Experiment 3: Preparation of Lidocaine This two-step synthesis involves the following conversion: 2,6-dimethylaniline α- chloro-2, 6-dimethylacetanilide Lidocaine. This synthetic scheme is shown in equation (1). Each step is discussed in more detail in each individual part of the lab along with experimental procedures. verall Reaction Sequence: NH 2 + C 2 H N H 2,6-dimethylaniline!-chloroacetyl chloride!-chloro-2,6-dimethylacetanilide ( CH 2 ) 2 NH diethylamine (1) N H N Lidocaine 1
Experiment Part A: Preparation of α-chloro-2, 6- dimethylacetanilide from 2,6-dimethylaniline. INTRDUCTIN 2, 6-Dimethylaniline is converted to α-chloro-2, 6-dimethylacetanilide by allowing it to react with α-chloroacetyl chloride, as shown in equation (2). NH 2 + C 2 H N H (2) 2,6-dimethylaniline!-chloroacetyl chloride!-chloro-2,6-dimethylacetanilide The nitrogen atom in 2, 6-dimethylaniline is a nucleophile because of the non-bonding pair of electrons on nitrogen. Nucleophilic substitution occurs selectively at the acyl carbon atom in α-chloroacetyl chloride because of the greater reactivity of nucleophiles toward acid chlorides compared to alkyl chlorides. The reasons for this selectivity are attributed to the differences in the electrophilicity of the two carbon atoms in α- chloroacetyl chloride. Electronically, the carbonyl carbon has two electron-withdrawing groups the oxygen doubly bonded to it and the - bonded to it. n the other hand, the carbon in CH 2 has only one electron-withdrawing group (-). Besides electronics, steric factors also play a role in this selectivity. It is easier for the nucleophile to attack the carbon of the planar carbonyl group in the acid chloride than to attack the tetrahedral carbon in the CH 2 group. The mechanism for the substitution reaction that occurs between an amine and an acid chloride may be found in lecture textbooks and may be discussed by your instructor. This reaction is carried out in glacial (anhydrous) acetic acid as the solvent. The acetic acid reacts with 2, 6-dimethylaniline to establish an equilibrium between it and its acetate salt, as shown in equation (3a) on the following page. As the reaction proceeds, hydrochloric acid is liberated so that 2, 6-dimethylaniline is partially converted into its hydrochloride salt [equation (3b)]. If this salt remains at the end of the reaction, it would contaminate the α-chloro-2, 6-dimethylacetanilide because both the hydrochloride salt and the α-chloro-2, 6-dimethylacetanilide are insoluble in cold acetic acid. The co-precipitation is avoided by adding an aqueous solution of sodium acetate to the warm reaction mixture and plays two key roles. (a) It reestablishes the reaction 2
shown in equation (3a) as an acid-base equilibrium. Sodium acetate, which is the salt of the weak acid acetic acid, serves to remove the H that is formed in the reaction and yield acetic acid, as shown in equation (3c) and to prevent co-precipitation of the hydrochloride salt. (b) The addition of the water contained in the aqueous sodium acetate solution changes the polarity of the solvent and causes the α-chloro-2, 6- dimethylacetanilide to precipitate. NH 2 + CH NH 3 C (3a) acetate salt NH 2 + H- NH 3 (3b) hydrochloride salt C + H- CH + acetate anion acetic acid (3c) Vacuum filtration is used to collect the product. The acetate salt of 2, 6- dimethylaniline and unreacted 2, 6-dimethylaniline are soluble in water and thus remain in the filtrate. After collecting the α-chloro-2, 6-dimethylacetanilide product by vacuum filtration, it is washed with several portions of cold water to remove residual amounts of acetic acid, unreacted 2, 6-dimethylaniline and the acetate salt of 2, 6- dimethylaniline. The α-chloro-2, 6-dimethylacetanilide is allowed to air-dry until the next lab period at which point a weight will be obtained. If desired, it can then converted without further purification to lidocaine in experiment 3B. 3
