Hydroperoxides, acid catalyzed mechanism

Mechanism of the formation of phenol by acid-catalyzed rearrangement of cumene hydroperoxide. 

The mechanism of metabolic degradation of indol-3-ylacetic acid (39) is a matter of debate. A possible route demonstrated in vitro includes oxidative decarboxylation to skatolyl hydroperoxide (40), catalyzed by horseradish peroxidase isoenzyme C (HRP-C), followed by rearrangement to 3-(hydroxymethyl)oxindole (41), as shown in equation 12 . 

Probtem 19.7 Give a mechanism for the acid-catalyzed rearrangement of cumene hydroperoxide involving an intermediate with an electron-deficient O (like R ). ... 

The mechanism proposed so far takes account of the induction period and initial stages of the reaction only, and it is difficult to see how it can account for the large amount of hydroperoxide decomposed by the sulfur compound. However, Tetralin hydroperoxide is decomposed catalytically by acids (5). Although in the absence of dilauryl thiodipropionate the decomposition of Tetralin hydroperoxide in the presence of acetic acid at 70 °C. was very slow, if the acid species is a much stronger acid than acetic—e.g., a sulfonic acid as seems likely from the nature of the products of the reaction, the rate of acid-induced decomposition may be comparable with the rate of decomposition by the sulfur compound. Some evidence that acid-induced decomposition does occur at some stage in the over-all reaction is found in the presence of an ortho substituted aromatic compound in the solid product of the reaction. The acid catalyzed decomposition of Tetralin hydroperoxide follows the path of Reaction 14 (5) to give y-(o-hydroxyphenyl)butyraldehyde. This forms a brown resin which is mainly the aldol of this aldehyde (cfthe resin obtained in this work). 

Problem 28.4 When a-phcnylcthyl hydroperoxide, CcHsCHCCH.OO-OH, undergoes acid-catalyzed rearrangement in H2 0, recovered unrearranged hydroperoxide is found to contain no oxygen-18. Taken with the other evidence, what does this finding tell us about the mechanism of reaction ... 

The origin of oxygen in cleavage products-Cl and XIII) of the hydroperoxide lyase reaction presumably occurs in a manner (Fig. 4) similar to the mechanism for acid-catalyzed rearrangement of the 13-... 

AA is probably generated via the intermediate formation of 2-hydroxycyclohexanone and 6-oxohexanoic acid. The mechanism may potentially include the formation of the enol tautomer of cyclohexanone, favored by the presence of an acid (Scheme 7.4 this may also be an alternative mechanism for cyclohexanone activation). The cyclohexen-l-ol formed is then oxidized to 2-hydroperoxycyclohexanone. The hydroperoxide generates 2-hydroxycyclohexanone, which is then cleaved to 6-oxohexanoic acid. The latter is then converted into AA via monoperoxyadipic acid this step is eventually catalyzed by cobalt. Scheme 7.5 shows the steps in the mechanism of reaction. 

In addition to the S(IV) —H202 reaction, the reactions of other peroxides such as peroxymonosulfate, peroxyacetic acid, and methyl hydroperoxide with S(IV) are also sensitive to specific-acid catalysis (Hoffmann and Calvert, 1985). The rate of oxidation of S(IV) by HSO / is comparable to the rate of oxidation of S( IV) by hydrogen peroxide (Betterton and Hoffmann, 1988). We have proposed a general mechanism for the ROOH-S(IV) reaction in which the rate-determining step involves the acid-catalyzed decomposition of a peroxide-bisulfite intermediate. The rate expression applicable for this mechanism is... 

Ethylene forms explosive mixtures in air the LEE and UEL values are 2.7% and 36% by volume of air, respectively. Its reaction with fluorine is explosively violent (AH = —112 kcal/mol), and violent with chlorine (AH = —36 kcal/mol). In the presence of sunlight or UV light, an ethylene-chlorine mixture will explode spontaneously. The reaction is explosive at room temperature over the oxides of mercury or silver (Mellor 1946, Suppl. 1956). Ethylene reacts vigorously with oxidizing substances. It reacts with ozone to form ethylene ozonide, H2C(03)CH2, which is unstable and explodes on mechanical shock. Acid-catalyzed addition of hydrogen peroxide may produce ethyl hydroperoxide, which is unstable and explodes on heat or shock ... 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

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