Відмінності між версіями «Kcal mol-1. The typical O bond strengths in Table five do not»

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Utilizing the identified pKas and reduction potentials for the quinones and semiquinones, the BDFEs (and BDEs) for a lot of hydroquinones is often calculated (Table six). The energy on the thermochemical cycles (Hess' Law) is illustrated by the calculation with the HQ?HQ- reduction potentials (Figure two), 1.64028E+14 which are tough to get straight due to the speedy disproportionation of semiquinone radicals.156 It really should also be noted that the BDFEs of these quinones usually do not necessarily reflect the 1e- quinone/semiquinone reduction potentials. For instance, tetrachloro-p-benzoquinone is 0.5 V extra oxidizing than pbenzoquinone,157 despite the fact that the average BDFEs are usually not also distinctive. One electron potentials for any wide variety of quinones in many diverse organic solvents are available in reference 157. The ortho-substituted quinone/catechol redox couple has reactivity and thermochemistry that's somewhat distinct in the para-quinone/hydroquinone couple. Ortho-quinones and catechols (1,2-hydroxybenzenes) are also key biological cofactors, essentially the most widely recognized of that are the catecholamines dopamine, epinephrine and norepinepherine.167 journal.pone.0174724 The antioxidant and anti-cancer activities of ortho-quinone derivatives, known as `catachins,' have not too long ago received considerable consideration.168 Regrettably, the data available for catechols are a lot more limited than those for hydroquinones, and therefore, the double square scheme in Figure 3 can't be absolutely filled in. Nevertheless, adequate outcomes are offered to show the important variations in between hydroquinones and catechols. The aqueous 2H+/2e- possible of catechol155 indicates an average O BDFE of 75.9 kcal mol-1, slightly higher than that of 1,4-hydroquinone (73.six kcal mol-1). In the recognized pKa from the semiquinone169 and the one electron potential of ortho-benzoquinone, the second BDFE is 65.4 kcal mol-1, applying eq 7. As a result, the initial BDFE in catechol should be 86.two kcal mol-1 in water. The second O BDFEs for the hydroquinone and catechol semiquinones are extremely similar, 65.5 kcal mol-1 and 65.4 kcal mol-1, respectively. The thermochemistry of catechols is distinctive from hydroquinones partially because of the availability of an internal S/bases. A couple of examples are listed in Table 20, with an hydrogen bond (Scheme 9). The first pKa of catechol (9.26170) is just not too various from the very first pKa in hydroquinone (9.85), and for both the second pKa isChem Rev. Author manuscript; available in PMC 2011 December 8.NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptWarren et al.Pagelarger, as anticipated for deprotonation of an anion. Nevertheless, the second pKa for catechol (13.4170) is two pKa units larger than that of hydroquinone (11.four), due to the fact the catecholate is stabilized by the strong intramolecular hydrogen bond. The intramolecular hydrogen bond seems to become more significant in the gas phase and in non-hydrogen bond accepting solvents exactly where it will not compete with hydrogen bonding to solvent. Theoretical function indicates that the intramolecular hydrogen bond in catechol has a free energy of about -4 kcal mol-1 and, importantly, that the analogous H ond within the monoprotonated semiquinone radical is about twice as powerful (Scheme 9).171,172 As a result the reactivity of catechols may be very unique in non-hydrogen bond accepting solvents vs.Kcal mol-1. The typical O bond strengths in Table five usually do not, however, Nsient flavosemiquinones, like those of most transient radicals, usually are not basic always parallel the person O bond strengths.