Experiment 28 the Reaction of the Butanols with Hydrobromic Acid

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Experiment 28 – The Reaction of the Butanols with Hydrobromic Acid

Goal To perform an acid-catalyzed nucleophilic substitution reaction that converts an alcohol into an alkyl halide.

Reading and Working Ahead You discussion should include examples of the possible SN1 and SN2 mechanisms.

Supplement your procedure with pictures and information contained in the following Operations.  Operation 6c – Heating Under Reflux  Operation 22 – Trapping Gases  Operation 25a – Simple Distillation  Operation 13 – Extraction (basis for washing liquids)  Operation 11b – Small-Scale Gravity Filtration  Operation 32 – Gas Chromatography These are mentioned in the procedure as well. I am just trying to emphasize the fact that this experiment is quite busy and uses a lot of techniques learned last semester.

Be sure to carefully read the section on “Understanding the Experiment” where you must pay attention to the identity of the upper/lower layer in the extraction process.

Calculations In the calculation section of your procedure, you will need to determine:  mass/volume of 1-butanol/2-butanol to be used in the experiment (since you will not know in advance which alcohol you will be assigned, prepare calculations for both)

 millimoles of HBr used in this experiment – calculated as follows:

10.0 mL ∣ 1.49 g soln ∣ 48 g HBr ∣ 1000 mmol ∣ = 88 mmol HBr ∣ 1 mL ∣ 100 g soln ∣ 80.9 g ∣

 everyone will use 72 mmol of concentrated H2SO4 (18 M) as the catalyst

 theoretical yield

If you need a refresher on how to perform the necessary calculations, refer to Appendix IV in the lab book.

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Procedure Notes You will complete the procedure through the simple distill, the first week. Work up of the solution and GC analysis will occur the second week.

These are some additional comments/changes to the procedure. They are not a complete summary of the procedure.

(1) Set the heating mantle and reflux condenser up on a lab jack, in preparation of converting to the simple distillation later in the lab. Water-cool the reflux condenser. (2) Once reflux begins (set the heating mantle control to ~75, and adjust from there), allow the reaction to proceed for 45 minutes, prior to switching to simple distillation. (3) While reflux is occurring, wash any dirty glassware, and assemble the apparatus for a simple distillation (refer to p. 660 in the lab book). Assemble everything from still head to receiver (25 mL round bottom flask). Grease joints, and use blue clips to hold everything together. (4) Once the 45 minutes of reflux is complete, perform the following steps quickly. Have the instructor or your neighbor assist you.  Turn off the water to the reflux condenser, drain and remove hoses.  Remove the condenser and quickly insert the still head of the simple distillation apparatus into the pot.  Attach the water hoses to the simple distillation apparatus. Once the simple distillation is complete, store the distillate in a 25 mL Erlenmeyer flask covered with parafilm. Label the container with and lab time.

Starting week 2: (Summer Session - all but GC analysis is performed in the first week). (5) During the washing steps, you do not need to stopper and shake the separatory funnel. Rather, you can swirl the solutions, 15-30 seconds for each step. The organic layer will switch from bottom to top during these various washings, so pay attention!! (6) Filter a portion of the organic layer through a pipet containing a bit of cotton (see p. 599). If necessary, store the product in a vial for later analysis. We will not perform the final simple distillation. (7) Determine the mass of your product. And analyze by gas chromatography.

Procedure for GC Operation: (1) To obtain a sample free of air bubbles, rapidly push/pull the syringe plunger while the needle is in the sample. Pull 0.5 l into the syringe.

(2) Pull the plunger back farther to pull the sample from the needle into the barrel of the syringe.

(3) Push the needle all the way into injector port A.

(4) Inject the sample, remove the syringe, and push “start” on the recorder, in rapid succession. Allow the recorder to run for 2 minutes before pushing the “stop” button.

(5) Advance paper by hitting “enter” repeatedly.

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Pre-lab Questions 1. Show the mechanism and stereochemistry for the following substitution reactions. (a) H D OH + HBr C H 3 7 (b) CH3

C2H5 OH + HBr

2. Use a CRC Index of Chemistry and Physics available in the lab to look up the following information (you can do this at the beginning of the lab). You will be looking under the section called Physical Constants of Organic Compounds. This is a large section, approximately in the center of the book. (a) the melting point of 1-bromobutane and 2-bromobutane 20 (b) the specific rotation  D of d-2-butanol and ℓ-2-butanol

Follow-up Questions 1. Explain the possible effect on your results if you had made the following errors in your experiment. (a) Forgot to add the sulfuric acid catalyst. (b) Refluxed the reaction mixture for only 20 minutes, prior to the distillation step. (c) Forgot to perform the wash with concentrated sulfuric acid in the separation step.

3. Show the mechanism and products (consider Zaitsev’s rule) for the elimination side reaction that may occur in the reaction of 2-butanol with HBr.

4. With the initial collection of product by steam distillation, the organic layer could have been the top layer or the bottom layer. Explain what composition of the product mixture would favor each scenario.

5. Write balanced equations showing how HBr and SO2 are consumed in the gas trap containing NaOH.