Sterically unhindered phenols

Analysis of kinetic models for chain reactions on the oxidation of organie substrates in the presence of BHT enables the mechanism with its participation to be compared widi that of the sterically unhindered phenols discussed in the preceding section. Besides, it allows die obtaining of further insight into the reason of the inhibitor efficieney and die reeeiving of useful information to prediet new stmctures of the efficient antioxidants. 

Let us eonsider the following specifie feature of the ethylbenzene oxidation inhibited by BHT, consisting in the substantial role of the chain propagation reaction, as a result of the interaction between the phenoxyl radicals and the hydroperoxide molecule (step 27). Examination of step rates enabled to conclude that in some cases the rates of steps (17) and (27) are commensurable, that is, the reaction of peroxyl radicals with BHT is close to equilibrium imder such conditions. To all appearance, when using ort/jo-substituted phenol (BHT) unlike the sterically unhindered phenols considered in the previous section, the role of step (27) must be minor due to its relatively small rate constant. However, one must reckon... 

As an example a model of die liquid-phase oxidation of the ethylbenzene in the presence of inhibitors, the iora-substituted phenols and the butylated hydroxytoluene, was selected. The identified dynamics of die value contribution of steps in the reaction mechanism is complicated. The dominant steps for die different time intervals of the inhibited reaction were determined. The inhibition mechanism of die ethylbenzene oxidation by sterically unhindered phenols is conditioned by establishing equilibrium (7.24) in the reaction of the chain carrier, the peroxyl radical, with the inhibitor s molecule (within sufficiently wide interval of the inhibitor s initial concentration), followed by the reaction radical s quadratic termination with the participation of the phenoxyl radical. The value analysis has established that the efficient inhibitor with low dissociation energy of the phenolic 0-H bond promotes shifting the mentioned equilibrium from the chain carrier to the direction of the phenoxyl radical formation. 

Sterically unhindered nucleophiles such as phenol give the more branched product. 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

Alkyl and aryl phosphite esters are also effective melt stabilizers. They are often used in combination with hindered phenols to give highly efficient melt stabilizing systems and to reduce discoloration of the polymer because of the oxidation products of the phenols present. Phosphites (particularly those derived from aliphatic alcohols and unhindered phenols) are, however, generally susceptible to hydrolysis. Consequently, moisture-sensitive phosphites affect adversely the handling characteristics (i.e., flow properties) of the additive package and are a source of other problems corrosion of metal surfaces, formation of dark colored spots, and gel formation. In practice, hydrolysis-resistant phosphites based on sterically hindered phenols are used, e.g., AOs 17 and 18, Table 1. 

Thus, the phenolic antioxidants containing one (TP-5) or two (TP-7) iso-bornyl substituents in the o-position, unlike TPh and unhindered phenols (with tert-butyl substituent at o-position, with methyl or cyclohexyl substituents at 0,0 -positions), are not regenerated in the mixture with the sterically hindered phenols the inhibiting effect of their mixtures is additive. 

Nonsymmetric allyl substrates normally undergo substitution at the least hindered allylic position, with a selectivity that depends on the size of the nucleophile (Scheme 5-6). Sterically unhindered nucleophiles such as phenol have a slight preference instead of the more branched product. ... 

The differential reactivity of the sterically hindered and unhindered isocyanate groups of tolylene-2,4-diisocyanate facilitates the stepwise conjugation of hapten (R) and protein (P) amino groups (Fig. 3, Rn 7). jd.jj -Difluoro-m,m -dinitrobenzene (DFDNB) reacts with numerous functionalities including primary and secondary amines, imidazoles, and phenols to yield mixtures of conjugated materials (Fig. 3, Rn 8). This reaction is apparently harder to control than the diisocyanate reactions, but it is much more versatile. 

RCO2H, R OH, DCC/DMAP, EtjO, 25°C, l-24h, 70-95% yield. This method is suitable for a large variety of hindered and unhindered acids and alcohols. The use of Sc(OTf)3 as a cocatalyst improves the esterification of 3° alcohols. Carboxylic acids that can form ketenes with DCC react preferentially with aliphatic alcohols in the presence of phenols whereas those that do not show the opposite selectivity. In some sterically congested situations the 0-acyl urea will migrate to an unreactive A-acyl urea in competition with esterification. Carbodi-imide I was developed to make the urea by-product water soluble and thus easily washed out. Isoureas are prepared from a carbodiimide and an alcohol which upon reaction with a carboxylic acid give esters in excellent yield. A polymer supported version of this process has been developed. This process has been reviewed. Note that DCC is a potent skin irritant in some individuals. 

The results of the comparison are presented in Table 7.17. The steric hindrance increases the activation energy for a thermally neutral reaction by 8.2, %2, and 24 kJ mol when oxygen-centerc nitrogen-craitered, and alkyl radicals are involved respectively. The data obtained with the aid of the parabolic model. The steric effect is manifested similarly also in the reactions of sterically hindered (Ar O) phenoxyl radicals with various substrates. The reactions of the sterically hindered diphenylpicryl radical (DPPH ) and the unhindered diphenylaminyl radical may serve as another example. In the reactions of DPPH- with phenols, the contribution of the steric effect to E ranges fix>m 23 to 30 kJ mof, i.e.., is very considerable. 

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