Hydrolysis of acetal

Via Reactions Affecting the Organic Moiety 10.2.1. Hydrolysis of Acetals 

Acetals of 2-formyl- or acetylphenyl dimethyl telluronium iodides were hydrolyzed in acidic medium to 2-formyl- or acetylphenyl dimethyl telluronium iodides.  

Dimethyl 2-FormyIphenyI Telluronium Iodide 10 g (2.1 mmol) of 2-(diethoxymethyl)phenyl dimethyl telluronium iodide are added to 100 m/of 4 molar hydrochloric acid, the mixture is heated with stirring for 2h, cooled, and the solid is filtered off yield 7.4g (90%) m.p. 125-130° (from glacial acetic acid or methanol). 

Similarly prepared were 2-acetylphenyl dimethyl telluronium iodide (90% yield m.p. 145-150°) and 4-acetylphenyl dimethyl telluronium iodide (80% yield m.p. 151 -154°) from the corresponding dimethylene acetals.  

The acetals and diacetals of various aldehyde or dialdehydes were reported to be hydrolyzed to the corresponding aldehydes in the presence of strongly acidic cation-exchange resins. Yields of aldehydes are given in Table 4.21.The hydrolysis with the cation-exchange resins takes place in an aqueous medium under milder conditions than in the presence of acids. 

The hydrolysis of the diethylacetals of acetaldehyde (I), croton aldehyde (II), ben-zaldehyde (III), citral (IV) was realized with stirring at 293 K in the presence of the cation-exchange resin. The tetraethyl diacetals of glutaraldehyde (VI), 1,2-diformyl-f n-dichlorocyclopropane (VII), and fumaraldehyde (VIII) are hydrolyzed in a similar manner. 

The hydrolysis of phenyl propagyl aldehyde diethyl acetal (V) is realized at 363 — 373 K with simultaneous distillation. In the case of the diacetals of unstable dialdehydes, e.g., malon aldehyde (IX), hydrolysis occurs in the presence of aniline hydrochloride, and 3-amilino-propenylanilinium chloride (X) is formed. 

The proposed mechanisms for acetal hydrolysis can be tested with the tools used to study other organic reactions. An A1 mechanism proceeding through transition structure II would lead to a carbocation centered on the departing R group, so racemization of a chiral center would be expected. On 

One method of generating enols involves photohydration of phenylacetylene, while another method utilizes the Norrish Type n cleavage of y-hydroxybutyrophenone. Photochemical reactions will be discussed in Chapter 12. 

Gibbs diagram for A1 hydrolysis of formaldehyde dimethyl acetal. (Adapted from reference 152.) 

In contrast to acetals, hemiacetals can revert to aldehydes and alcohols through both acid and base catalysis. Some catalysts incorporate both acid and base functions and can serve as bifunctional catalysts. This type of 

A small positive or negative charge might be present, depending on the relative rates of bond making and bond breaking in the transition structure. 

---

The disadvantages attending the use of acetic anhydride alone are absent when the acetylation is conducted in aqueous solution according to the following procedure. The amine is dissolved in water containing one equivalent of hydrochloric acid, slightly more than one equivalent of acetic anhydride is added to the solution, followed by enough sodium acetate to neutralise the hydrochloric acid, and the mixture is shaken. The free amine which is liberated is at once acetylated. It must be pointed out that the hydrolysis of acetic anhydride at room temperature is extremely slow and that the free amine reacts much more readily with the anhydride than does the water this forms the experimental basis for the above excellent method of acetylation. 

The acetylation reaction, [1], is carried out in pyridine to avoid the hydrolysis of acetic anhydride by water. After the acetylation is complete, water is added to convert the remaining acetic anhydride to acetic acid, [2]. 

Optically Active Alcohols and Esters. In addition to the hydrolysis of esters formed by simple alcohols described above, Hpases and esterases also catalyze the hydrolysis of a wide range of esters based on more complex and synthetically useful cycHc and acycHc alcohols (Table 5). Although the hydrolysis of acetates often gives the desirable resolution, to achieve maximum selectivity and reaction efficiency, comparison of various esters is recommended. 

The relative rates of hydrolysis of acetate, chloro-, dichloro-, and trichloroacetates have been compared and give the following relative rates 1 760 1.6 x 10 10. ... 

Historically, simple Vz-alkyl ethers formed from a phenol and a halide or sulfate were cleaved under rather drastic conditions (e.g., refluxing HBr). New ether protective groups have been developed that are removed under much milder conditions (e.g., via nucleophilic displacement, hydrogenolysis of benzyl ethers, and mild acid hydrolysis of acetal-type ethers) that seldom affect other functional groups in a molecule. 

Hammett treatments show good correlations with large negative p values for the hydrolysis of acetals of aromatic aldehydes. This is consistent with the development of a positive charge at the carbonyl center in the rate-determining step. 

