Hydroquinone dianion

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

Formal potentials can be defined on different levels of conditions Thus the formal potential of the -> quinhydrone electrode may be defined (I) as including (a) the standard potential of the hydroquinone di-anion/quinone system, (b) the two acidity constants of the hydroquinone, and (c) the activity coefficients of the hydroquinone dianion and quinone, or, (II), it may also include (c) the pH value. In the latter case, for each pH value there is one formal potential, whereas in the first case one has one formal potential for all pH values, and an equation describing the dependence of the electrode potential as a function of that formal potential and the individual pH values. Formal potentials are strictly thermodynamic quantities, and no kinetic effects (e.g., by electrochemical -> irreversibility) are considered. 

The reduction of benzoquinone in NMA at a platinum wire electrode has been studied143). In unbuffered solutions the reduction gave two fast, one-electron waves of almost equal height ( 1 2 = —0.04 V and —0.38 V versus a normal hydrogen reference electrode). It was proposed that these represented the formation of the radical anion and of the hydroquinone dianion, respectively. In buffered solutions an irreversible two-electron reduction (probably to hydroquinone) was found. It thus appears that certain radical anions may be much more stable in NMA than in water. 

It is proposed that the semiquinone is an active developing agent, estimated by Umberger [64] to be 10 times as active as the hydroquinone dianion. The low sulfite level in Lith developers allows the p-benzoquinone (Q) to build up, which boosts the semiquinone concentration via the dismutation reaction. The increased semiquinone level accelerates development which produces more p-benzoquinone and semiquinone according to Eqs. (46), (47),... 

Alkylation. The topological analogies outlined in the previous section are substantiated further by considering the chemistry of metal dithienes. We first discuss the alkylation of anionic species. This reaction is equivalent to the alkylation of the hydroquinone dianion and aflFords members of a class of new coordination compounds (14). With simple alkyl halides the d metal dithiene dianions aflFord the 1,4-S-dialkylderiva-tives (16). With a,a>-dibromoalkanes at high dilution new chelates of type 17 were obtained for x = 5-12 (3). With smaller values of x or... 

The absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

HjCat represents catechol or hydroquinone, Q quinone, SQ semiquinone anion, HCat- catechol monoanion, and Cat2 catechol dianion DTBQ represents 3,5-di-rerr-butyl-o-quinone, o-Q o-benzoquinone, p-Q p-benzoquinone, TCQ tetrachloro-o-benzoquinone, and TFQ tetrafluoro-o-benzoquinone. 

Equimolar amounts of diphenyl tellurium benzenesulfonimides and 1,4-dihydroxybenzene in anhydrous chloroform at 20° for 30 min yielded, among other compounds, a pale-yellow substance that is insoluble in benzene and consists of a diphenyl tellurium group and the dianion of hydroquinone ... 

The 5,8-dihydroxy-4(3//)-quinazolinones possess three protons capable of dissociation in aqueous buffer, viz. N3—H and the protons of the 5- and 8-hydroxyl groups (Scheme 107). Initial acid dissociation of the N3 proton does not take place at low pH because electron-rich substituted quinazolin-4(3//)-ones possess pK values greater than 10. The first dissociation is from the 5-hydroxyl which affords an anion (697) which is stabilized by internal hydrogen bonding expressed by pK 7.3 (7.8). Cleavage of the halogen-carbon bond in the 2-substituent in (697) involves a rate-determining reaction of the hydroquinone monoanion and dianion species, since there is an approximate 100-fold difference in rate constants between the precursor 2-chloro (696 X = Cl) and... 

The product and mechanistic studies of oxidation of important pyrocatechols and hydroquinones by oxygen in an alkaline medium189-19S give information on the types of radical intermediates formed. It was found by means of flow ESR methods196 197 that the radical CXXXVII arises in aquenous alkaline medium in the oxidation of 2-tert-butylhydroquinone CXXXVI, which is one of the most efficient hydroquinone antioxidants. As a secondary radical, the dianion radical CXLI was detected in a static arrangement, which is formed via intermediates... 

CXXXIX and CXL (R = H). The reaction proceeds analogously in aqueous-alcoholic medium (in intermediates CXXXIX and CXL, R = Me or Et). Analysis of ESR spectrum of the secondary radical as well as the model oxidation of 2-tert-butyl-6-hydroxy-hydroquinone CXLII confirmed the structure of the dianion radical CXLI. Also the oxidation of other 2-tert-alkyl-, 2-sec.alkyl-, or 2-n-alkylhydroquinone proceeds by the same mechanism1981. The oxidation can be carried out preparati-vely194,195,198) 2-hydroxy-6-alkyl-l,4-benzoquinonesof type CXLIII are formed in the yield of 65 to 87%. 

Reaction of [Fe" salen] with p-quinones gives, in contrast, the binuclear complexes [Fe2(salen)2Q]. It was shown by a variety of properties (magnetic measurements and IR, ESR and Mossbauer spectra) that the compounds consist of high-spin Fe" ions bridged by the dianion Q (structure 33) of the hydroquinone which transmits weak ( — J< 6cm ) antiferromagnetic interaction. This structure has been confirmed, for the case of Q = the dianion of hydroquinone, by a single crystal structure determination. ... 

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