How do quinones work?

How do quinones work?

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Quinones are electrophilic Michael acceptors stabilized by conjugation. Moreover, depending on the quinone. In addition, the site of reduction. As a result, reduction can either rearomatize the compound. Or break the conjugation.

In this experiment, the Hammett rate constant (ρ) was measured by Voltammetric Analysis of Benzoquinones.² In this research project, Acetonitrile, Dimethylformamide, and Tetrahydrofuran was used as solvent.² 1,4-Benzoquinone, 2,5 dimethyl-p-benzoquinone, tetrachloro-p-benzoquinone, 2-dichloro-5,6-dicyano-1,4-benzoquinone, and Duroquinone was tested in the research project. Then, the Cyclic voltammetry (CV) was used to determined anodic peak potential (E^a) and the cathodic peak potential (E^c).¹ Finally, the Hammett rate constant (ρ) was calculated, and the value in Acetonitrile was 6.31. In the Dimethylformamide was 5.91, and in the Tetrahydrofuran was 5.07. 

Introduction

In this experiment, quinones can be used by hydroquinones in a two-step electron transition.¹ The transfer potentials for those two steps was measured by cyclic voltammetry.^1,2 The standard reduction potential energy(E⁰’) which was scanned from +0.8V to -1.8V was obtained from¹

                                                            E⁰’ = (Ea+Ec)2                                                                            (1)

Then, the standard Hammett free energy relationship for equilibrium data was shown² 

                                                                  logKlogK0=                                                                           (2)

Where K was the equilibrium constant of the reaction for quinones.^1,2 Then, the standard reduction potential energy (E⁰’) was found by K by the equation¹

                                                              log K = nFE02.303RT                                                                    (3)

In equation (3), n was the number of moles of electrons which was transferred. F was Faraday’s constant, which was 9.65×10⁴ C/mol. R was the gas constant which was 8.314 J/mol K, and T was the room temperature. Then, equation (2) and (3) was produced together as equation (4)

                                              log K – log K₀ = (nFE02.303RT) – (nFEo’2.303RT) = σρ                                        (4)

In that case, when ΔE⁰ =E⁰ -E₀⁰, equation (4) could be written 

                                                      ΔE⁰ =E⁰ -E₀⁰ = (nF2.303RT)ρ∑σ                                                        (5)

Where ∑σ was the sum of substituent effects in the compound. 

Experimental

This research project was supported by reference 1 and 2. First of all, 25ml of solution was made by each benzoquinone with Acetonitrile. In that case, enough mass for each solution was weighed out as 2.0mM benzoquinone, and 0.10M of TBA was used. When the solutions were prepared, the cyclic voltammetry cell was used to determine anodic peak potential (E^a) and the cathodic peak potential (E^c). Before the set up was used, Nitrogen gas was purged for 5 mins to clean air in the system. The system was set up to scan from +0.80V to -1.80V, with a scan rate of 50mV/s. Repeat again. Also, this procedure was repeated by different solvents which were Dimethylformamide and Tetrahydrofuran. Finally, LUMO energy of each benzoquinone was found by PC Spartan. 

Results

Reduction potentials (E) and the sum of the substituent effects in the substituted compounds (∑σ) by Acetonitrile as solvent was measured in this experiment in Table 1. Also, an add on experiment for LUMO energy was measured in Table 2. 

Table 1.       Measured the anodic peak potential (E^a), the cathodic peak potential (E^c), standard reduction potentials(E⁰), ΔE⁰’, and ∑σ. The result value was shown.

Table 2. Measured ∑σ and LUMO Energy, and the result value was shown.

ΔE⁰’ was plotted versus ∑σ, and the resulting graph was shown in Figure 1. In Figure 1, there was a point which was the 1,4-benzoquinone (E₀), so that point wasn’t included in the result, and a new result graph with a correlation coefficient (R²) of 0.99¹ which was shown in Figure 2. The equation of the line y = 0.3734x – 0.0274 where y = ΔE⁰’, x = ∑σ, and the slope = 2.303RTnF = 0.3734 J/mol C. In that case, the final result of ρ was 6.31 (5.54% error)^1,2

Figure 1. This graph of ΔE⁰’ vs. ∑σ showed the relationship for ρ.

