Styrene oxides hydrolysis, mechanism

In a study by Berti et al., acid-catalyzed hydrolysis of styrene oxide was reported to occur with 67% inversion and 33% retention at the benzyl carbon.45 In a later study, it was reported that the styrene glycol product formed in the acid-catalyzed hydrolysis of chiral styrene oxide is completely racemic, which would indicate an A-l mechanism.46 As these two results indicate quite different mechanisms for this reaction, the glycol product from acid-catalyzed hydrolysis of chiral styrene oxide was converted to its bis-( + )-a-(methoxy-a-trifluoromethyl)phenylacetate diester derivative, and the composition of the diastereomeric diester mixture was determined by H NMR.47 This study agreed with those of Berti et al. and showed that acid-catalyzed hydrolysis of styrene oxide occurs with 67% inversion and 33% retention at the benzyl carbon. Acid-catalyzed methanolysis of styrene oxide is reported to occur with 89% inversion at the benzyl carbon.48 The fact that the diol product from acid-catalyzed hydrolysis of chiral styrene oxide is not completely racemic demonstrates that the lifetime of the carbocation is not sufficiently long for it to become symmetrically solvated. 

Acid-catalyzed hydrolysis of oxiranes has been reviewed extensively (59CRV737, 60QR317, B-64MI 103-01, B-72MI104-01). The kinetics and mechanism of the acid-catalyzed hydrolysis of styrene oxides... 

The proposed mechanism, which is based on the double carbonylation of styrene oxide, is shown in Scheme 6.2. The generation of an acylcobalt carbonyl complex from the reaction of cobalt tetracarbonyl anion with an alkyl halide is followed by reaction with a thiirane. This species can undergo carbonylation, the thioester function can undergo hydrolysis to reveal a sulfido nucleophile, and intramolecular cydization then produces thietan-2-one. The thietan-2-one can undergo ring cleavage and the mercapto acid results by protonahon. 

Based on a reasoned discussion of the following experimental data, propose a mechanism for the hydrolysis of substituted styrene oxides in acidic and basic media. 

With the data we have discussed until now we have plenty of information to propose a reasonable mechanism for the HsC -catalyzed hydrolysis of styrene oxides. First, we know that it is a specific acid-catalyzed reaction and second, the intermediacy of a carbocation seems very likely. When writing a reaction mecha-... 

Clearly, the OH (MeO in the methanolysis reaction) is acting as a nucleophile (not as a base) in the process. In consequence, the hydrolysis (methanolysis) of styrene oxides in basic medium is a nucleophile-catalyzed reaction, not a base-catalyzed process. This is an important point to be considered when proposing a reaction mechanism, because the nucleophile has to be involved in the rate-determining step. 

As expected, the hydrolysis (solvolysis) of / -substituted styrene oxides follows a different mechanism in acidic or basic conditions. At low pH values the reaction is a specific acid-catalyzed process and a carbocation is involved as intermediate. Under basic conditions, the hydrolysis is nucleophile-catalyzed and the attack of the nucleophile takes place at both carbons of the oxirane ring. The a-attack (more hindered position) is preferred when strong /7-electron-donating substituents are placed in the styrene ring. /7-Electron-withdrawing substituents favor the P-attack (less-hindered position). 

Iso-UP has ester bonds only in the main chain where hydrolysis occurs, so a part of reaction products from the main chain dissolves into the solution. While the crosslink formed by styrene remains unaffected because of its stable C-C bonding. As a result, the corroded surface layer resists the diffusion of NaOH solution. This mechanism is just like an oxidation of the metal at high temperature with formation of thick, cohered oxide scale, and can be expressed by similar relation of Wagner s parabolic law as shown in Equation 2. The concept of corrosion in metals can be applied in this case too. 

PVAc, PVA and PVB homopolymers as well as the different copolymers mentioned above all have a similar chemical motif in common. They exhibit an all carbon-carbon single bond backbone, which needs to be broken at some point in a potential biodegradation mechanism. With respect to the backbone, poly(vinyl ester)s are closely related to poly(olefin)s, poly(styrene)s and poly(acrylate)s. These three are known not to be biodegradable. Instead, they usually decompose by the impact of UV radiation, oxidation and hydrolysis reactions, which are not considered to be biological degradation. 

The kinetics of the chromyl chloride oxidation of the carbon-carbon double bond of styrene have been examined using stopped-flow spectrophotometry/ the product being a chromyl chloride-styrene adduct. The rate is first order with respect to each reactant and a linear free-energy relationship (Hammett correlation) has been shown to hold for a variety of substituted groups. The data are consistent with a mechanism involving electrophilic attack by the chromium(vi) species on the double bond to yield the adduct, which decomposes on hydrolysis to give phenylacetalde-hyde and benzaldehyde as the major reaction products ... 

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