.36 & 6.10 & 103.4 & 128.0 & 139.6 & 145.0 \\ \hline \end{array} $$ What conclusions can be drawn about electron distribution in the cation and anion from the NMR data and how does it relate to the HMO pattern?

Step-by-Step Solution

Answer

The cation shows electron deficiency at C(1) and C(2), whereas the anion shows electron richness there. This aligns with the HMO pattern, where cationic phenalene is electron-deficient and anionic phenalene is electron-rich at these sites.

Step 1: Analyze Cation Proton NMR

In the cation, the chemical shifts for protons at C(1) and C(2) are 9.38 ppm and 8.55 ppm, respectively. The high ppm values indicate that these protons are deshielded, suggesting an electron-depleted environment around them due to electron-withdrawing effects.

Step 2: Analyze Anion Proton NMR

For the anion, the chemical shifts for protons at C(1) and C(2) are significantly lower, 5.36 ppm and 6.10 ppm. These lower values indicate that the protons are more shielded, suggesting an electron-rich environment around these carbon atoms due to electron-donating effects.

Step 3: Analyze Cation Carbon NMR

The carbon shifts in the cation at C(1) is 155.5 ppm, which is relatively high, suggesting significant deshielding. Other carbons also show shifts that indicate an electron-deficient characteristic, with C(2), C(3a), and C(9b) being somewhat less deshielded but still showing higher shifts typically for sp2 hybridized carbons involved in cationic systems.

Step 4: Analyze Anion Carbon NMR

In the anion, C(1) exhibits a lower shift at 103.4 ppm, indicating more electron density around this carbon. The higher chemical shifts for the other carbon atoms compared to the cation, like C(9b) and C(3a), indicate that these areas are electron-rich, in line with an anionic species.

Step 5: Relate NMR Data to HMO Pattern

The HMO (Hückel Molecular Orbital) pattern suggests regions of electron density and deficiency for neutral phenalene. The NMR data implies that the cation has reduced electron density, especially around C(1) and C(2), matching an electron-deficient HMO. In contrast, the anion data shows increased electron density, suggesting these sites correspond to electron-rich areas in its HMO.

Key Concepts

Electron Distribution

Electron distribution is a key concept in understanding how electrons are arranged around atoms or groups within a molecule. In NMR spectroscopy, the electron distribution around specific nuclei can be analyzed by examining chemical shifts, which are changes in the resonance frequencies.
In the case of the phenalene cation and anion, the electron distribution is indicative of the electron-weaker (cation) or electron-richer (anion) environment around particular carbon atoms.
- **Cation:** Here, the protons and carbons exhibit high chemical shifts, suggesting that electrons are pulled away, creating an electron-poor environment. As a result, the nuclei become deshielded, which is observed in higher ppm values. - **Anion:** Contrarily, lower chemical shifts in the anion's spectra indicate that electrons donate towards certain nuclei, creating electron-rich surroundings. This results in increased shielding and thus lower ppm values.
This contrast between these environments in NMR data allows the determination of the electron distribution in these charged species.

Hückel Molecular Orbital (HMO) Theory

Hückel Molecular Orbital (HMO) theory is a method used to determine the energy levels in conjugated systems, providing insights into molecular stability and electronic arrangements. It considers r - the overlap of r - r - electron clouds r - and energy levels of r - the r - molecular r - orbitals. In the context of phenalene, the HMO theory assists us in visualizing the sites that are likely to be electron-rich or electron-poor in its neutral state.

- **Cation(HMO):** The HMO pattern points to specific sites experiencing electron deficiency when a positive charge is present. This correlates with the NMR data for the cation, where high chemical shifts reflect electron scarcity. - **Anion (HMO):** On the other hand, the anionic form, as indicated by the HMO theory, shows how the extra electron density enhances electronic shielding, resulting in lower chemical shifts. The practical application of HMO theory in NMR spectral analysis emphasizes how molecular orbitals influence chemical shifts by highlighting electron-rich and deficient areas.

Cation and Anion Analysis

Cation and anion analysis involves examining how positive and negative charges respectively alter the electron environment within a molecule. With NMR spectroscopy, these changes manifest as different chemical shifts.

- **Cation:** When analyzing the phenalene cation, the - - - NMR data tells us that the positive charge pulls electron density - - - away from certain regions, causing them to become electron-deficient. This effect leads to deshielding and higher ppm values for these atoms. - **Anion:** Conversely, the phenalene anion allows for an electron cloud donation towards specific regions, supplementing electron density around those atoms. Consequently, these areas become electron-rich, seen as lower chemical shifts due to shielding effects. The varying electron distribution patterns between cations and anions substantially affect molecular environments, with tangible observations made through NMR data.

Chemical Shifts

Chemical shifts in NMR are integral to understanding molecular environments, especially when analyzing charged species such as cations and anions.

- **Deshielding:** Occurs when electrons are pulled away from a nucleus, increasing the chemical shift (higher ppm). This is observed in the phenalene cation, where electron-withdrawing effects lead to high ppm values for protons and carbons. - **Shielding:** If a nucleus gains electron density, it becomes more shielded, thus lowering the chemical shift (lower ppm). In the phenalene anion, the NMR data shows this effect with lower ppm values. In summary, chemical shifts provide a window into the electron distribution and dynamics around nuclei in a molecule. They are crucial in discerning the electron environments in phenalene's cation and anion forms, revealing how charges influence nuclear magnetic properties and overall molecular structure.