Figure 2. This graph of ΔE⁰’ vs. ∑σ showed the relationship without 1,4-benzoquinone (E₀) for ρ.

How do quinones work?

Also, ΔE⁰’ was plotted versus LUMO Energy, and the result graph was shown in Figure 3. 

Figure 3. This graph of ΔE⁰’ vs. LUMO Energy showed the relationship.

Also, another reduction potentials (E) and the sum of the substituent effects in the substituted compounds (∑σ) by dimethylformamide as solvent was measured in this experiment in Table 3. 

Table 3.       Measured the anodic peak potential (E^a), the cathodic peak potential (E^c), standard reduction potentials(E⁰), ΔE⁰’, and ∑σ. The result value became shown.

ΔE⁰’ was plotted versus ∑σ, and the resulting graph was shown in Figure 4. The equation of the line y = 0.3503x – 0.0793 where y = ΔE⁰’, x = ∑σ, and the slope = 2.303RTnF = 0.3503 J/mol C. In that case, the final result of ρ was 5.91 (7.22% error)^1,2

Figure 4. This graph of ΔE⁰’ vs. ∑σ showed the relationship without 1,4-benzoquinone (E₀) for ρ.

Figure 5. This graph of ΔE⁰’ vs. ∑σ showed the relationship for ρ.

Finally, another reduction potentials (E) and the sum of the substituent effects in the substituted compounds (∑σ) by dimethylformamide as solvent was measured in this experiment in Table 5.

Table 4. Measured the anodic peak potential (E^a), the cathodic peak potential (E^c), standard reduction potentials(E⁰), ΔE⁰’, and ∑σ. The result value became shown.

ΔE⁰’ plotted versus ∑σ, and the resulting graph became shown in Figure 6. The equation of the line y = 0.2985x – 0.1551 where y = ΔE⁰’, x = ∑σ, and the slope = 2.303RTnF = 0.2985 J/mol C. In that case, the final result of ρ was 5.07 (25.55% error)^1,2

Figure 6. This graph of ΔE⁰’ vs. ∑σ showed the relationship without 1,4-benzoquinone (E₀) for ρ.

Figure 7. This graph of ΔE⁰’ vs. ∑σ showed the relationship without 1,4-benzoquinone (E₀) for ρ.

Discussion

The value of LUMO energy became measured by PC Spartan with the PM3 Hamiltonian. In this experiment, there were many kinds of substituent groups. For example, in Table 2, Duroquinone had -0.68∑σ and -1.48 eV LUMO energy, and 2,5-dimethyl-p-benzoquinone had -0.34 ∑σ and -1.60 eV LUMO energy. Duroquinone had 4 methyl groups, and 2,5-dimethyl-p-benzoquinone had 2 methyl groups. In methyl, hydrogen, chloro and cyano groups, the methyl group was the most electron donating group, and cyano was the most electron withdrawing group. In this experiment, more methyl groups meant more negative charge became donated into the ring. In addition, other electrons were hard to attach on the molecule. So Duroquinone had higher LUMO energy, other electrons were hard to attach on it, and 2,5-dimethyl-p-benzoquinone had lower LUMO energy than Duroquinone, so other electrons were easier to attach to it. 

Here was another sample that could explain the result. Furthermore, donating groups were negative, and it affected orbital energies up. Then LUMO energy would go up. Also, withdrawing groups were more positive, and it affected orbital energies down. Then LUMO energy would go down. So Tetrachloro-p-benzoquinone’s LUMO energy was -2.18 eV, and 2-Dichloro-5,6-dicyano-1,4-benzoquinone had LUMO energy which was -2.86 eV. When those two molecules became scanned by Cyclic Voltammetry, there were some peaks in the graph. Because tetrachloro-p-benzoquinone had more LUMO energy, so peaks for 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone would present quicker than tetrachloro-p-benzoquinone because the LUMO energy was lower. 

References : How do quinones work?

1. Heffner, J.E., Raber, J.C., Moe, O.A.; Wigal, C.T. J. Chem. Educ. 1998, 75, 365-7.
2. Heffner, J.E., Wigal, C.T.; Moe, O.A. Electroanalysis 1997, 9, 629-32.

Sample Calculations

How do quinones work?

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