Trimeric paraldehyde

Nicotinic acid Nicotinic acid, pyridine-3-carboxylic acid (20.2.9) is synthesized industrially by heating a paraldehyde trimer of acetaldehyde, under pressure with ammonia, which leads to the formation of 2-methyl-5-ethylpyridine, followed by oxidation with nitric acid which gives the desired product [22-25]. 

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). 

Paraldehyde (acetaldehyde trimer, 2,4,6-trimethyl-l,3,5-trioxane) 12,5 , 124 , d 0.995, n 1.407. Washed with water and fractionally distd. 

When catalyzed by acids, low molecular weight aldehydes add to each other to give cyclic acetals, the most common product being the trimer. The cyclic trimer of formaldehyde is called trioxane, and that of acetaldehyde is known as paraldehyde. Under certain conditions, it is possible to get tetramers or dimers. Aldehydes can also polymerize to linear polymers, but here a small amount of water is required to form hemiacetal groups at the ends of the chains. The linear polymer formed from formaldehyde is called paraformaldehyde. Since trimers and polymers of aldehydes are acetals, they are stable to bases but can be hydrolyzed by acids. Because formaldehyde and acetaldehyde have low boiling points, it is often convenient to use them in the form of their trimers or polymers. 

Chemical/Physical. Oxidation in air yields acetic acid (Windholz et ah, 1983). In the presence of sulfuric, hydrochloric, or phosphoric acids, polymerizes explosively forming trimeric paraldehyde (Huntress and Mulliken, 1941 Patnaik, 1992). In an aqueous solution at 25 °C, acetaldehyde is partially hydrated, i.e., 0.60 expressed as a mole fraction, forming a gem-diol (Bell and McDougall, 1960). Acetaldehyde decomposes at temperatures greater than 400 °C, forming carbon monoxide and methane (Patnaik, 1992). 

In 1972, Eiter and his group reported the synthesis of a-alkoxy dialkyl N-nitrosamines (11),which can be obtained easily in 20-50 g quantities. This synthetic scheme works well when formaldehyde was used. In those cases when higher aliphatic aldehydes are used (e.g. acetaldehyde), the yields decreased to 3-5%. The a -alkoxy dialkyInitrosamines always contained the trimeric paraldehyde as impurity. When acetaldehyde and... 

Ethanal (acetaldehyde) polymerizes under the influence of acids to the cyclic trimer, paraldehyde, and a cyclic tetramer, metaldehyde. Paraldehyde has been used as a relatively nontoxic sleep-producing drug (hypnotic). Metaldehyde is used as a poison for snails and slugs, Snarol. Ketones do not appear to form stable polymers like those of aldehydes. 

Acetaldehyde boils near room temperature, and it can be handled as a liquid. Acetaldehyde is also used as a trimer (paraldehyde) and a tetramer (metaldehyde), formed from acetaldehyde under acid catalysis. Heating either of these compounds provides dry acetaldehyde. Paraldehyde is used in medicines as a sedative, and metaldehyde is used as a bait and poison for snails and slugs. 

When cyclopentanecarbaldehyde is prepared, it is a colourless liquid. On standing, particularly with traces of acid, it forms the crystalline trimer. The trimer is a stable six-membered heterocycle with all substituents equatorial Acetaldehyde (ethanal) forms a liquid trimer called paraldehyde , which reverts to the monomer on distillation with catalytic acid. More interesting is metaldehyde , the common slug poison, which is an all-as tetramer (2,4,6,8-tetramethyl-l,3,5,7-tetroxocane) formed from acetaldehyde with dry HC1 at below 0°C. Metaldehyde is a white crystalline solid that has all the methyl groups pseudoequatorial, and it reverts to acetaldehyde on heating. 

In order to obtain high polymers rather than the trimer paraldehyde, the acetaldehyde polymerization must be carried out at low temperatures, and a proper balance of initiator activity and choice of solvent must be maintained. With a reasonably active initiator, increasing the polymerization temperature or the polarity of the solvent favors trimerization over polymerization. 

SYNS ACETALDEHYDE, TRIMER ELALDEHYDE PARACETALDEHYDE PAR. L PARALDEHYD (GERMAN) PARALDEIDE (ITALIAN) PCHO ... 

