Experiment 8

2nd Order Nucleophilic Substitution: The Preparation of 1-bromobutane from 1-butanol

Reading (in Zubrick): Ch 11 (all); Ch 22 (pp 228-229); Chap 29 (all)

Outline: Synthesize 1-bromobutane from 1-butanol using an S_N2 reaction. Separate and purify the product using simple distillation, and determine its relative purity by measuring its index of refraction.

Background

This experiment is the first one in which you will carry out a reaction with the specific purpose of making a new compound; that is, it is the first synthetic reaction you perform. It is your responsibility to read the procedure carefully ahead of time, and look up (in Zubrick) any laboratory techniques required. You must understand the procedure well enough to be able to go into lab, get set up, and execute the necessary steps quickly, efficiently, and accurately. Be sure to plan ahead so that you and your partner can assemble the next apparatus you will need while the previous step is still in progress.

Nucleophilic substitutions are one of the most generally useful classes of synthetic organic reactions. This experiment introduces this class of reactions, specifically the 2nd order nucleophilic substitutions, or S_N2.

As you learned in lecture, this general class of reactions requires three things: a nucleophile, an electrophile, and a leaving group. In order for the reaction to proceed via the S_N2 mechanism, the nucleophile should be in the "good" to "excellent" range, the electrophile must be unhindered (methyl or 1°), and the leaving group should also be in the "good" to "excellent" range.

In applying these criteria to the transformation we wish to make here (in acid, water is the leaving group),

CH₃CH₂CH₂CH₂-OH₂⁺ + Br^- → CH₃CH₂CH₂CH₂-Br + H₂O

we see that the first two are satisfied (bromide is an excellent nucleophile and the electrophile is a 1° alkyl group), but hydroxide is a poor leaving group, due to its negative charge and its high basicity.

The central question becomes, "how can we make OH^- into a better leaving group? We have a few choices: 1) react the alcohol with p-toluenesulfonyl chloride (abbreviated TsCl), which will convert the —OH group into a sulfonic acid ester, making it a much better leaving group--then react the ester with sodium bromide to produce 1-bromobutane; 2) react the alcohol with phosphorus tribromide (PBr₃), which converts the —OH into a "—P(OH)X₂" leaving group (where X is either —Br or —OH), and which also produces free bromide ions which react with the electrophile, replacing the new leaving group, all in one reaction mixture; or 3) using a strong acid to protonate the —OH group in the presence of the bromide ion, which changes the leaving group from hydroxide to water, and allows the bromide to react with it in the same mixture. We will use the last of these methods.

Procedure

Safety: 1-butanol and 1-bromobutane are both flammable liquids and irritants-- no flames will be allowed, and wear gloves while handling them. Concentrated sulfuric acid is strongly corrosive and toxic--wear gloves while handling it, and be sure to wash your gloves and your hands immediately after handling it. Sodium bromide, sodium bisulfite and calcium chloride are all irritants--gloves are recommended.

Required Equipment:

1. Obtain a 250-mL round bottom flask from the upper cupboard next to the prep lab door, or check out this round bottom flask from the stockroom (consult your instructor for whether you will be checking out the flask).  Make certain your flask, and all other glassware is clean.
2. Obtain a cork ring from one of the drawers to sit the flask in, when it is not being heated.  
3. Make an ice-water bath in a 600 mL beaker.

Experimental Procedure:

1. Add 20.0 g of sodium bromide and 15 mL of water into the 250-mL round bottom flask.
2. Swirl the flask until all (or most) of the sodium bromide is dissolved (some crystals remaining will be okay, since they will dissolve when the flask is heated).
3. Add 15 mL of 1-butanol to this mixture.
4. Place the flask, containing the above reaction mixture, in the ice-water bath (make sure the flask cannot tip over!!).
5. Slowly add 15 mL of concentrated sulfuric acid to the contents of the flask by pouring about 1 mL at time to the flask.  Let the acid run down the inside wall of the flask (you can use the side arm to add the acid).
6. Gently swirl the flask to mix the contents after each addition (be careful, and don't add the acid too fast).
7. Assemble the flask into a reflux apparatus using only the condenser tube attached to the flask (a drying tube is not used). Don't forget to add a few boiling stones.
8. Reflux the mixture for 45 minutes, making sure the reflux vapor ring does not rise above the halfway point of the condenser. When the reflux is over, turn off the heat and allow the condensate in the condenser to drain back into the flask.
9. After the reflux setup as cooled somewhat, use the reaction flask as the distillation pot for a simple distillation.
10. Heat reaction mixture and collect everything that has a boiling point of up to about 105°C. When the temperature reaches 105°C, continue to boil the mixture and collect the distillate.  You should test the distillate every 2 or 3 minutes to see if it is still water-insoluble (add the distillate to a tube of water, and see if a separate, organic phase is sill present). When there appears to be no more water insoluble droplets coming across, turn off the heat. Once the distilling flask has cooled, dispose of its strongly acidic waste in the appropriate waste container. The distillate contains both water and the product, 1-bromobutane, with a little sulfuric acid mixed in.
11. Using a separatory funnel, wash the distillate with about 20 mL of a 5% NaCl solution.  (How do you know which layer contains your organic product? Is the organic layer always the top layer?). If any pink color (due to the presence of Br₂) forms during this process, add about 0.1 g of sodium bisulfite to the mixture to remove it.
12. Collect the organic layer (is it on top or bottom?).  Record the volume of the crude 1-bromobutane.  Dry your product with anhydrous Na₂SO₄ (add enough salt to remove the cloudiness, indicating water removal).  
13. Store you liquid product until the next lab period in a tightly closed container (you must use a lid or ground-glass stopper).

Purify the 1-bromobutane by simple distillation.

Pour the clear, dry liquid into a 50- or 100-mL round bottom flask.  Be careful not to let any solid crystals go into the distillation pot.

Discard any forerun but start collecting the sample that distills above 90-95^oC.  Continue to collect the 1-bromobutane up to its boiling point.

Collect your distillate in a previously weighed flask or vial.

Measure the mass, volume, and index of refraction of the purified product.

 Put all of the 1-bromobutane in a sample vial labeled with your name: name of compound, mass, volume, index of refraction, and names of people in the group. Wrap the cap with parafilm and give the vial to your instructor.

Questions:

1. What is the difference between Na₂SO₄ (sodium sulfate) and Na₂SO₃ (sodium sulfite)? The Na₂SO₃ is used to convert Br2 into Br-, but the anhydrous Na₂SO₄ is used to dry your product.
2. What is the refractive index of your purified product?
3. What was the boiling point?
4. Explain why the upper phase, after reflux, contained the 2-bromobutane, even though its expected density is 1.276 g/cm³, and the density of water is ~1.00 g/cm³.  Think what is in the aqueous solution (e.g., what is the density of salt water?)
5. What is the theoretical yield of 1-bromobutane?  What is your actual yield of 1-bromobutane?  What is your percent yield of 1-bromobutane?
6. What is the purpose of the anhydrous Na₂SO₄?  Why is it important that none of the solid Na₂SO₄ be included in the final distillation of your 1-bromobutane?

Useful physical constants:

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Copyright © Donald L. Robertson (Modified: 09/18/2006)