REAGENT/PRDUCT TABLE: Reagents MW (g/mol) MP (ºC) BP (ºC) Density 2, 6-dimethylaniline 121.18 214/739mm 0.984 alpha-chloroacetyl chloride 112.94-22 10-106 1.418 acetic acid 60.0 16.2 116-118 1.049 Product alpha-chloro-2, 6-dimethylacetanilide 197.6 MW (g/mol) EXPERIMENTAL PRCEDURE FR YUR SAFETY 1. α-chloroacetyl chloride is a noxious chemical and must be kept in the hood. When it is used, transfer it to the conical vial in the hood (Step 3). 2. Wear gloves at all times when handling α-chloroacetyl chloride. 3. Avoid inhaling α-chloroacetyl chloride. 1. Heat a 0-mL beaker containing about 40 ml of water and a couple of boiling stones on the hot plate set to about 3 until the water is gently boiling. This beaker will be used as a hot water bath. 2. Take a clean, dry, pre-weighed ml conical vial with spin vane with a cap and add 0.0 ml of 2, 6-dimethylaniline (density 0.984 g/ml) to the vial. Reweigh this capped vial to determine the weight of 2, 6-dimethylaniline you added to it. Add about 1 ml of glacial acetic acid to the same vial. Stir this mixture gently with the tip of a clean microspatula, to ensure the contents in the vial are mixed. 3. btain a clean dry small sample vial with a cap and take a weight on them. In the hood, place 0.0 ml of α-chloroacetyl chloride (density = 1.42 g/ml) in the vial, cap it and and reweigh to determine the weight of the α-chloroacetyl chloride you added. [Note: You must transfer the α-chloroacetyl chloride to this vial in the hood.] Add about 1 ml of glacial acetic acid to this vial, and stir this mixture gently with the tip of a clean microspatula, to ensure mixing. 4. Take the ml conical vial containing 2,6-dimethylaniline/glacial acetic acid mixture to the hood, and using a dry pipette, transfer the α-chloroacetyl chloride/glacial acetic acid mixture to the ml conical vial. Attach an air condenser to the conical vial, clamp the air condenser and lower the conical vial so it is about one-half immersed in the hot water bath. (See Section A.3 and Figure A.3) Start magnetic stirring, and after the water starts to boil, continue heating for about 30 minutes. 4
Safety Note: Place the small sample vial that had contained the α chloroacetyl chloride/glacial acetic acid in the dirty vial beaker in the hood.. After heating the reaction mixture in the conical vial for 30 minutes, cool it to room temperature by placing the conical vial (air condenser still attached) in a beaker of cold tap water. In the hood, remove the air condenser and pour the contents of the reaction vial into a clean, dry 30-mL beaker. Add about 6. ml (measured with a graduated cylinder) of 0.4 M NaAc at one time to the beaker. This causes the α chloro-2, 6-dimethylacetanilide to form as a heavy precipitate. Stir the mixture with a stirring rod, and cool the beaker in an ice water bath for about minutes. 6. Collect the solid by vacuum filtration using a Hirsch funnel (see Section A.2, Figure A.2). Add about 1 ml of ice water to the beaker that contained the reaction mixture and pour it over the solid on the filter to wash the 2, 6-product. Add a second 1 ml portion of ice water to the funnel. Keep the vacuum applied to the filter flask and filter containing the solid for about 10 minutes to remove as much solvent as possible. Break up the solid on the funnel with a clean, microspatula and leave the funnel containing your product in a small beaker in your locker to dry until the next lab period. At that time, you will need to get a weight on your compound and record this in your notebook. No melting point will be taken on this product. WASTE DISPSAL Place the filtrate contained in the filter flask in the aqueous acid waste bottle.
8 80 7 70 6 60 %Transmittance 0 4 40 3 30 2 20 1 10 4000 300 3000 200 2000 100 1000 Wavenumbers (cm-1) Date: Wed Dec 16 10:44:9 2009 (GMT-0:00) 2,6-dimethylaniline Scans: 4 Resolution: 4.000
90 8 80 7 70 6 60 %Transmittance 0 4 40 3 30 2 20 1 10 0 4000 300 3000 200 2000 100 1000 Wavenumbers (cm-1) Date: Wed Dec 16 10:49:17 2009 (GMT-0:00) alpha-chloroacetyl chloride Scans: 4 Resolution: 4.000
NH 2 0.16 0.08 0.16 0.48 6 4 PPM 3 2 1 0
N H 0.07 0.21 0.14 0.42 8 7 6 PPM 4 3 2 1 0