Hydroquinone, as antioxidant

There are many examples in the literature on applications of 1,4-hydroquinones as polymer stabilizers and as antioxidants. The natural ubiquinols are 2,3-dimethoxy dialkyl derivatives of these hydroquinones and these natural compounds are now known to be of great importance in biological systems. We select a few examples of 1,4-hydroquinones as antioxidants to illustrate the effect of structure (e.g. substituents) on their reactivity, but especially to emphasize the role that hydrogen bonding plays in the reactivity of catechols, 1,4-hydroquinones and methoxy derivatives. 

Styrene Dry cinnamic acid, together with a little hydroquinone as antioxidant, is vigorously boiled in a round-bottomed flask carrying a fractionating column attached to a descending condenser, the dimensions of the assembly being such that the temperature at the head of the column reaches 120°. Styrene collects in a receiver to which also a little hydroquinone has been added as stabilizer this material is distilled in steam, and the colorless oil obtained is dried over calcium chloride and distilled under diminished pressure (b.p. 44—46740 mm yield 38 41 %). 

Figure 3-33 H NMR spectrum of cyanoacrylate and hydroquinone as antioxidant, 300 MHz, solvent CDCl3 enzene-d6(5 l) "SSL".

Suzuki and Wada explored the Knoevenagel condensation of malononitrile and methyl cyanoacetate under solvent-free conditions (Scheme 2.45) [40]. Different alkaline earth carbonates and fluospar were used as mild catalysts for the condensation of 2 equiv. of malononitrile and 1 equiv. of aromatic aldehyde to obtain products in high yields, after short milling (30 min. Table 2.42). When methyl cyanoacetate was ball milled with aldehydes, the reaction occurred with the stereoselective formation of (fl-cyanocinnamic esters 133. However, significant oxidative loss of aldehydes was encountered. The addition of a small amount of hydroquinone as antioxidant was found to solve the problem and drastically increase yields. 

To 7 g (20 mmol) of the mixture of 3 and its regioisomers in a 50 ml four-necked flatbottomed flask equipped with a Hickmann distilling head, mechanical stirrer, thermometer and gas inlet, were added 20 ml of a solution of 15 % NaOAc in 90 % aq. acetic acid and a trace of hydroquinone as antioxidant. The mixture was heated to 95-100°C for 3h, during this time 6 ml of a mixture of methanol, ethanol and some acetic acid were distilled off (TLC control silica 60 F254, hexane/Et20 2 1). 

Dr. Berger omitted the x in this term. If there is an x in the first equation there should also be an x in the second equation. If we refer to a phenolic antioxidant (on which Dr. Berger concentrates his arguments), such as hydroquinone, as AH2 (with AH being the semiquinone radical), then the two equations would be... 

By performing the oxidation at elevated temperatures the phenols which would ordinarily yield polymers are converted instead to diphenoquinones. These quinones are readily reduced to the corresponding hydroquinones, compounds which promise to be useful as antioxidants and polymer intermediates. 

The activities of all compounds from the hydroquinone series studied as antioxidants in isotactic polypropylene were lower than that of pyro-catechol. A similar conclusion was reached even when comparing both groups of substances as stabilizers in y-irradiated isotactic polypropylene... 

The presence of antioxidants in eluents and extraction solvents Antioxidants can be readily oxidized electrochemically and generate high background currents or interfering broad peaks. Thus, eluents and extraction solvents containing such compounds should be either avoided or purified before use. For example, ethers, such as diethyl ether, diisopropyl ether, and tetrahydrofuran are likely to contain up to 0.1% (w/v) pyrogallol or quinol (hydroquinone) as stabilizer. If the stabilizer is removed, peroxides will form and their concentration will increase with time unless the solvent is stored under nitrogen. Not only do peroxides present a hazard from explosion, but they may also oxidize susceptible analytes. Methyl f-butyl ether (MTBE), on the other hand, is stable to oxidation. 

The amine antioxidants discussed above have the problem that they discolor the rubber products. They are not suitable for products intended to be light colored. With the development of synthetic rubber in the 1930s and 1940s, which was initially white from its polymerization reactors, it became even more desirable to develop non-staining antioxidants [ 1 ]. Substances such as hydroquinone, resorcinol, 1-naphtol, and 2-naphthol had been claimed as antioxidants early in the century [6, 7]. By the 1940s tris(nonyl phenyl)phosphite (Polygard )... 

