Question

Prepare a detailed flow sheet for the isolation and purification of 1-bromobutane. Indicate how each reaction byproduct is removed, and which layer is expected to conatain the product in each separation step

Answer 1

In this experiment 1-butanol is converted to 1-bromobutane by an S_N2 reaction. In general, a primary alkyl bromide can be prepared by heating the corresponding alcohol with (1) constant-boiling hydrobromic acid (47% HBr), (2) an aqueous solution of sodium bromide and excess sulfuric acid, which is an equilibrium mixture containing hydrobromic acid, or (3) a solution of hydrobromic acid produced by bubbling sulfur dioxide into a suspension of bromine in water. Reagents (2) and (3) contain sulfuric acid at a concentration high enough to dehydrate secondary and tertiary alcohols to undesirable by-products (alkenes and ethers).

workup:

Remove the 1-bromobutane with a Pasteur pipette, and place it in a dry test tube. Add 1 mL of concentrated sulfuric acid, and mix the contents well by flicking the tube. The acid removes many unreacted starting material as well as any alkenyl or ethereal by-products. Allow the two layers to separate completely, and then remove the sulfuric acid layer. The relative densities given above will help identify the two layers. An empirical method of distinguishing the layers is to remove a drop of the lower layer into a test tube of water to see whether the material is soluble (H₂S0₄) or not (1-bromobutane). Separate the layers and wash the 1-bromobutane layer with 1 mL of 3 M sodium hydroxide solution (density 1.11 g/mL) to remove traces of acid, separate, and be careful to save the proper layer. In experiments of this type, it is good practice to save all layers until the product is in hand.• Dry the cloudy 1-bromobutane by adding anhydrous magnesium sulfate and mixing until the liquid clears and the magnesium sulfate no longer clumps together. After 5 min, decant the dried liquid into the dry 25-mL round-bottomed flask. Weigh the final product and determine your percent yield.