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Lab Manual: The Sn2 Reaction - Procedure: Synthesis of 1-Bromobutane

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Introduction

This experiment utilizes SN2 chemistry to convert 1-butanol (n-butanol) to 1-bromobutane (n-bromobutane). The nucleophile for the reaction is Br- ions. The nucleophile in this lab is generated from an aqueous solution of sodium bromide. The sulfuric acid acts as a catalyst in converting the OH functional group of butanol, to a better leaving group (water).

The by-products for this lab are 1-butene, di-n-butyl ether, and the starting alcohol (butanol). Any by-products are removed by extraction using concentrated sulfuric acid. The product can be removed from sulfuric acid, since it does not mix with H2SO4 and will remain as a separate layer.

Procedure: Synthesis of 1-Bromobutane.

Take a 25-mL round-bottomed flask. Dissolve 2.66 g of sodium bromide (solid, corrosive) in 3.0 mL of DI water and 1.60 g of 1-butanol. Add a couple of boiling chips to the round bottom flask. Cautiously, with steady swirling, add 2.2 mL (3.8 g) of concentrated sulfuric acid drop wise to the solution (conc. H2SO4 is extremely corrosive and will cause severe burns if you get any on you skin). You should notice that the NaBr will dissolve once you start to heat the reaction mixture. Reflux the reaction using a sand bath for 45 min. Make sure reactants don't evaporate out of the condenser during the reflux (make sure that water is flowing through the condenser, throughout the reflux).

Stick a wad of glass wool on the top of the condenser to prevent vapor from escaping. As time proceeds you should see the formation of your product as a separate layer on top of the aqueous layer. After the 45 minutes of reflux, distill (simple distillation) the product (from ~60-95 0C) into a large test-tube until no more water-insoluble droplets are seen to collect in the test-tube. One lab partner should wash the round-bottomed flask and the distillation apparatus as soon as the distillation is done and the apparatus has cooled down.

The other lab partner should move the distillate to a small disposable test-tube. Rinse the larger test-tube with about 1mL of water and transfer it to the same small disposable test-tube containing your product. Note that the 1-bromobutane now forms the lower layer.

Remove the 1-bromobutane from above, with a plastic pipette, and place it in another dry small test-tube. Add 1mL of concentrated sulfuric acid, and mix the contents well by flicking the tube. The acid will remove unreacted starting material and any by-products.

Allow the two layers to separate completely, and then remove the sulfuric acid layer (density: 1.841 g/mL; bottom layer) using a new plastic pipette. Wash the 1-bromobutane layer (density 1.275 g/mL) with 1mL of 3 M NaOH (density 1.11 g/mL).

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