Metaldehyde

Acetaldehyde also polymerises gradually on keeping, yielding the liquid paraldehyde (CH3.CHO)3, which boils without decomposition at 124°. If a little hydrogen chloride is passed, with cooling, into acetaldehyde the likewise polymeric metaldehyde crystallises out. 

Experiment 8.—To 5 c.c. of freshly distilled acetaldehyde in a moderate-sized conical flask one drop of concentrated sulphuric acid is added with cooling. When the vigorous reaction is over, the paraldehyde produced is shaken in a small separating funnel with water in order to remove the sulphuric acid, and the polymeride, which is insoluble in water, is separated if necessary by extraction with ether. After being dried with a little calcium chloride the substance is distilled from a small distilling flask. Boiling point 124°. 

Conversely paraldehyde can be reconverted into acetaldehyde by adding a few drops of concentrated sulphuric acid and distilling from the water bath through a column. 

By this means fresh acetaldehyde can be prepared whenever required. 

Experiment 9.—Pure paraldehyde is tested by the previously described aldehyde reactions with ammoniacal silver nitrate, fuchsine-sulphurous acid, and bisulphite solutions. All are negative. 

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PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

Dissolve 2 ml. of acetaldehyde in 5 ml. of dry ether, cool in a freezing mixture of ice and salt, and pass in dry hydrogen chloride gas for 30-60 seconds. The solid polymer, metaldehyde, may separate in a short time, otherwise cork the tube and allow it to stand for 10-15 minutes. Filter ofiF the crystals. 

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. 

Metaldehyde is flammable and a dangerous fire risk. It is highly toxic and a strong irritant to skin and mucous membranes (Refs 3 4). 

Metal deactivation, in antidegradant selection, 22 787 Metal deactivators antioxidants, 3 115 in gasoline, 22 407 for lubricating oil and grease, 15 221 Metaldehyde, 2 103... 

At low temperatures a second polymeric form of acetaldehyde, the beautifully crystalline metaldehyde, is formed. 

Experiment 10.—A few bubbles of hydrogen chloride are passed into a solution of a few cubic centimetres of acetaldehyde in twice as much absolute ether kept cold in a freezing mixture. After a short time the metaldehyde separates in magnificent needle-shaped crystals which are filtered with suction and washed with ether. The filtrate yields a second crop on repeating the treatment. 

Like paraldehyde, metaldehyde can be preserved, and, when freshly prepared, is odourless. It also has no aldehydic properties. On keeping, however, a distinct odour of acetaldehyde becomes evident —a sign that here also an equilibrium is slowly being established. Metaldehyde can be completely depolymerised by heating. Molecular weight determinations (in phenol) show that metaldehyde is tetra-molecular (Hantzsch) the examination of the space lattice of crystals by the method of Laue and Bragg points to the same conclusion (Mark). 

Metaldehyde is prepared on a technical scale for use as fuel ( solid methylated spirits ). 

Like formaldehyde, acetaldehyde easily forms polymers, in this case paraldehyde and metaldehyde. Paraldehyde will form when hydrochloric or sulfuric acid is added to acetaldehyde. Polymerization of acetaldehyde to mec-aldehyde occurs in the gaseous phase in the presence of aluminum oxide or silicon dioxide catalyst. 

Metaldehyde is a white solid that will also decompose partially to acetaldehyde, but only if it is heated above 175°F. It is hazardous, flammable, and of little commercial use. 

ChemicaPPhysical Metaldehyde was converted to acetaldehyde by heating to 150 °C for 4-5 h or by the reaction of concentrated hydrochloric acid (6 M) for a couple of minutes (Booze and Oehme, 1985). 

Booze, T.F. and Oehme, F.W. Metaldehyde toxicity a review, Vet Human Toxicol, 27 11-19,1985. 

Iwata, Y, Carman, G.E., Dinoff, T.M., and Gunther, F.A. Metaldehyde residues on and in citrus fruits after a soil broadcast of a granular formulation and after a spray application to ciUus Uees. J. Agric. Food Chem., 30(3) 606-608, 1982. 

Shih CC, Chang SS, Chan Y-L, Chen JC, Chang MW, Tung MS, Deng JF, Yang CC, Acute metaldehyde poisoning in Taiwan, Vet Human Toxicol 46 140—143,... 

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