Polymers of Acetaldehyde*
Paraldehyde
Paraldehyde, 2,4,6-trimethyl-1,3,5-trioxane [123-63-7], C6H12O3 , Mr 132.161, is a cyclic trimer of acetaldehyde:
Properties. Paraldehyde is colorless and has an ethereal, penetrating odor.
bp 124.35 °C mp 12.54 °C Critical temperature tcrit 290 °C Solubility in 100 g water at 13°C 12 g at 75°C 5.8 g bp 124.35 °C mp 12.54 °C Critical temperature tcrit 290 °C Solubility in 100 g water at 13°C 12 g at 75 °C 5.8 g
In the solid state, four different crystal forms exist; transition points: 230.3 K, 147.5 K, 142.7 K.
Paraldehyde is miscible with most organic solvents.
Density d204 0.9923 Refractive index n20D 1.4049 Viscosity at 20°C 1.31 mPa · s Heat of combustion at constant pressure 3405 kJ/mol Heat capacity cp at 25°C 1.947 J g–1 K–1 Entropy (l) at 25°C 2.190 J g–1 K–1 Free energy (l) at 25°C 276.4 J/g Heat of vaporization 41.4 kJ/mol Latent heat of melting 104.75 J/g Heat of formation from acetaldehyde (calculated from combustion enthalpies) –113.0 kJ/mol The equilibrium 3 acetaldehydeparaldehyde is 94.3 % on the paraldehyde side at 150 °C.
Production. Paraldehyde is produced from acetaldehyde in the presence of acid catalysts, such as sulfuric acid, phosphoric acid, hydrochloric acid, or acid cation exchangers. In the homogeneous reaction, acetaldehyde is added, with stirring and cooling, to paraldehyde containing a small amount of sulfuric acid. After the addition is completed, stirring is continued for some time to establish the equilibrium; the sulfuric acid is exactly neutralized with a sodium salt, such as sodium acetate, sodium carbonate, or sodium bicarbonate; the reaction mixture is separated into acetaldehyde, water, and paraldehyde by fractional distillation [57].
For continous production, liquid acetaldehyde at 15 – 20 °C or acetaldehyde vapor at 40 – 50 °C is passed over an acid cation exchanger [58]. Conversion is greater than 90 %. Acetaldehyde and paraldehyde are separated by distillation. For depolymerization, acetaldehyde is slowly distilled off in the presence of acid catalysts. Paraldehyde also can be decomposed in the gas phase. Catalysts are HCl, HBr, H3PO4 , or cation exchangers. The reaction is first order. Other catalysts described in the literature are Al2O3 , SiO2 , ZnSO4 , and MgSO4 [59].
Uses. Paraldehyde is used in chemical synthesis as a source of acetaldehyde whereby resin formation and other secondary reactions are largely eliminated. Such synthetic reactions are, for instance, used for the production of pyridines and chlorination of chloral. Between 1939 and 1945 paraldehyde was used as a motor fuel.
Metaldehyde
Metaldehyde [9002-91-9], C8H16O4 , Mr 176.214, is the cyclic tetramer of acetaldehyde:
Properties. Metaldehyde forms tetragonal prisms, mp (closed capillary) 246.2 °C, sublimation temperature (decomp.) 115 °C, heat of combustion at constant volume 3370 kJ/mol.
Metaldehyde is insoluble in water, acetone, acetic acid, and carbon disulfide.
Depolymerization of metaldehyde to acetaldehyde begins at 80 °C, and is complete above 200 °C. Depolymerization takes place faster and at lower temperatures in the presence of acid catalysts, such as dilute H2SO4 or H3PO4. Metaldehyde does not show the typical acetaldehyde reactions. It is stabilized by ammonium carbonate or other weakly basic compounds which neutralize acidic potential catalysts.
Production. Metaldehyde is obtained in addition to large amounts of paraldehyde during polymerization of acetaldehyde in the presence of HBr and alkaline earth metal bromides, such as CaBr2 , at temperatures below 0 °C. However, yields are scarcely higher than 8 %. Yields of 14 – 20 % have been reported when working in the presence of 7 – 15 % of an aliphatic or cyclic ether at 0 – 20 °C [60]. Insoluble metaldehyde is filtered out. Acetaldehyde is then distilled from the filtrate following depolymerization of the paraldehyde and is returned to the polymerization. Recycling of the large amounts of acetaldehyde results in losses that increase the process costs.
Uses. Metaldehyde in pellet form is marketed as a dry fuel (Meta). Mixed with a bait, metaldehyde is used today as a molluscicide.
Polyacetaldehyde
Polyacetaldehyde [9002-91-9] is a high-molecular-mass polymer with an acetal structure (polyoxymethylene structure):
By using cationic initiators, mainly an amorphous polymer is obtained. Temperatures below –40 °C are preferred in this case. Above –30 °C, mainly paraldehyde and metaldehyde are produced. The initiator activity also depends on the solvent used. Suitable initiators include H3PO4 in ether and pentane, as well as HCl, HNO3 , CF3COOH, AlCl3 in ether, and particularly BF3 in liquid ethylene [61]. Al2O3 and SiO2 also seem to be good initiators [62].
