REFERENCE: Introduction to Organic Laboratory Techniques, 3rd Ed. Pavia, Lampman and Kriz
Live Steam Distillation pp.582-587
OBJECTIVES: The student should be able to:
1. Handle bromine safely in the laboratory.
These are the third and fourth steps in a multi-step synthesis of p-bromoaniline. The amine group has such a strong activating effect on the benzene ring that direct reaction of aniline with bromine gives 2,4,6-tribromoaniline.
Thus the monobromo compound must be made by a less direct route. Advantage is taken of the fact that acetylated aniline (acetanilide or N-pheneylacetamide) is considerably less reactive than aniline, is readily prepared from aniline, and the acetyl group can easily be removed by hydrolysis.
The separation of p-bromoaniline from the reaction mixture involves a live steam distillation. It is important to understand both the principle and the technique before beginning this experiment. These two steps should be considered as one experiment in all lab book entries and in submitting result cards. The flow sheet for isolation of the products is to be provided by the student.
1. Reactants: Bromine and acetanilide from Experiment 13.
2. Reactants: water, 50 mL and concentrated hydrochloric acid, 25 mL
Calculate and use the theoretical amount of bromine required assuming 100% yield of acetanilide from the previous experiment.
In calculations round mole values to 3 decimal places, mass values to 2 decimal places and volume values to whole numbers.
The crude, dry acetanilide prepared in Experiment 131 is dissolved in glacial acetic acid2 in an erlenmeyer flask3. A solution of bromine4,5 in glacial acetic acid is placed in a small dropping funnel for addition to the acetanilide solution6. The addition is done slowly and with continuous, thorough mixing7. The mixture is occasionally swirled for an additional 10 minutes, then poured into 250 mLs of cold water8, stirred and the product filtered out9.
The p-bromoacetanilide10 is placed in a 500 mL round bottom flask and water and concentrated hydrochloric acid added. The mixture is refluxed until the solid dissolves and for an additional 15 minutes. The mixture isthen made strongly alkaline with a 20% sodium hydroxide solution11 and the amine steam distilled12,13. The distillation maybe done rapidly, just so liquid is not splashed over and uncondensed vapors do not pass through the condenser. The distillation is continued until no more solid forms in the distillate flask.
The product is then filtered out of the distillate and pressed as dry as possible on the filter. A mixture of 100 mL each of ethyl alcohol and water and 7 mL of concentrated hydrochloric acid is prepared and added slowly to the crude p-bromoaniline until all of the p-bromoaniline just dissolves. It may not be necessary to use all of the solvent mixture to dissolve the p-bromoaniline. If it is necessary to decolor the solution, add about 1 g of decolorizing charcoal, and warm on a steam bath for 10 mintues14. The charcoal is filtered out through fluted filter paper and the filtrate poured slowly with vigorous stirring into a mixture of 45 mL of 10% sodium hdyroxide solution and 75 g of ice15. The product is filtered out, washed with cold water and dried16. The melting point of the dried product is determined and IR and NMR spectra run.
1. The bromination should be done in the hood.
2. Remember that glacial acetic acid is highly corrosive to biological materials. If this reagent comes in contact with the skin, it must be washed off immediately with a large quantity of water.
3. If necessary, the mixture may be warmed gently on a steam bath to complete the dissolving of the solid. The solution should not be above 40°C when the bromine solution is added.
4. Bromine, both the liquid and the vapor, is highly corrosive and irritating and must be handled with extreme care. A face shield, long sleeves, apron and gloves will be worn during the handling of bromine. If liquid is spilled on the skin, it must be removed immediately, first by copious streams of water, then by working with soap and water. Report all difficulties with bromine to the instructor immediately.
5. The instructor or instructional assistant will measure out the desired amount of bromine.
6. To avoid accidents, it is important that the funnel be securely supported.
7. There will be a noticeable evolution of heat and hydrogen bromide. If there is a marked warming of the reaction mixture, it should be cooled in cold water.
8. If the mixture shows an appreciable bromine color, solid sodium bisulfite should be added until the color is just removed.
9. Alcohol is a suitable solvent for recrystallization of a small portion of the p-bromoacetanilide. A melting point should be run on the purified p-bromoacetanilide and the purified p-bromoacetanilide turned in with a result card reporting the melting point.
10. This material need not, of course, be dry.
11. Do not add the 20% sodium hydroxide solution, if the steam distillation will not be done until the next laboratory period.
12. In the steam distillation, the mixture must be hot. The heat of neutralization serves this purpose initially. The mixture must be strongly alkaline at this point. A demonstration set up of a live steam distillation apparatus is found in the laboratory and the instructor or instructional assistant will demonstrate the procedure.
13. The product, being a solid at room temperature, may collect in the condenser. It can be removed by temporarily stopping the flow of water in the condenser until the solid has melted.
14. Avoid changing the composition of the solvent by boiling off any appreciable quantity of alcohol.
15. If only a small amount of the ethyl alcohol, water and hydrochloric acid solvent was used to dissolve the p-bromoaniline, less than 45 mL of the 10% sodium hydroxide solution should be used. Consult the Instructor or Instructional Assistant as to the amount to use.
16. p-Bromoaniline, like most other aromatic amines, undergoes darkening due to oxidation when left standing in air. If it must be kept from one period to the next it should be protected from the air. This method of purification is used because the alternative recrystallization from alcohol results in appreciable loss of product.
TPS May 2003