Sterically hindered phenol

AcCl, NaOH, dioxane, Bu4N HSO, 25°, 30 min, 90% yield. Phase-transfer catalysis with tetra-n-butylammionium hydrogen sulfate effects acylation of sterically hindered phenols and selective acylation of a phenol in the presence of an aliphatic secondary alcohol. 

S Ar reacdons also provide an important strategy for the preparadon of various kinds of diaryl ethers -Dinitrobenzene reacts v/ith even sterically hindered phenols to give the corresponding diaryl ethers fEq 9 3 ... 

Sterically hindered phenols and other additives containing thioesters, phosphites, phosphonites and hindered amine moieties were analysed by FAB-MS and LD-FTMS. The laser desorption technique was preferred for analysis of polymer additives because of undesirable fragmentation from FAB experiments [93]. 

Primary anti-oxidants. These are sterically hindered phenols and amines (HALS) and aromatic secondary amines. They intercept the radicals as they are formed ... 

Synergism based on the mixture of a chain-breaking antioxidant (sterically hindered phenol) and a hydroperoxide decomposer (organic sulfides or phosphites). 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

The addition of an acceptor decreases the rate of POOH decomposition. The increase of added [InH] creates a tendency for k-% to decrease to the kA value, i.e., Ax —> A d at [InH] —> DC. Acceptors, which do not react with hydroperoxide groups, were used sterically hindered phenols and stable nitroxyl radicals (TEMPO) were found to be efficient acceptors. The ratio kinA(2kt)m can be calculated from the values Ax and A d according to the formula ... 

VA Roginskii. Oxidation of polyolefines inhibited by sterically hindered phenols, Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1983. 

We see that the BDE values of monosubstituted phenols vary from 338 kJ mol 1 (Y = o-NH2) to 382kJ mol (Y p-CF ). These values are sufficiently lower for sterically hindered phenols (2,6-bis(l,l-dimethylethyl)-4-Y-phenols). A few examples are as follows [4]. 

The BDE values of O—H bond of hydroperoxide depend on the substituent near the hydroperoxide group (see Chapters 2, 7, 8, and 9). The higher the value of D(ROO—H) the faster the exothermic reaction of the peroxyl radical with phenol. The values of AH of reactions of different R02 with several of the monosubstituted phenols (ArjOH) and sterically hindered phenols (Ar2OH) are collected in Table 15.1. 

Parameters of Phenols (A OH) and Sterically Hindered Phenols (Ar2OH) Reactions with Free Radicals in IPM Model [33,34]... 

As the reaction temperature increases, the equilibrium constant diminishes, since complex formation is accompanied by heat liberation. Sterically hindered phenols form loose complexes because of the impeding effect of voluminous alkyl substituents in the ortho-position. Hydrogen bonding reduces the activity of phenols, which was first observed in the studies of the effects of cyclohexanol and butanol on the inhibitory activity of a-naphthol in cyclohexane [9]. This phenomenon was investigated in detail with reference to the oxidation of methylethylketone [10]. The k7 values for some inhibitors of the oxidation of ethylbenzene and methylethylketone are given below (333 K) [10,46] ... 

We came across the influence of the steric factor earlier when discussing the difference in the reactivities of sterically nonhindered phenols (Ar OH) and sterically hindered phenols (Ar2OH) in their reactions with peroxyl radicals. The same factor, namely, the influence of... 

CH2C12 (25 ml) over a period of ca. 30 min. The mixture is stirred for 120 h (for sterically hindered phenols), filtered, and evaporated to yield the phenyl ester. 

Brewis M, Clarkson GJ, Humberstone P, Makhseed S, McKeown NB (1998) The synthesis of some phthalocyanines and napthalocyanines derived from sterically hindered phenols. Chemistry-A European Journal 4 1633-1640. 

Sterically hindered phenol (24) was elec-trochemically oxidized to afford a phenox-enium ion (25) that reacted with O- and A-nucleophiles to yield a cyclohexadienyl-protected amino acid (26) (Scheme 9) [64]. 

Sonolysis provokes electron-transfer reactions in which hindered phenols act as donors (Aleksandrov et al. 1995). Steric hindrance does not allow a donor and an acceptor to come closer and thus, prevents or significantly hampers overlapping of the corresponding orbitals. Acoustic field effect helps in overcoming this hindrance. 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

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