Synthèse alcool allylique

[inviteef6d3de2]

Bonjour, je me suis lancé récemment dans la synthèse de l'alcool allylique à partir de glycérol et d'acide oxalique mais je me suis heurté à deux reprises au même problème lors de la purification ... pourquoi mon produit distille t'il à 100-103°C (colonne de vigreux) alors que l’azéotrope eau-alcool allylique est de 88°C.

J'ai suivi cette procédure à la lettre la 1ere fois et la deuxième fois avec une hydrolyse sous reflux (1H):

A mixture of 500g anhydrous oxalic acid and 500g of glycerol was heated in a partial vacuum on a water bath for 4-5h (or longer) until formic acid ceased to distill over. The mixture was then gradually heated to 240°C under ordinary pressure, the flask being fitted with a fractionating column. At 220-225°C, CO2 was given off and a mixture of approximately equal amounts of allyl alcohol and allyl formate distilled over leaving in the distillation flask a residue containing somewhat 50% of the glycerol originally used. Practically no acrolein was produced. The distillate was treated with 50g NaOH in 1000ml water (to hydrolyze the formate), allowed to stand for 12h at room temp, and finally distilled. The first 300 ml of distillate contained all the allyl alcohol, which after fractionation yielded 200-210g of a allyl alcohol/water mixture (bp 87-88°C) which may be dehydrated using anhydrous potassium carbonate yielding approximately 150g of anhydrous allyl alcohol.

J'ai rapidement fais un test d'oxydation avec KMnO4 et ça s'oxyde très bien, pas eu le temps de faire un dosage.

Auriez vous une hypothèse sur le produit que j'ai synthétisé ?
Merci d'avance pour toutes vos réponses
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[moco]

Bonjour,

Cette synthèse n'a pas l'air de provoquer des torrents d'enthousiasme parmi ceux qui l'ont commentée sur le Net. On lit par exemple que personne ne connaît bien l'équation de la réaction, ni son mécanisme.
Voici ce qu'on peut lire par exemple, en tapant "Allylic alcohol synthesis" via Google

The well-known process of Tollens for the production of allyl alcohol (Annalen, 1870, 156, 129) is open to very serious objections and is probably based on a false conception of the reaction. À very large excess of glycerol is used and the amount of oxalic acid lost as carbon monoxide is also very large, consequently the yield is very poor. Chattaway (T., 1914, 105, 151; 1915, 107, 407) demonstrated that it was essentially by the decomposition of a dioxalin that allyl alcohol was produced.
The oxalins are initially formed in varying proportions according to the conditions of the experiment. At higher temperatures (110°) the hydrogen oxalins give carbon dioxide and formins. Dioxalin and monoformin dioxalin, at higher temperatures still (220-225°), decompose into carbon dioxide and allyl alcohol and allyl formate respectively, whilst at still higher temperatures (24á°) monoformin gives glycerol and carbon monoxide. Formic acid is produced by the action of an excess of oxalic acid on the formins and distils during the second stage of the reaction. From this it is apparent that a relatively large amount of oxalic acid must be used in the preparation of allyl alcohol.
Chattaway (loc. cit.) drew these conclusions partly from a study of the action of ammonia and aniline on the products of the reaction and partly from the properties of the ethyl oxalates; the results of comparative experiments given below completely confirm the above scheme in every detail.
The experiments also clearly demonstrated the four consecutive reactions, and it was possible to convert nearly the whole of the glycerol in one operation into allyl alcohol and to account for more than 90 per cent of the oxalic acid as free and combined formic acid, carbon dioxide, and water. The net loss, therefore, is negligible.
From the point of view of the student the usual methods of preparing formic acid and allyl alcohol are very uninteresting and unsatisfactory with regard to yield, time, and manipulation. Also the simple equation given in the text-books gives no clue as to where the loss occurs.

[HarleyApril]

Tu trouveras le mode opératoire en français dans les comptes rendus de l'académie des sciences janvier-juin 1869 p266 et suivantes (sur Gallica).
http://visualiseur.bnf.fr/CadresFene...25&I=266&M=tdm
Selon eux, il se forme 20% d'alcool en masse par rapport à la masse d'acide oxalique engagé.

Je t'ai également trouvé ceci
http://www.orgsyn.org/demo.aspx?prep=CV1P0042
Bonne lecture, lis aussi les notes en fin de texte ...

Fais attention quand même, la réaction semble dégager pas mal de gaz, sois sûr qu'il y a une bonne échappatoire pour lui (risque de débordement).

Cordialement