Experiment #1 - Preparation of Heptyl Bromide

R-CH₂-OH      +        NaBr      +           H₂SO₄      -------->    R-CH₂-Br   +        NaHSO₄   +     H₂O   (1)

Primary alcohols are readily converted to alkyl bromides using sodium bromide and sulfuric acid (eq 1). The mechanism involves S_N2 substitution by bromide ion on the protonated alcohol as discussed in the Klein textbook, chapter 12.9.

In this experiment, you will use this reaction to prepare n-heptyl bromide, which will in turn be used as the starting material for Expt 2.

Procedures

Running the reaction.

Reagents. Add 30 mmol of 1-heptanol to a 25-mL round bottom flask equipped with a magnetic stirbar. Add 4.8 g NaBr and 5 mL H₂O to the flask. Cool the mixture on ice and then add 4.0 mL H₂SO₄(conc) dropwise to the stirring reaction mixture. Attach a reflux condenser and reflux the solution for 1 h. Cool briefly before going on to the work up procedure. Do not let the mixture cool all the way to room temperature before going on to the extraction procedure. If you do then the byproduct salts will precipitate making the extraction procedure more difficult.

Work up the reaction.

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. 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 test tube. Use a Pasteur pipet to 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. After the layers separate, transfer the organic layer 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. 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 over anhydrous Na₂SO₄. 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 reaction vial. Weigh the vial again to get your crude yield of product.

Distillation. See Mohrig, Technique 13. Assemble a Hickman distillaton apparatus as shown in Figure 13.12. Raise the temperature of the aluminum block slowly until product begins to distill and then 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. Obtain a proton NMR spectrum using chloroform-d as the solvent. A C-13 NMR spectrum may also be obtained if time allows. 

2. Prepare a sample for GC-MS by adding one or two drops of the NMR sample solution to a clean vial containing 2-3 mL of purified CH₂Cl₂.

3. Obtain an IR spectrum. The lab assistant will assist with this.

4. Turn in the product in a capped vial labeled with your name and lab section, product structure and name and distillation bp range.

Report

Make sure the spectra are correctly interpreted and fully labeled! You may want to schedule a time for your team to hold a consultation with the instructor if you are at all puzzled by the spectra.

Make sure to report the yield, theoretical yield, and percentage yield of the product in your results table. Make sure the Results and Discussion address all of the areas listed in the Guidelines for Lab Reports.