The SN2 Reaction: 1-Bromobutane

Lab 7: The SN2 Reaction: 1-Bromobutane

From K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston. p247; revised 2/22/02

Prelab Exercise: Write a detailed flow sheet/ flow chart for the isolation and purification of 1- bromobutane. Designate how each minor-product is removed from the major product and which layer holds the product in each experimental step performed.

H2SO4 OH Br n-butanol NaBr 1-Bromobutane bp 118°C den 0.810 bp 101.6°C den 1.275 MW 137.03 n 1.439 MW 74.12 n 1.399

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).

\ Clamp Water Out

Condenser Water in

Round bottom flask with boiling chips

Transformer Gray Heating Mantle with sand

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). NaOH neutralizes any traces of H 2SO4 in the organic product layer. At this point, your product layer will turn milky. Let the two layers separate out and remove the top NaOH layer with a new plastic pipette. Please save all layers until you are absolutely sure that you have the product in hand.

Dry 1-bromobutane (product) by adding a small amount of anhydrous magnesium sulfate and mixing (glass rod) until the liquid clears. After 5 min, pour out the dried liquid into clean, dry and pre-weighed flask. Weigh the final product. Show your limiting reagent and calculate the theoretical yield and determine your percent yield (You need to show all work to get credit!). Post Lab Questions

1 Write complete reaction mechanisms for formation of 1-bromobutane and also show how 1- butene and di-n-butyl ether are formed.

2. Why is it necessary to remove water before weighing the 1-bromobutane?