Acetaldehyde from paraldehyde

Assemble the simple fractional distillation apparatus shown in Fig. II, 16, 1 the round-bottomed flask should have a capacity of 200 or 250 ml. and the conical flask 100 ml. (Alternatively, a long all-glass 

To obtain pure acetaldehyde, the product must be redistilled. Clean and dry the 200-250 ml. flask first used, immerse it in cold or ice water pour in the crude acetaldehyde rapidly, attach the fractionating column, etc. Immerse the receiver in crushed ice. Heat the flask gently in a water bath and adjust the temperature so that the aldehyde distils slowly and at a uniform temperature. The temperature recorded at the top of the column may depend partly upon the temperature of the laboratory, if this is above 21°. Pure acetaldehyde boils at 21°. 

---

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. 

Tetrahydroharman, m.p. 179-80°, has been prepared by a number of workers by a modification of this reaction, viz., by the interaction of tryptamine (3-)5-aminoethylindole) with acetaldehyde or paraldehyde and Hahn et al. have obtained a series of derivatives of tetrahydronorharman by the use of other aldehydes and a-ketonic acids under biological conditions of pH and temperature, while Asahina and Osada, by the action of aromatic acid chlorides on the same amine, have prepared a series of amides from which the corresponding substituted dihydronorharmans have been made by effecting ring closure with phosphorus pentoxide in xylene solution. 

The lowest-cost synthetic pyndine base, 2-mcthyl-5-cthylpyridinc, is made in a liquid-phase process from paraldehyde (derived from acetaldehyde) and aqueous ammonia 111 the presence of ammonium acetate at approximately 102- 190 atmospheres and 220-280cC in 70-80% yield. Minor byproducts include 2- and 4-picoline. 

If nA is the number of moles of paraldehyde at time t and nAO, the moles of paraldehyde at t = 0, then at time t, moles of acetaldehyde from stoichiometry are ... 

Acetaldehyde may be conveniently prepared by distilling from paraldehyde in the presence of a trace of sulfuric add an efficient fractionating column should be used. 

This compound, as distinguished from paraldehyde and metaldehyde, is a true aldehyde, in that it cannot be converted back to acetaldehyde. Aldol loses water easily, and is converted into an unsaturated aldehyde ... 

Acetaldol 121,122 Acetaldehyde (100 g), freshly prepared from paraldehyde, is added slowly to ice-cold water (200 ml) the solution is cooled to —12° and an ice-cold solution of potassium cyanide (2.5 g) in water (100 ml) is added cautiously at such a rate that the temperature does not exceed — 8°. The mixture is kept for a further 2 h in the freezing mixture and then for 30 h at 0°, after which it is saturated with sodium chloride and extracted several times with ether. When dried and fractionated, the extracts afford 40-45 % of the aldol at 80-90°/ 20 mm. The pure product boils at 83°/20 mm. 

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. 

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. 

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. 

Fulminate can be prepared from acetaldehyde instead of from alcohol, and from substances which are convertible into acetaldehyde, such as paraldehyde, metaldehyde, dimethyl- and diethyl-acetal. Methyl alcohol, formaldehyde, propyl alcohol, butyralde-hyde, glycol, and glyoxal do not yield fulminate. ... 

In a reaction which is mechanistically related to the Skraup reaction an a,/ -unsaturated carbonyl compound, generated by way of an acid-catalysed aldol condensation, reacts with a primary aromatic amine in the presence of acid to yield a quinoline derivative (Doebner-Miller reaction). For example, when aniline is heated with paraldehyde (which depolymerises to acetaldehyde during the reaction) in the presence of hydrochloric acid the final product is 2-methyl-quinoline (101) (quinaldine, Expt 8.40). Retrosynthetic analysis for the 1,2-dihydroquinoline reveals crotonaldedhyde as the unsaturated carbonyl component which is in turn formed from acetaldehyde (see Section 5.18.2, p. 799). 

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. 

Thus acetone, acetaldehyde, and esters condense to dimers in the first stage. From aldehydes we also can obtain six-membered rings such as trioxane from CU2O, paraldehyde from CHaCHO, etc. Dienes often give dimers reversibly. 

---