Thermal and Photochemical Reactions of Aromatic Compounds in Trifluoroacetic Acid, Involving Carbocations and Radical Cations.

Eberson, Lennart; Radner, Finn; Jørgensen, Even H.; Pettersen, Anita; Spielbüchler, P.; Pedersen, J. Boiden; Krogsgaard‐Larsen, Povl

doi:10.3891/acta.chem.scand.46-0630

Cited by 23 publications

(6 citation statements)

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“…Trifluoroacetic acid is able to oxidize a number of aromatic compounds to yield radical cations, a reaction that is accelerated by light (Eberson and Radner, 1992). Whether a thermal electron transfer reaction is possible or not is less clear, but if it does take place, it is very slow (Eberson and Radner, 1992). The solutions in the present experiments were kept in the dark of the cell holder and exposed to the analyzing light for only a short period every 15 min.…”

Section: Discussionmentioning

confidence: 99%

“…The carotenoid radical cation formed by reaction with a number of different radicals also absorbs strongly in the same spectral region (Edge et al, 1997). Trifluoroacetic acid is able to oxidize a number of aromatic compounds to yield radical cations, a reaction that is accelerated by light (Eberson and Radner, 1992). Whether a thermal electron transfer reaction is possible or not is less clear, but if it does take place, it is very slow (Eberson and Radner, 1992).…”

Section: Discussionmentioning

confidence: 99%

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Kinetics and Mechanism of the Primary Steps of Degradation of Carotenoids by Acid in Homogeneous Solution

Mortensen

Skibsted

2000

J. Agric. Food Chem.

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The kinetics of reaction between trifluoroacetic acid as an acid of medium strength and the carotenoids beta-carotene, zeaxanthin, canthaxanthin, and astaxanthin has been examined in detail including the effects of dioxygen, acid concentration, and carotenoid structure. Reaction between acid and carotenoid leads to species absorbing in the red and near-infrared (NIR) spectral regions, intermediates that subsequently disappear. ESR experiments clearly show that these species are not carotenoid radicals, although their NIR absorption is similar to the absorption of carotenoid radical cations. Under most reaction conditions, the disappearance of carotenoids follows pseudo-zero-order kinetics, whereas the reaction order is >1 with respect to acid, and the long-lived (hours) intermediates are suggested to be mono- (700 nm) and diprotonated carotenoid ( approximately 950 nm). Acid induces cis/trans-isomerization via the protonated intermediates, which also decay to nonradical species with shorter conjugated systems-most probably carotenoid esters. Slow protonization of the methine carbon is the primary step in the degradation, but dioxygen increases the rate as a result of formation of a charge-transfer complex with the carotenoids as indicated by a red-shift of the NIR absorption bands. Carotenoids with carbonyl groups (astaxanthin and canthaxanthin) have slower rates of degradation than beta-carotene and zeaxanthin, indicating preferential nondegradative protonation of the carbonyl groups.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Kinetics and Mechanism of the Primary Steps of Degradation of Carotenoids by Acid in Homogeneous Solution

Mortensen

Skibsted

2000

J. Agric. Food Chem.

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“…This is the first time that a mixed trimer of thiophene and pyrrole was oxidized to its radical cation and observed by means of EPR spectroscopy. Despite the very short lifetime of 6 •+ due to oligomerization through the very reactive 5,5‘‘ positions, its EPR spectrum in fluid solution has been observed in TFA due to the good properties of this acid either as a solvent of very low nucleophilicity to stabilize cationic species or as a mild oxidant of substrates with relative low oxidation potentials. An increasing stability of these radical cation species is afforded when the extreme positions are blocked by alkyl groups. , Some more stable and better characterized radical cation spectra are now in progress.…”

Section: Discussionmentioning

confidence: 99%

Molecular and Electronic Structures of Heteroaromatic Oligomers: Model Compounds of Polymers with Quantum-Well Structures

