Competitive Nucleophiles

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Competitive Nucleophiles With 1-Butanol
In this experiment you will determine the nucleophile strength of bromide ion vs. chloride ion in their reaction with 1-butanol in acidic solution. The reaction is shown below. The reagents are a convenient source of HBr and HCl. The reaction is an SN2 reaction with H2O as the leaving group and Cl- and Br- as the nucleophiles. The molar amounts of Cl - and Br- are equal, so the better nucleophile will lead to more product. You will analyze the amounts of 1-bromobutane and 1-chlorobutane by GC and NMR. We will also examine the same reaction with 2-methyl-2-propanol, using a reaction carried out by the lab assistant or instructor.

Consider that the nucleophiles may be influenced by interactions with the solvent. These interactions are much more significant in protic solvents (e.g., water and alcohol) than in aprotic solvents (e.g., acetone or DMSO).

Experimental Procedure

Assemble an apparatus for reflux using a 25-mL round-bottom flask, a reflux condenser, and a stir bar. Using a 10-mL graduated cylinder, obtain 10.0 mL of the solvent-nucleophile mixture (NH4Br + NH4Cl + H 2SO4) which is in an Erlenmeyer flask in the hood. Pour the mixture immediately into the round bottom flask. Add 0.75 mL of 1-butanol to the round bottom flask, attach the condenser, start the water circulating and the stir bar spinning. Turn on the heating mantle and heat the mixture at gentle reflux (look for the reflux ring in the condenser) for about one hour. After the heating period, lift the round-bottom flask from the heat and allow it to cool. Do not remove the condenser until the flask is cool. You can immerse the flask in cold tap water to help it cool (do not remove the condenser). You should see an organic layer at the top of the reaction mixture. Add ~1 mL of pentane to the mixture and gently swirl the flask. The purpose of the pentane is to increase the volume of the organic layer so it's easier to separate.

Using a Pasteur...
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