Hydroquinone, retardation

In addition, this reaction is an auto-catalyzed reaction, as demonstrated by the reaction between / -bromotoluene and CuCN, which at 250°C completes only 15% within the first 60 min, while 60% more product is formed in the subsequent 30 min of reaction. Moreover, the presence of a small amount of nitrile effectively reduces the induction period. Some nitrogenous bases such as tertiary amine and pyridine also promote this reaction. It has been found that CUSO4 has a catalytic effect for this reaction, whereas hydroquinone retards the formation of aromatic nitrile, indicating that this reaction involves the divalent copper ion. However, this reaction also has a few associated problems the requirement for extreme reaction conditions (150-280°C) a harsh work-up process by heating with HCl and FeCb a stoichiometric amount of cuprous cyanide which leads to the formation of equivalent amount of copper halide which may complicate the product separation and cause the problem of waste disposal as well. ... 

Benzaldehyde is easily oxidised by atmospheric oxygon giving, ultimately, benzoic acid. This auto-oxidation is considerably influenced by catalysts tiiose are considered to react with the unstable peroxide complexes which are the initial products of the oxidation. Catalysts which inhibit or retard auto-oxidation are termed anti-oxidants, and those that accelerate auto-oxidation are called pro-oxidants. Anti-oxidants find important applications in preserving many organic compounds, e.g., acrolein. For benzaldehyde, hydroquinone or catechol (considerably loss than U-1 per cent, is sufficient) are excellent anti-oxidants. 

Propionaldehyde [123-38-6] M 58.1, b 48.5-48.7 , d 0.804, n 1.3733, n S 1.37115. Dried with CaS04 or CaCl2, and fractionally distd under nitrogen or in the presence of a trace of hydroquinone (to retard oxidation). Blacet and Pitts [J Am Chem Soc 74 3382 1952] repeatedly vacuum distd the middle fraction until no longer gave a solid polymer when cooled to -80°. It was stored with CaS04. 

Some other inhibitors from the patent literature include hydroquinone [129], ionoP [130], and quinone [131]. Other inhibitors used to stabilize MMA include butylated hydroxy toluene (BHT), phenothiazine, methylene blue, hydroxy-diphenylamine and di-/jc/<3-napthol [132]. Several good reviews of inhibition and inhibitors have been written [133-136]. The mechanisms of inhibition are subtle and complicated. For example, it has been reported that highly purified benzo-quinone acts as a retarder rather than an inhibitor [137]. It has been proposed... 

Efforts to achieve a retardation of cross-linking in elastomers are based on the general assumption of a radical mechanism for retardation cross-linking and the possibility of its inhibition by a deactivation of the reactive macromolecular radical [33]. These compounds generally contain one or more labile hydrogen atoms, which after, donation of this atom, will form relatively inactive radicals. Typical antirad agents are quinones, hydroquinones, and aromatic amines (phenyl and napthylamines). 

A possible mechanism proposed by Kuivila was based on the fact that retardation by hydroquinone has been observed 72) (see however 73)) and that optically active alkyl halides RX have been transformed into racemic RD 72). 

The reaction is retarded by the addition of the radical chain scavengers 2,6-di-/-butyl-4-methylphenol and hydroquinone. The oxidation rate is not affected by the oxygen pressure (Table 2), indicating that re-oxidation of Ce(III) in 1 is not rate limiting. The oxidation rate is first order in formaldehyde and goes through a maximum with increasing concentration of 1. All these phenomena are consistent with a chain radical mechanism of oxidation (11,12). 

Acceptors of peroxyl radicals (phenols, hydroquinones, aromatic amines) retard hydrocarbon oxidation, terminating the chains (see Part II). 

The early practical application of antioxidants was connected with the development of rubber production. The rubber is easily oxidized in air, and the first antioxidants were empirically found and used to stabilize it [1]. Empirical search for antioxidants was performed by Moureu and Dufresse [2] during the First World War. These researchers successfully solved the problem of acrolein stabilization by the addition of hydroquinone. They explained the retarding action of the antioxidant in the scope of peroxide conception of Bach and Engler (see Chapter 1). They proposed that the antioxidant rapidly reacts with the formed hypothetical moloxide and in such a way prevents the autoxidation of the substrate. 

For example, Melville [26] studied the ultrasonically induced polymerisation of monomers such as styrene, methyl methacrylate and vinyl acetate in the presence and absence of polymethyl methacrylate and found that the polymerisation rates ( 1 % conversion/h) were not substantially increased by the presence of polymer. He concluded, in contrast to Driscoll, that the degradation of polymer was not the major source of radical production. Using hydroquinone as an inhibitor, he was able to deduce, from retardation times, that the rate of radical production was 2 X 10 mol dm s. A typical value for radical production using as an example the thermal initiation of AZBN (10 mol dm ) at 60 °C is 2 x 10" mol dm s" ... 

Antioxidant. Substances that retard or inhibit autoxidation at moderate temperatures and pressures. Commonly Icnown, commercial antioxidants are aromatic amines, alkylated phenols, cresols, and hydroquinones. 

Uses Toning baths retarder in hydroquinone developer used to buffer acidity of acid solutions. Notes Keep in a tightly sealed bottle in a cool place. 

Acrolein is a substance which alters easily and in it the characteristic properties of the aldehydes—the tendencies to polymerisation and oxidation—are very pronounced. Polymerisa tion transforms acrolein into an amorphous white mass, insoluble in water and alcohol, which no longer has the irritating properties of acrolein and is known as disacryl. In order to prevent, or rather retard, this polymerisation the acrolein should be left in a somewhat impure condition, as it seems that the impurities have the property of inhibiting the change. Substances which have been found to be especially good stabilisers for acrolein are phenol, hydroquinone, benzoic acid, etc., which even if present to the extent of 1-2% check the polymerisation for many months (Moureu). 

Preparation.1 Butadiene and sulfur dioxide in the ratio of 1 2 and with the addition of 1% of hydroquinone to retard poly sulfone formation are allowed to react at 100° for 12 hrs, in a steel bomb or at room temperature for 2-3 weeks in pressure... 

---