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Synthése de la lidocaïne


Lire le protocole suivant extrait du Journal of Chemical Education (Vol 76. No. 11 November 1999).
The Preparation of Lidocaine - Thomas J. Reilly
Department of Chemistry and Biochemistry, Loyola Marymount University, 7900 Loyola Blvd.,
Los Angeles, CA 90045-8225;
This synthesis of 2-(diethylamino)-N-(2,6-dimethyl-phenyl)acetamide, commonly known as Lidocaine or
Xylocaine, has been used in our introductory organic chemistry laboratory for several years in support of lecture
material on reactions of amines. It effectively illustrates the acylation and alkylation of amines and dramatically
emphasizes the difference in rate between the two processes. Procedures are simple, yields are high, and the known
pharmacological properties of this widely used local anesthetic successfully capture the attention of most students.
The procedure given here is based on the original patent (1) with modification to permit isolation of the crude
product by average students in a single three-hour laboratory period.
All reagents were used as supplied by Aldrich Chemical Co. Seriously discolored samples of 2,6-dimethylaniline
may be used with only slight reduction in yield.
CAUTION : 2,6-Dimethylaniline is toxic and readily absorbed through the skin. Chloroacetyl chloride is toxic and
corrosive. These reagents should be dispensed directly into the reaction vessel from an automatic-delivery pipet. If
these reagents are poured from the bottle, the students must wear gloves and work in an efficient hood.

2,6-Dimethylaniline (3.0 mL, 2.9 g, 24.4 mmol) is added to 15 mL of glacial acetic acid in a 125-mL
Erlenmeyer flask followed by chloroacetyl chloride (2.0 mL, 2.85 g, 25.1 mmol) and 25 mL of half-saturated
aqueous sodium acetate.
CAUTION: Glacial acetic acid is corrosive and can cause serious burns.
Precipitation of the amide is virtually instantaneous. The product is stirred thoroughly with 60 mL of cold water
and isolated by vacuum filtration. It should be pressed as dry as possible in the Buchner funnel and used
immediately in the next step.
The amide is placed in a 50-mL round-bottom flask containing diethylamine (7.5 mL, 5.29 g, 72.5 mmol) and 25
mL of toluene and refluxed for one hour. The reaction mixture is cooled to room temperature and transferred to a
separatory funnel, where it is washed 4× with 50-mL portions of water to remove diethylamine hydrochloride and
excess diethylamine. The organic layer is extracted with one 20-mL portion of 3 M hydrochloric acid and washed
once with 20 mL of water. The combined aqueous extracts are placed in a 125-mL Erlenmeyer flask, cooled to 10
°C in an ice bath, and neutralized by addition of 3 M sodium hydroxide in portions with stirring while maintaining
the temperature below 20 °C.
The product separates as a granular white solid and is isolated by vacuum filtration. It is washed with cold water,
pressed dry, and air-dried as long as possible. The yield of dry product is 4.1 grams, mp 64–66 °C (lit. 67–69 °C),
71.1% based on 2,6-dimethylaniline.
1H NMR (300 MHz, CDCl3) triplet 1.13 6H, singlet 2.23 6H, quartet 2.68 4H, singlet 3.22 2H, singlet
7.08 3H, singlet 8.92 (broad) 1H.
Lidocaine is highly soluble in all common organic solvents but it can be recrystallized from warm hexane using 1
mL of solvent per gram of crude product. The product crystallizes in large colorless spars, mp 65–67 °C (lit. 67–69
I would like to thank the John Stauffer Charitable Trust for a generous grant that facilitated the purchase of the
Brucker AC 300 used to acquire the 1H NMR data.
Literature Cited
1. Lofgren, N. M.; Lundquist, B. J. Alkyl Glycinanilides. U.S. Patent 2 441 498, May 11,1948.
Auteur : Brigitte VRAY
Académie de LYON

Equations de cette synthèse :

1ère étape :

Données expérimentales :

Questions :

A propos de la réaction :
1. Quels sont les groupes caractéristiques identifiables :  dans le réactif A et le produit C de la 1ère étape ?  dans le réactif E de la 2ème étape ? 2. D’après le texte, comment nomme-t-on la molécule E ? 3. La première étape correspond à une réaction de : 4. La deuxième étape correspond à une réaction de : Auteur : Brigitte VRAY
Académie de LYON
Sécurité :
Quelles précautions faut-il prendre lors de cette synthèse ? A propos du protocole lors de la 2ème étape :
6. Choisir dans la liste suivante la verrerie que l’on doit utiliser pour mesurer le volume V = 25 mL de - bécher 50 mL - pipette jaugée 25 mL - éprouvette graduée 10 mL - éprouvette graduée 50 mL 7. a) Parmi les montages suivants, quel est celui qui correspond au montage utilisé ? Montage (1)
Montage (2)
Montage (3)
b) Nommer ce montage. c) Quel est l’intérêt de ce type de montage ? d) Ce montage est-il nécessaire lors de la 1ère étape ? Justifier. 8. Pourquoi la température de fusion du produit sec après la 1ère filtration n'est-elle que de 64-66 °C ? 9. a) Quel est l'intérêt de la recristallisation ? b) Quel solvant est utilisé ? Pourquoi à chaud ? c) Pourquoi le volume de solvant doit-il être précis et minimal ? 11. Interpréter le spectre RMN proposé.
12. Le texte indique : « 2,6-dimethylaniline (3.0 mL, 2.9 g, 24.4 mmol) ». Quelles relations lient ces valeurs. Vérifier les valeurs données. 13. Montrer que la quantité de matière de lidocaïne que l’on devrait théoriquement obtenir est 24,4 mmol. Quelle serait alors la masse de lidocaïne obtenue ? 14. Le texte indique : « The yield of dry product is 4.1 grams, 71.1% based on 2,6-dimethylaniline». b) Quelle masse de lidocaïne est obtenue expérimentalement ? c) Montrer que la quantité de matière de lidocaïne que l’on devrait théoriquement obtenir est 24,4 mmol. Quelle serait alors la masse de lidocaïne obtenue ? 15. Définir et calculer le rendement de cette synthèse. Auteur : Brigitte VRAY
Académie de LYON


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