Paraldehyde

Paraldehyde (2,4,6-trimethyl-1,3-5-trioxane, boiling point 125.4°C), a cyclic trimer of acetaldehyde, is formed when a mineral acid, such as sulfuric acid, phosphoric acid, or hydrochloric acid, is added to acetaldehyde. Paraldehyde can also be formed continuously by feeding liquid acetaldehyde at 15 to 20°C over an acid ion-exchange resin. 

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

---

On acetylation it gives acetanilide. Nitrated with some decomposition to a mixture of 2-and 4-nitroanilines. It is basic and gives water-soluble salts with mineral acids. Heating aniline sulphate at 190 C gives sulphanilic add. When heated with alkyl chlorides or aliphatic alcohols mono- and di-alkyl derivatives are obtained, e.g. dimethylaniline. Treatment with trichloroethylene gives phenylglycine. With glycerol and sulphuric acid (Skraup s reaction) quinoline is obtained, while quinaldine can be prepared by the reaction between aniline, paraldehyde and hydrochloric acid. 

OH groups are in the para or 1,4 position to each other. This use of the prefix is confined to disubstituted benzene derivatives in such cases as para-hydrogen and paraldehyde the prefix has no uniform structural significance and is always written in full. 

It is convenient to replace the volatile free acetaldehyde by paraldehyde, which by dissociation in the reaction-mixture generates acetaldehyde in situ. 

Add 60 ml. of concentrated hydrochloric acid with shaking to 30 ml. of aniline in the flask, cool the mixture to about 50, and then add 45 ml. of paraldehyde and some fragments of unglazed porcelain assemble the apparatus without delay in a fume-cupboard with the condenser inclined at an angle of about 60°. 

Aldehydes. Formaldehyde, metaformaldehyde, acetaldehyde, paraldehyde, chloral hydrate, benzaldehyde, salicylaldehyde (and other substituted benzaldehydes). 

Acetaldehyde, CH3CHO, b.p. 21°, is generally used in aqueous solution, which has also a characteristic odour paraldehyde, (CH3CHO)3, is a liquid polymer, b.p. 124°, slightly soluble in water, odour similar to that of acetaldehyde, but less intense. Chloral, CCI3CHO, a liquid, is almost invariably encountered as the stable solid hydrate , CCl3CH(OH)2, m.p. 57 . Both have a characteristic odour the hydrate is readily soluble in water. 

Action of sodium hydroxide, (a) Warm 1 ml. of acetaldehyde with a few ml. of cone, (c.g., 30%) NaOH solution. A yellow resin, having a characteristic odour of bad apples, is formed. Paraldehyde slowly gives a yellow resin. 

With paraldehyde and the aromatic aldehydes (being insoluble in water), it is advisable to warm the mixture gently on a water-bath, shaking the tube vigorously from time to time to break up the oily globules of the aldehyde. 

To I ml. of paraldehyde in a test-tube add 5 ml. of dil. HjSO and distil gently into cold water contained in another test-tube (Fig. 68, p. 320), An aqueous solution of acetaldehyde is thus obtained. 

Almost insoluble in cold water. Higher alcohols (including benzyl alcohol), higher phenols (e.g., naphthols), metaformaldehyde, paraldehyde, aromatic aldehydes, higher ketones (including acetophenone), aromatic acids, most esters, ethers, oxamide and domatic amides, sulphonamides, aromatic imides, aromatic nitriles, aromatic acid anhydrides, aromatic acid chlorides, sulphonyl chlorides, starch, aromatic amines, anilides, tyrosine, cystine, nitrocompounds, uric acid, halogeno-hydrocarbons, hydrocarbons. 

TEST Formaldehyde (solution) Acetal d yde (solution) Paraldehyde Chloral hydrate Benzaldehyde Salicylaldehyde... 

The acetaldehyde should be freshly distilled (b.p. 20-5-21°). It can be conveniently prepared by depolymerising pure dry paraldehyde (see Section 111,65). 

Acetaldehyde, b.p. 21°, undergoes rapid pol5unerisation under the influence of a little sulphuric acid as catalyst to give the trimeride paraldehyde, a liquid b.p. 124°, which is sparingly soluble in water. The reaction is reversible, but attains equilibrium when the conversion is about 95 per cent, complete the unreacted acetaldehyde and the acid catalyst may be removed by washing with water ... 

Quinoline derivatives may be synthesised by heating aii aromatic amine with an aldehyde or a mixture of aldehydes in the presence of concentrated hydrochloric or sulphuric acid this synthesis is known as the Doebner - Miller reaction. Thus aniline and paraldehyde afford 2-methylquinohne or quinaldine. 

In a 1-litre round-bottomed flask, fitted with a condenser and trap (compare Fig. II, 13, 8), place 62 g. (61 ml.) of aniline. Cool the flask in an ice bath, add 120 ml. of concentrated hydrochloric acid slowly, followed by 90 g. of paraldehyde swirl the contents of the flask to ensure thorough mixing. Remove the flask from the ice bath and shake it frequently at... 

Paraldehyde Alkalies, HCN, iodides, nitric acid, oxidizers... 

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. 

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids. 

Anesthesia. Materials that have unquestionable anesthetic properties are chloral hydrate [302-17-0] paraldehyde, dimethoxymethane [109-87-5] and acetaldehyde diethyl acetal. In iadustrial exposures, however, any action as an anesthesia is overshadowed by effects as a primary irritant, which prevent voluntary inhalation of any significant quantities. The small quantities which can be tolerated by inhalation are usually metabolized so rapidly that no anesthetic symptoms occur. 

A thkd method utilizes cooxidation of an organic promoter with manganese or cobalt-ion catalysis. A process using methyl ethyl ketone (248,252,265—270) was commercialized by Mobil but discontinued in 1973 (263,264). Other promoters include acetaldehyde (248,271—273), paraldehyde (248,274), various hydrocarbons such as butane (270,275), and others. Other types of reported activators include peracetic acid (276) and ozone (277), and very high concentrations of cobalt catalyst (2,248,278). 

---