Preparation of n-Butyl Bromide

This is an adaptation of the procedure published by Pavia, et. al.¹ All figures and page references are from the Mohrig, Hammond, and Schatz textbook.²

Running the reaction.

Apparatus. Set up a simple reflux apparatus using a 25-mL round bottom flask as shown in Fig 7.1a. Use a heating-mantle sand bath Fig 6.4a as the heat source. Set the sand bath on a magnetic stirrer. Make sure to plug the heating mantle into a variable voltage electrical outlet (a "Variac") rather than a full voltage (105 V) outlet.

Reagents. Add 2.8 mL of 1-butanol to the reaction flask. (Hint: weigh the graduated cylinder before the addition with the alcohol and then again after the addition so you can get an accurate mass of alcohol actually used.) Add 4.8 g NaBr and 5 mL H₂O to the flask. Cool the mixture on ice before slowly (dropwise) adding 4.0 mL H₂SO₄(conc) using a Pasteur pipet. (Caution: concentrated sulfuric acid causes severe burns if gotten on the skin. Wear gloves and if contacted by it immediately rinse the affected area of your skin with large amounts of water.)

Reaction period. Add a magnetic stirbar and replace the ice bath with the sand bath you set up earlier. Heat the reaction mixture using an applied temperature of about 145° C. The solution should be refluxing gently throughout the reaction period. Continue heating for 60-75 min and then cool briefly before going on to the work up procedure. Do not let the mixture cool all the way to room temperature. Go on to the extraction procedure while the solution is still warm. If you do not then byproduct salts will precipitate making the extraction procedure more difficult.

Work up the reaction.

For this part please see Chapter 8.6-8.7 in Mohrig for hints about how best to carry out the extraction operations.

Extraction part 1 - separate organic and aqueous phases. Observe the reaction flask; there should be two immiscible liquid layers, a large aqueous layer on the bottom and a smaller organic layer floating on top. (In some cases there might actually be two organic layers, one of which is the product and the other unreacted alcohol.) Remove as much as possible of the bottom aqueous layer using a pipet. Ignore any solids that form; if they are drawn into the pipet simply discard them with the aqueous layer. Once the volume of liquid in the flask is around 10 mL or less and it is difficult to remove more bottom layer as above, then transfer the entire contents of the flask to a small test tube. Remove the bottom layer from the test tube and discard with the previous aqueous material.

Warning: even though the above procedure instructs you to discard the bottom layer, it is an important rule of working in the organic lab that no discards are made until the lab has been completed and the final product has been obtained. When instructed to discard anything you should always save it in a labeled flask or beaker until the end of the lab when you are sure that it is not needed and only then discard it.

Extraction part 2 - wash with sulfuric acid. Make sure to wear protective gloves during this part. Prepare 4 mL of 9M H₂SO₄ by adding 2 mL H₂SO₄(conc) to 2 mL water. (Caution: always add the acid to the water and be prepared for vigorous heat evolution. It is best to do the acid dilution in a large test tube with cooling from an ice bath.) Add the prepared 9M H₂SO₄ to the test tube conatining the organic layer from the reaction. Cap the tube and shake it gently, venting frequently to prevent unsafe pressure build up in the tube. Set the tube down in a safe place and allow the layers to separate. Any remaining alcohol should react and dissolve into the acid layer so there should now be only one organic layer. The top layer should be the organic layer, but to be safe test both layers after the next step by adding a drop of water and observing whether it dissolves immediately or not. Remove the bottom layer with a pipet and transfer to a new test tube.

Extraction part 3 - wash with H₂O - Add 4 mL H₂O to the organic layer, cap and shake gently with frequent venting. Allow the layers to separate. Because alkyl halides are always more dense than water, this time the bottom layer is expected to be the organic layer. Use a pipet to transfer as much as possible of the organic layer to a dry 5 or 10 mL Erlenmeyer flask.

Drying. Dry the liquid in the Erlenmeyer over anhydrous Na₂SO₄. (See Mohrig, Chapter 8.8-8.9) After the product appears dry (allow at least 20 min) (it should appear crystal clear when dry) transfer the liquid using a dry Pasteur pipet to a preweighed dry 5-mL conical vial. Weigh the vial again to get your crude yield of product.

Distillation. See Mohrig, Chapter 11. Assemble a short-path distillaton apparatus similar to that shown in Figure 11.9. The main difference will be that you are using a conical vial with a magnetic spin vane (see p 27) rather than a round bottom flask and stir bar. Raise the temperature of the aluminum block slowly until it is in the vicinity of 140-150° C at which point the n-butyl bromide should begin to distill. Maintain heating until most of the liquid in the vial has distilled out. Make sure to carefully observe and note the range of temperatures at which the distillate comes over. Weigh the distillate (your final product) and obtain the required spectra.

 

(1) Introduction to Organic Laboratory Techniques A Microscale Approach, 4th edition by Pavia, Lampman, Kriz, and Engel; Brooks/Cole - Thomson Learning: Pacific Grove, CA, 2007.

(2) Techniques in Organic Chemistry, 2nd ed. by Mohrig, Hammond, and Schatz. Freeman, New York, 2006.