Depolymerization of paraldehyde

Polymerization. Paraldehyde, 2,4,6-trimethyl-1,3-5-trioxane [123-63-7] a cycHc trimer of acetaldehyde, is formed when a mineral acid, such as sulfuric, phosphoric, or hydrochloric acid, is added to acetaldehyde (45). Paraldehyde can also be formed continuously by feeding Hquid acetaldehyde at 15—20°C over an acid ion-exchange resin (46). Depolymerization of paraldehyde occurs in the presence of acid catalysts (47) after neutralization with sodium acetate, acetaldehyde and paraldehyde are recovered by distillation. Paraldehyde is a colorless Hquid, boiling at 125.35°C at 101 kPa (1 atm). 

Of the preceding oxides, only those having an acid strength equal to or greater than that corresponding to an H0 of - 3.0 were catalytically active for the depolymerization of paraldehyde. 

Depolymerization of paraldehyde occurs in the presence of acid catalysts, and, after neutralization with sodium acetate, acetaldehyde and paraldehyde are recovered by distillation. 

The solution is cooled to prevent loss of acetaldehyde by vaporization. Commercial acetaldehyde may be redistilled just before it is used, or acetaldehyde may be prepared by depolymerization of paraldehyde.2... 

Write a plausible reaction mechanism for the trimerization of acetaldehyde to paraldehyde with a trace of acid. How does this mechanism compare to the acid-catalyzed depolymerization of paraldehyde ... 

Since the polymerization of acetaldehyde to paraldehyde has both forward and reverse directions, the depolymerization of paraldehyde to acetaldehyde follows the reverse mechanism of paraldehyde formation. 

In 1950, Walling (I) discovered that the solid surface of some metal sulfates changes the color of Hammett indicators, suggesting a new series of solid acids, but did not consider their usage as a possible catalyst. In 1957, Tanabe et al. 2,3) were able to affirm that the solid acidity of the metal sulfates is an intrinsic one, not arising firom any impurities, and they found that it increased remarkably on heat treatment, attained a maximum value, and then decreased at higher temperatures. Since then, many of the metal sulfates heat-treated at optimum temperatures have been used as solid catalysts for various acid-catalyzed reactions. Among those reactions are the depolymerization of paraldehyde, the polymerization of propylene and of aldehydes, the hydration of propylene, the formation of formaldehyde from methylene chloride,... 

Fig. 2. Effect of heat treatment on acidic property and catalytic activity of nickel sulfate. Dotted line shows the catalytic activity for depolymerization of paraldehyde (see Section V, A.).

Before we discuss characteristic features of a metal sulfate catalyst, it is to be noted that the model reaction should be one which has as straightforward a mechanism as possible, preferably in a homogeneously catalyzed reaction. This is the only way we can critically evaluate the efficiency of the present solid catalyst system. The depolymerization of paraldehyde was most extensively studied in view of the foregoing criterion. For the homogeneous acid catalysis of the depolymerization of paraldehyde, there are ample data given by Bell and his associates in nonaqueous solvent (by proton acid as well as Lewis acid) and also in aqueous solution (55,56). Since most Hammett indicators change their color when adsorbed on the surface acids of both Bronsted and Lewis type, it is fortunate that this depolymerization proceeds easily by acids of both types. Evidently the dotted line in Fig. 2 shows excellent... 

Correlation of the activity with the acidity of this strength is observed, such as illustrated in Fig. 2 for the NiS04 catalyzed depolymerization of paraldehyde. 

The depolymerization of paraldehyde catalyzed by nickel sulfate and cupric sulfate was shown to follow enzyme kinetics. In Fig. 7, the recip-... 

Thus, the depolymerization of paraldehyde with the metal sulfate may be represented by a scheme analogous to an enzymatic reaction, as shown below ... 

Silica-alumina has a very strong acidity of pK —8.2, but metal sulfates do not show such a strong acidity, at what ever temperature they are heat-treated. Silica-alumina, whose acidity (0.6 mmole/gm) at pK +3.3 is much larger than the acidity (0.1 mmole/gm) of nickel sulfate at the same acid strength, was found to be much less catalytically active than the latter for the depolymerization of paraldehyde (57) and for the esterification of anhydrous phthalic acid (11). This might be interpreted thus acid of moderate strength is effective but too strong acid does not act as catalyst for the reactions. 

Cogan,R.,Pipko,G. and A.Nir. Simultaneous Forced Convection, Diffusion and Reaction in a Porous Catalyst III- Depolymerization of paraldehyde. Chem.Eng.Sci. 37(1982)147-151... 

Preparation, (a) Measure 20 ml. (20 g.) of paraldehyde into a 50-ml. round-bottomed flask, add a cooled mixture of 0.5 ml. each of coned, sulfuric acid and water, attach a fractionating column, condenser, and ice-cooled receiver, and heat gently with a microburner at such a rate that acetaldehyde distils at a temperature not higher than 35°. To avoid charring of the mixture, continue only until about half of the material has been depolymerized. 

Metaldehyde [9002-91-9] a cycHc tetramer of acetaldehyde, is formed at temperatures below 0°C in the presence of dry hydrogen chloride or pyridine—hydrogen bromide. The metaldehyde crystallizes from solution and is separated from the paraldehyde by filtration (48). Metaldehyde melts in a sealed tube at 246.2°C and sublimes at 115°C with partial depolymerization. 

By contrast, when the dimer is dissolved in water, diethyl ether, acetic acid, or paraldehyde, it is depolymerized completely, so that there are no crystallization nuclei left in the solution. Hence, it is difficult to recrystallize the dimer from such solutions. When crystallization finally does take place, it leads to form (II) unless a crystal of form (I) is used as a primer. 

Fio. 6. Correlation of BSK intensity with acidities at pK = — 3.0 and catalytic activity (the first-order rate constant for paraldehyde depolymerization at 30°). O ESR intensity O acidity 0 first-order rate constant. 

A. The mechanism of toxicity is not well understood. Metaldehyde, like paraldehyde, is a polymer of acetaldehyde, and depolymerization to form acetaldehyde may account for some of its toxic effects. Further metabolism to acetone bodies may contribute to metabolic acidosis. 

Corresponding to the maximum acidity, the catalytic activities for paraldehyde depolymerization and m-2-butene isomerization show maxima at the composition of 10% M0O3. The active sites for butene isomerization are poisoned by ammonia but not by CO2, indicating that the acid sites are the active sites for the reaction. This is in contrast to the catalytic behavior of the single component oxide M0O3 in which the active sites are poisoned by CO2 but not by ammonia. 

---