The polymer has a rubber-like consistency and is soluble in common organic solvents. It depolymerizes at room temperature, liberating acetaldehyde. It evaporates completely within a few days or weeks. Acidic compounds accelerate depolymerization, and amines (e.g., pyridine) stabilize polyacetaldehyde to a certain extent. A complete stabilization (as, for instance, in the case of polyformaldehyde) has not yet been achieved, so the polymer is still of no practical importance.
Copolymers with propionaldehyde, butyraldehyde, and allylacetaldehyde also have been produced [63]. Crystalline, isotactic polymers have been obtained at low temperatures (for example, –75 °C) by using anionic initiators [64]. Suitable initiators are alkali metal alkoxides, alkali metals, or metal alkyls in hydrocarbon solvents. The products are insoluble in common organic solvents but have an acetal structure like the amorphous polymers' [65]. Polymerization of acetaldehyde to poly(vinyl alcohol), which in contrast to polyacetaldehyde has a pure carbon backbone, has not yet been achieved [66].
Toxicology and Occupational Health
Acetaldehyde. At higher concentrations (up to 1000 ppm), acetaldehyde irritates the mucous membranes. The perception limit of acetaldehyde in air is in the range between 0.07 and 0.25 ppm [67], [68]. At such concentrations the fruity odor of acetaldehyde is apparent. Conjunctival irritations have been observed after a 15-min exposure to concentrations of 25 and 50 ppm [69], but transient conjunctivitis and irritation of the respiratory tract have been reported after exposure to 200 ppm acetaldehyde for 15 min [69], [70]. The penetrating odor, the low perception limit, and the irritation that acetaldehyde causes, give an effective warning so that no serious cases of acute intoxication with pure acetaldehyde have been reported. Acute acetaldehyde intoxication can also be observed following combined ingestion of disulfiram (Antabuse) and ethanol [73]. In animal experiments at high concentrations (3000 – 20 000 ppm), pulmonary edema and a narcotic effect become evident. The clinical course is similar to alcohol intoxication. Death occurs by breath paralysis or — with retardation — by pulmonary edema.
For rats, the LC50 (30 min inhalation) is 20 500 ppm [71]. No studies on subchronic or chronic toxicity of acetaldehyde in humans are available. Investigations of sister chromatid exchange in cell cultures [72] and in human lymphocytes [74] and studies of single- and double-strand breaks in human lymphocytes incubated with acetaldehyde [75] have revealed mutagenic effects of acetaldehyde. Long-term exposures of Syrian golden hamsters to concentrations in the range 1650 – 2500 ppm have resulted in inflammatory hyperplastic and metaplastic alterations of the upper respiratory tract with an increase in carcinomas of the nasal mucosa and the larynx [76]. Male and female Wistar rats were exposed to aldehyde concentrations of 0 ppm, 750 ppm, 1500 ppm for 6 h daily, 5 d per week for 52 weeks. In the highest dose group, the initial aldehyde concentration of 3000 ppm was reduced to 1000 ppm in the course of the study due to toxic effects. The dose-dependent effects found were increased mortality in all dose groups and delayed growth in the middle- and highest-dose group. Adenocarcinomas were observed at all three investigated concentrations. An increased rate of squamous epithelial carcinomas was only seen at 1500 ppm or more. Histological signs of irritation were observed in the larynx region in most animals from the medium- and high-dose groups [77]. Concomitant exposure to acetaldehyde also considerably increases the number of tracheal carcinomas induced by instillation of benzo[a]pyrene [72]. This suggests that chronic tissue injury is a prerequisite for tumor formation by acetaldehyde. Tumors probably do not develop if doses are not sufficient to cause tissue necrosis. In male Wistar rats exposed to 150 ppm or 500 ppm for 6 h per day, 5 d per week for 4 weeks, morphological changes in the olfactory epithelium were observed in the high-dose group [78]. However, long-term toxicity data at lower exposure concentrations are not yet available.
Currently, the TLV is 25 ppm (STEL/ceiling value) [79], and the MAK is 50 ppm [80]; the latter value is preliminary. At 50 ppm acetaldehyde, no irritation or local tissue damage in the nasal mucosa is observed. Because the mechanism of action is assumed to be analogous to that of formaldehyde, acetaldehyde is regarded as a suspected carcinogen [80].
When taken up by the organism, acetaldehyde is metabolized rapidly in the liver to acetic acid. Only a small proportion is exhaled unchanged. After intravenous injection, the halflife in the blood is approximately 90 s [81].
Paraldehyde acts as a sedative with few side effects. Ingested paraldehyde partly is metabolized to carbon dioxide and water and partly is exhaled unchanged. It generates an unpleasant odor in the expired air and therefore is not much used.
Metaldehyde decomposes slowly to acetaldehyde in the presence of acids, so ingestion may cause irritation of the gastric mucosa with vomiting. As characteristic signs of a metaldehyde intoxication, especially in children, heavy convulsions (sometimes lasting several days) have been reported, with lethal outcomes, after ingestion of several grams of metaldehyde [82]. For these reasons, those molluscicides and solid fuels which contain metaldehyde must be kept away from children.
References
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* Source: (Gerald Fleischmann, Reinhard Jira, Hermann M. Bolt, Klaus Golka) Ullmann's Encyclopedia of Industrial Chemistry ©2002 Wiley-VCH Verlag GmbH, Weinheim, Germany
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