Experiment #6 - Preparation of an Assigned Alkyl Bromide by an S_N2 Reaction

R-CH₂-OH      +        NaBr      +           H₂SO₄      -------->    R-CH₂-Br   +        NaHSO₄   +     H₂O

Primary alcohols are readily converted to alkyl bromides using sodium bromide and sulfuric acid. The mechanism involves S_N2 substitution by bromide ion on the protonated alcohol as shown for n-butyl bromide in the lab manual. (Pavia, Expt 23)

In this experiment, you will adapt the lab text's semi-microscale procedure for making n-butyl bromide (Pavia, Expt 23) to the preparation an assigned alkyl bromide as detailed in the table below.

1. Determine the name of the alcohol to be used as the starting material to prepare your alkyl bromide. Look up physical constants for this compound in a chemistry reference book or on the Internet. (Hint: use the molecular formula rather than the name to search for the compound.).

2. Prepare your table of reactants and products. Use the same amount of your alcohol in moles as the moles of n-butyl alcohol called for in the book procedure. (Use the exact same amounts as specified in the book for all other reactants.) (If your alcohol is a liquid at room temperature then you will want to measure it out by volume. Make sure to calculate the volume to use and include it in your table.)

3. Because you will be carrying out a distillation of the product alkyl bromide, you need to include the literature bp of this compound in your table (Your bromide will not distill at the same temperature recommended in the procedure for n-butyl bromide.) Please also be aware that many times compounds with higher boiling points are difficult to distill and, therefore, are distilled at reduced pressure which causes the bp to be lower. Because of this, often times you may find a literature bp reported as 78²⁰ (for example). In such a case the superscripted number represents the pressure in torr and cannot be ignored. You should do your best to find the normal bp (bp at 1 atm or 760 torr) for your product. Also include the literature refractive index of the product if you can find it.

4. If you think that your starting alcohol may present significantly more steric hindrance that n-butyl alcohol would, then it would be reasonable to use a longer reaction period (reflux time) than that specified in the handout procedure.

5. Make sure your planned procedure is written specifically for your assigned reaction.

1. Prepare a sample for NMR. Label the NMR tube with your group name. Have the instructor or lab assistant help you obtain the proton NMR spectrum. 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. Measure the refractive index of the purified product.

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 the Conclusions address all of the areas listed in the Guidelines for Lab Reports. http://course1.winona.edu/tnalli/f09/labreport.htm