Hydrolysis, acetal

This reaction is simply the reverse of the reaction by which acetals are formed—acetal formation is favored by excess alcohol acetal hydrolysis by excess water Acetal for matron and acetal hydrolysis share the same mechanistic pathway but travel along that pathway m opposite directions In the following section you 11 see a clever way m which acetal formation and hydrolysis have been applied to synthetic organic chemistry... 

Basing your answers on the general mechanism for the first stage of acid catalyzed acetal hydrolysis... 

Poly(vinyl alcohol) used to manufacture the poly(vinyl acetal)s is made from poly(vinyl acetate) homopolymer (see Vinyl polymers, vinyl alcohol polymers Vinyl POLYMERS, vinyl acetate polymers). Hydrolysis of poly(vinyl acetate) homopolymer produces a polyol with predominandy 1,3-glycol units. The polyol also contains up to 2 wt % 1,2-glycol units that come from head-to-head bonding during the polymeri2ation of vinyl acetate monomer. Poly(vinyl acetate) hydrolysis is seldom complete, and for some appHcations, not desired. For example, commercial PVF resins may contain up to 13 wt % unhydroly2ed poly(vinyl acetate). Residual vinyl acetate units on the polymer help improve resin solubiHty and processibiHty (15). On the other hand, the poly(vinyl alcohol) preferred for commercial PVB resins has less than 3 wt % residual poly(vinyl acetate) units on the polymer chain. 

Fig. 7. Combined sulfur during preparation of cellulose acetate hydrolysis of sulfate and esters (6). Acetylation schedule A, mixer charged with linters and acetic acid B, minor portion of catalyst added C, began cooling to 18°C D, acetic anhydride added and continued cooling to 16°C E, significant portion...

Successive introduction of two methyl groups at ring carbon increases the hydrolysis rate by a factor of 10 in each step, indicating cation formation in the transition state as in acetal hydrolysis. Equilibrium protonation before hydrolysis becomes evident from an increasing rate of hydrolysis with a decreasing pH value (Table 3). Below pH 3 no further increase of rate is observed, so that protonation is assumed to be complete. 

KCN, 95% EtOH, 20° to reflux, 12 h, 93% yield.Potassium cyanide is a mild transesterification catalyst, suitable for acid- or base-sensitive compounds. When used with 1,2-diol acetates hydrolysis proceeds slowly until the first acetate is removed. ... 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Fig. 8.2. Contour plot showing a favOTed concerted mechanism for the first step in acetal hydrolysis, in which proton transfer is more complete in the transition state than C—O bond breaking.

This would disperse the positive charge over several atoms and diminish the sensitivity of the reaction to substituent effects. The p values that are observed are consistent with this interpretation. Whereas p is —3.25 for acetal hydrolysis, it is only —1.9 for hemiacetal hydrolysis. ... 

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

An alternate method of producing the 21-hydroxy-20-ketone consists in lithium aluminum hydride reduction of the dimethyl acetal, hydrolysis to the 20-hydroxy-21-aldehyde and rearrangement, preferably via the bisulfite addition product... 

NaBH4, LiCl, diglyme. A diacylated guanidine was not deacylated under these conditions, whereas the usual basic conditions for acetate hydrolysis also resulted in guanidine deacylation. ... 

The cleavage process occurs by p-quinonemethide formation after acetate hydrolysis. 

Acyloxybenzyl esters were designed to be released under physiological conditions. Porcine liver carboxyesterase efficiently releases the phosphate by acetate hydrolysis and quinonemethide formation. In a diester, the first ester is cleaved faster than the second. ... 

Naturally occurring compounds called cyanogenic glycosides, such as lotau-stralin, release hydrogen cyanide, HCN, when treated with aqueous acid. The reaction occurs by hydrolysis of the acetal linkage to form a cyanohydrin, which then expels HCN and gives a carbonyl compound-fa) Show the mechanism of the acetal hydrolysis and the structure of the cyanohydrin that results. 

The mechanism, which involves initial formation of a hemiacetalj is the reverse of that given for acetal hydrolysis (10-6) ... 

A detonation took place while eliminating the excess of vinyl acetate by distillation, under reduced pressure, that was explained as before (formation of peracetic acid after acetate hydrolysis). 

Cyclohexane 3.2 OS-phosphate buffer pH 7.2 (1/1) Wheatgerm lipase Prenyl acetate hydrolysis — 0.68 7... 

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

In the case of the salt of a weak acid and a weak base HAB, e.g. ammonium acetate, hydrolysis of both the ions occurs ... 

This is known as specific add catalysis, specific in that H30 is the only acidic species that catalyses the reaction the reaction rate is found to be unaffected by the addition of other potential proton donors (acids) such as NH4 , provided that [H3Offi], i.e. pH, is not changed, indirectly, by their addition. The mechanism of the above acetal hydrolysis is believed to be,... 

Scheme 5.14 One-pot cascade involving acid-catalyzed acetal hydrolysis followed by the addition of methyl vinyl ketone (MVK) in an amine-catalyzed Baylis-Hillman reaction. The acid and base used were synthesized as in Scheme 5.13 [30],...

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