Numerous examples of couplings between hydroquinone, paro-quinone or toluquinone and a linear terpenic chain have been observed in Dictyotales and Fucales. Most prenylquinones and prenylated hydroquinones have a diterpenic chain and a methyl on the aromatic ring. The analogy with tocopherols and vitamin K is clear, and these compounds may be active as antioxidants and/or antiradicals (Isler and Montavon, 1965 Fisch et al, 2003). The following examples are intended to illustrate these observations. 

Butylated Hydroxyanisole. 2- and 3-/ i -Butyl-4-methoxyphenol (butylated hydroxyanisole (BHA)) is prepared from 4-methoxyphenol and tert-huty alcohol over siUca or alumina at 150°C or from hydroquinone and tert-huty alcohol or isobutene, using an acid catalyst and then methylating. It is widely used in all types of foods such as butter, lard, and other fats, meats, cereals, baked goods, candies, and beer as an antioxidant (see Antioxidants Eood additives). Its antioxidant properties are not lost during cooking so that flour, fats, and other BHA-stabiLized ingredients may be used to produce stabilized products. 

Antioxidants are not important only to the health conscious food manufacturers also rely on these chemicals to maintain the shelf life of their products. Synthetic antioxidants such as butylated hydroxyanisole, butylated hydroxytoluene, propyl gallate and tert-butyl hydroquinone were widely used in food processing to control oxidation and maintain food quality. However, as these synthetic antioxidants are suspected to be carcinogenic they now have restricted use in food (Madahavi and Salunkhe, 1995). Therefore, natural antioxidant sources, especially of plant origin, are of great interest to the food industry. 

The rare example of synergistic action of a binary mixture of 1-naphthyl-A-phcnylaminc and phenol (1-naphthol, 2-(l,l-dimethylethyl)hydroquinone) on the initiated oxidation of cholesterol esters was evidenced by Vardanyan [34]. The mixture of two antioxidants was proved to terminate more chains than both inhibitors can do separately ( > /[xj). For example, 1-naphtol in a concentration of 5 x 10 5 mol L-1 creates the induction period t=170s, 1 -naphthyl-A-phenylamine in a concentration of 1.0 x 10-4 mol L 1 creates the induction period t = 400s, and together both antioxidants create the induction period r = 770 s (oxidation of ester of pelargonic acid cholesterol at 7= 348 K with AIBN as initiator). Hence, the ratio fs/ZfjXi was found equal to 2.78. The formation of an efficient intermediate inhibitor as a result of interaction of intermediate free radicals formed from phenol and amine was postulated. This inhibitor was proved to be produced by the interaction of oxidation products of phenol and amine. 

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

Aniline is an aromatic amine used in the manufacture of dyes, dye intermediates, rubber accelerators, and antioxidants. It has also been used as a solvent, in printing inks, and as an intermediate in the manufacture of pharmaceuticals, photographic developers, plastics, isocyanates, hydroquinones, herbicides, fungicides, and ion-exchange resins. It is produced commercially by catalytic vapor phase hydrogenation of nitrobenzene (Benya and Cornish 1994 HSDB 1996). Production of aniline oil was listed at approximately 1 billion pounds in 1993 (U.S. ITC 1994). Chemical and physical properties are listed in Table 1-2. 

Inhibitors can be injected into the system in order to kill active species present, for example, by neutralizing the catalyst or by capturing free radicals in a polymerization. For example, the Lewis acid, BF3-complex can be killed using gaseous NH3 since the inactive compound BF3 NH3 is formed, and the reaction stops for lack of active centers. An antioxidant such as hydroquinone can be used to capture peroxide radicals to control reactions involving vinyl-type monomeric substances. 

The mechanism of formation of PCBs from the trichlorobenzene solvent involves hydrogen abstraction by the urea present, leading to the formation of trichlorophenyl radicals. Two such radicals can then combine (Scheme 2.2) to form a PCB molecule (2.36). This reaction can be suppressed by adding antioxidants such as hydroquinone or sodium phosphite to yield hydrogen (H ) radicals, which convert (Scheme 2.3) the trichlorophenyl radicals back to the parent substance (2.37). 

Mecfianism of Action The mechanism of action is not fully understood. Monobenzone maybe converted to hydroquinone, which inhibits the enzymatic oxidation of tyrosine to DOPA it may have a direct action on tyrosinase, or it may act as an antioxidant to prevent SH-group oxidation so that more SH groups are available to inhibit tyrosinase. Therapeutic Effect Depigmentation in extensive vitiligo. 

---