To a hot solution of 190 g. (1.77 moles) of p-toluidine in 300 g. of concentrated hydrochloric acid is slowly added 225 g. (1.9 moles of trimer) of paraldehyde with constant stirring, and the mixture is then heated on a water bath for 1 hour. The material is extracted with ether, and the aqueous layer is then made alkaline with sodium hydroxide solution with cooling. The precipitated oil is removed by ether extraction, and the ether solution is dried over sodium sulfate and distilled. The distillate boiling in the range 150-220°/200 mm. 

It should be mentioned that under acidic conditions higher aldehydes, especially acetaldehyde with H2SO4, undergo very readily cyclo-trimerization to paraldehyde at temperatures below 0°C a small amount of the cyclic tetramer, metaldehyde, is also formed. The reaction is very fast and is accompanied by a rapid rise of temperature. As a consequence of this, the cationic acetaldehyde polymerization to linear polymers must always be carried out under conditions to avoid the trimerization using solvents of low dielectric constant, very low temperatures and initiators of appropriate activity. High dielectric constant solvents, high temperature, and very active initiators cause exclusively trimer formation. 

Paraldehyde is the cyclic trimer of acetaldehyde, a colorless or slightly yellow-colored liquid. It has been used as an anticonvulsant, but because of its adverse effects and because it is difficult to use it has been replaced by more modern agents. However, it is still sometimes used to treat status epilepticus that is resistant to first-line drugs (1). The usual adult rectal dose is 10-20 ml. 

The trimer of trichloroacetaldehyde (compare paraldehyde, p. 621) exists in two forms, N and O, which give the following nmr data. 

Metaformaldehyde The cyclic trimer of formaldehyde is usually called trioxane, but is sometimes called metaformaldehyde or trioxymethylene. The hydrated linear polymer form of formaldehyde, (CH20)n H20, is called paraformaldehyde. The trimer of acetaldehyde is called paraldehyde,... 

In the course of the reactions described in section (I) the complex silver and bismuth ions are reduced to the metal by the -CHO group of the aldehyde. No reaction occurs if this group is not present (paraldehyde). In reaction (II) the SO3 group of the fuchsinesulfurous acid is split off and adds to the aldehyde to form an a-hydroxysulfonic acid. The color of the fuchsine itself is now visible. All the normal reactions of an aldehyde are observed if acetaldehyde is set free from its trimer paraldehyde. Sulfuric acid catalyses both the forward and back reactions in this equilibrium (eqn. 1) ... 

There is no good carbonous adsorbent for aldehyde molecules. It is well known that acetaldehyde molecules eissociate to form trimer (paraldehyde) in the presence of H2SO4 ... 

Acetaldehyde undergoes polymerization reactions similar to formaldehyde. Acetaldehyde reacts with itself in the presence of H2S04 at room temperature to form paraldehyde, a trimer, though at 0 °C it forms methaldehyde, a tetramer. 

C]. Reacts violently with oxidizers. Capable of reacting as both a weak base and a weak acid. Decomposes on contact with acids, forming hydroxylamine and acetaldehyde. Forms explosive peroxides on contact with air, acids. Attacks various alkali metals (i.e., lithium, sodium, potassium, mbidium, cesium, francium). When exposed to air forms unstable peroxides may explode or decompose violently during distillation. ACETALDEHYDE TRIMER (123-63-7) see paraldehyde. 

For the preparation of an acetal, the carbonyl compound is generally used directly. However, where the aldehyde is rather volatile, a polymeric form is preferred. For example, the trimer, paraldehyde, is often used instead of acetaldehyde. In other cases, the carbonyl compound has been used as its diethyl or dimethyl acetal. It has been found that, whereas the paraldehyde-sulfuric acid reagent often yields the seven-membered, oxido-diethylidene ring (2), 1,1-dimethoxyethane or 1, l-diethoxyethane, and an acid catalyst, afford only the six-membered, ethylidene ring. 

Beilstein Handbook Reference) Acetaldehyde, trimer p-Acetaldehyde AI3-03115 BRN 0080142 DEA No. 2585 EINECS 204-639-8 Elaldehyde HSDB 3375 NSC 9799 Paracetaldehyde Paral Paraldehyd Paraldehyde Paraldehyde Draught (BPC 1973) Paraldehyde Enema (BPC 1973) Paraldeide RCRA waste number U182 1,3,5-Trimethyl-2,4,6-trioxane 2,4,6-Trimethyl-1,3,5-trioxane 2,4,6-T rimethyl-1,3,5-trioxaoyolohexane ... 

In the presence of an acid catalyst, acetaldehyde forms a trimer known as paraldehyde. Because it induces sleep when it is... 

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 ... 

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