The overall reaction stoichiometry having been established by conventional methods, the first task of chemical kinetics is essentially the qualitative one of establishing the kinetic scheme in other words, the overall reaction is to be decomposed into its elementary reactions. This is not a trivial problem, nor is there a general solution to it. Much of Chapter 3 deals with this issue. At this point it is sufficient to note that evidence of the presence of an intermediate is often critical to an efficient solution. Modem analytical techniques have greatly assisted in the detection of reactive intermediates. A nice example is provided by a study of the pyridine-catalyzed hydrolysis of acetic anhydride. Other kinetic evidence supported the existence of an intermediate, presumably the acetylpyridinium ion, in this reaction, but it had not been detected directly. Fersht and Jencks observed (on a time scale of tenths of a second) the rise and then fall in absorbance of a solution of acetic anhydride upon treatment with pyridine. This requires that the overall reaction be composed of at least two steps, and the accepted kinetic scheme is as follows. 

Sketch the reaction progress diagram for the pyridine-catalyzed hydrolysis of acetic anhydride. 

Direct detection of an intermediate. A nice example, the pyridine-catalyzed hydrolysis of acetic anhydride, was discussed in Chapter 1. Spectroscopic techniques are of great value, because they do not perturb the kinetic system, and because they are selective and sensitive. If the intermediate can be detected, the time course of its appearance and disappearance may be followed. 

Figure 4-12. Stopped-flow study of the pyridine-catalyzed hydrolysis of acetic anhydride, showing the formation and decay of the acetylpyridinium ion intermediate. Initial concentrations were 0.087 M pyridine, 2.1 x im M acetic anhydride the pH was 5.5 ionic strength, 1.0 M temperature, 25 C. Five hundred data points tabsorbance at 280 nm) were measured in I s. The smooth curve is a ht to Eq. (3-27). Source Data of D. Khossravi and S.-F. Hsu, University of Wisconsin.

Figure 4-12 shows a stopped-flow study of the pyridine-catalyzed hydrolysis of acetic anhydride. The absorbance-time curve reveals tbe formation and decay of the reactive intermediate acetylpyridinium ion. 

A further observation is the fact that differences in rates of nitration between the reagents prepared at different temperatures tended to zero as the water concentration of the added nitric acid was decreased to zero73. It has been argued that, since the acid-catalysed hydrolysis of acetic anhydride must be very rapid at 25 °C and removes water which initially competes with acetic anhydride and acetyl nitrate for protons, this removal permits equilibria (30) and (31) to be displaced towards products. The more anhydrous the nitric acid, the less important is this initial hydrolysis of the acetic anhydride and so the difference in the nitrating power of the differently prepared mixtures becomes less. When reagents are mixed at low temperatures, the hydrolysis of the anhydride is very slow, but once this is accomplished, formation of the protonated acetyl nitrate and subsequent nitration is rapid as observed73. 

This is a chloromethylation reaction and is consequently listed at 11-24. However, in the course of the reaction formaldehyde is generated from the acetal. This reaction is not listed at 10-6 (hydrolysis of acetals), because it is not really a preparation of formaldehyde. 

Hydrolysis of Acetals, Enol Ethers, and Similar Compounds ... 

The hydrolysis of acetals and ortho esters is governed hy the stereoelectronic control factor previously discussed (see A and B on p. 427) though the effect can generally be seen only in systems where conformational mobility is limited, especially in cyclic systems. There is evidence for synplanar stereoselection in the... 

Enol ethers are readily hydrolyzed by acids the rate-determining step is protonation of the substrate. However, protonation does not take place at the oxygen but at the p carbon, because that gives rise to the stable carbocation 104. After that, the mechanism is similar to the A1 mechanism given above for the hydrolysis of acetals. 

Chromium (III) oxide hydrate had been introduced into acetic anhydride causing a very violent hydrolysis of acetic anhydride and the spreading of the products. 

HYDROL - Batch Reactor Hydrolysis of Acetic Anhydride... 

BATCH HYDROLYSIS OF ACETIC ANHYDRIDE EXAMPLE OF REACTION WITH HEAT EFFECTS UNDER ADIABATIC CONDITIONS... 

A good deal of heat is evolved when the hydrochloric acid is added to the reaction mixture, owing to the hydrolysis of acetic anhydride. The reaction mixture will become excessively hot unless it is cooled in an ice bath. 

A. Corma, M. J. Climenti, H. Garcia, and J. Primo, Formation and hydrolysis of acetals catalysed by acid faujasites, Appl. Catal., 59 (1990) 333-340. 

First determine [OH ] resulting from the hydrolysis of acetate ion. 

The hydrolysis of acetic anhydride is being studied in a laboratory-scale continuously stirred tank reactor (CSTR). In this reaction acetic anhydride [(CH3C0)20] reacts with water to produce acetic acid (CH3COOH). 

Consider the hydrolysis of acetic anhydride carried out in dilute aqueous solution in a batch reactor ... 

---