The SN2 Reaction: 1

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 1bromobutane. Designate how each minor-product is removed from the major product and
which layer holds the product in each experimental step performed.
bp 118°C den 0.810
MW 74.12 n 1.399
bp 101.6°C den 1.275
MW 137.03 n 1.439
This experiment utilizes SN2 chemistry to convert 1-butanol (n-butanol) to 1-bromobutane (nbromobutane). 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
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).
Water Out
Water in
Round bottom
with boiling
Gray Heating
Mantle with
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 ~6095 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 H2SO4
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 1butene and di-n-butyl ether are formed.
2. Why is it necessary to remove water before weighing the 1-bromobutane?