et al. 1998

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Conformational preferences of 2,2′-bithiophene, 2-(2-thienyl)pyrrole, and N-methyl-2-(2-thienyl)pyrrole have been investigated by means of computational methods. Calculations were performed at the ab initio HF/6-31G(d) and MP2/6-31G(d) levels and, additionally, with the density functional B3-LYP/6-31G(d). The results indicate that 2-(2-thienyl)pyrrole behaves similarly to the 2,2′bithiophene. Thus, two minimum energy conformations were found for each compound, which correspond to anti-gauche and syn-gauche. Such minima are separated by barriers of about 1.7 and 1.3 kcal/mol at the HF and MP2 levels. On the contrary, the preferences found for N-methyl-2-(2-thienyl)pyrrole were different, giving an almost negligible energy barrier between the two minimum energy conformations. Furthermore, at the MP2 level the anti-gauche and syn-gauche minimum energy conformations present an inter-ring dihedral of 135°and 68°, respectively, displaying deviations greater than those found in 2,2′-bithiophene and 2-(2-thienyl)pyrrole. The conformational analysis was extended to the tricyclic compound N-methyl-2,5-di(2-thienyl)pyrrole. For this molecule, a contour map of the conformational energy as a function of the inter-ring dihedral angles was computed at the HF/6-31G(d) level. Minimum energy conformations were subsequently computed at the same level of theory. Results were in agreement with those obtained for the bicyclic compound N-methyl-2-(2-thienyl)pyrrole. Thus, the inter-ring dihedral angles of the minimum energy conformations present a large distortion with respect to the planarity, and such minimum energy conformations are separated by almost negligible energy barriers. Finally, N-hexyl-2,5-di-(2-thienyl)pyrrole radical cation was observed by EPR spectroscopy. This is the first time that a mixed trimer of thiophene and pyrrole is oxidized to its radical cation and detected by means of EPR spectroscopy. The spectrum seems to be symmetric, which is accounted for by a fast interconversion between conformers. This supports the small energy barrier calculated between minima for the bicyclic and tricyclic compounds. Overall, the results presented in this work indicate that N-methylpyrrole rings are able to induce large rotational deffects in [...-(pyrrole) n -(thiophene) n -...] block copolymers.

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“…These excellent properties of the TTFA−TFA system are due not only to the stabilization effect of radical cations by TFA but to the low nucleophilicity of the system which prevents any further reaction on the primary radical cations . Thus, if mercury(II) trifluoroacetate, also a good oxidant, is used instead of thallium(III) trifluoroacetate, some substrates undergo, after being oxidized to their radical cations, further mercuriation on the aromatic ring .…”

Section: Introductionmentioning

confidence: 99%

Thallium(III) Trifluoroacetate−Trifluoroacetic Acid in the Chemistry of Polythiophenes. 2. Treatment of 3-Alkylthiophenes and Electron Paramagnetic Resonance Results

et al. 1997

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The treatment of thiophene and 3-alkylthiophenes with thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) gives insoluble and dark powdery solids with oxygen content and electrical conductivities ranging from 10-4 to 10-6 Ω-1 cm-1. Polar and short fractions are negligible. All of them show semiconductivity (10-3−10-6 Ω-1 cm-1) when doped in iodine atmosphere. Electron paramagnetic resonance (EPR) spectra of either as-synthesized or I2-treated solids display characteristic single and broad lines (ΔHpp, 1.84−7.4 G) with Lorentzian shapes and g-values in the range 2.0028−2.0038. Infrared spectra show characteristic C−H out-of-plane deformations (780 cm-1 for polythiophene and 820−825 cm-1 for poly(3-alkylthiophenes)) in addition to a strong peak at 1650−1690 cm-1 which has not been conclusively assigned. EPR spectra of some disubstituted and tetrasubstituted 2,2‘-bithiophene radical cations have been observed and their g-values and coupling constants assigned when the corresponding parent compounds are photolyzed with ultraviolet light in TFA. Photolysis of 3-alkylthiophenes in TFA in the EPR instrument gave the radical cations of 4,4‘-dialkyl-2,2‘-bithiophenes. In no case, were EPR signals of the isomeric 3,3‘-dialkyl- or 3,4‘-dialkyl-2,2‘-bithiophene radical cations observed, indicating that dimerization of 3-alkylthiophenes occurs through the less sterically hindered 5-position. The presence of two doublet species corresponding to both conformers, syn and anti, in the radical cations is associated with a large barrier to rotation about the C(2)−C(2‘) bond.

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Thermal and Photochemical Reactions of Aromatic Compounds in Trifluoroacetic Acid, Involving Carbocations and Radical Cations.

Cited by 23 publications

References 0 publications

Kinetics and Mechanism of the Primary Steps of Degradation of Carotenoids by Acid in Homogeneous Solution

Kinetics and Mechanism of the Primary Steps of Degradation of Carotenoids by Acid in Homogeneous Solution

Molecular and Electronic Structures of Heteroaromatic Oligomers: Model Compounds of Polymers with Quantum-Well Structures

Thallium(III) Trifluoroacetate−Trifluoroacetic Acid in the Chemistry of Polythiophenes. 2. Treatment of 3-Alkylthiophenes and Electron Paramagnetic Resonance Results

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