Substitution Reactions of Alkyl Halides
Purpose: To examine the relative rates of various alkyl halides under Sn1 and Sn2 conditions.
We will be examining Sn2 reactions with the Finkelstein reaction. The conditions for this are NaI in acetone. NaI is soluble in acetone but the products of the reaction; NaCl and NaBr are not. We will watch the reaction by looking for the first appearance of the solid salts. Acetone is a good solvent for Sn2 reactions because it is a polar aprotic solvent.
We will be examining Sn2 reactions with an ethanolic solution of silver nitrate. The silver ion coordinates with a lone pair on the halogen which begins to weaken the carbon-halogen bond. In the rate determining step, the carbocation and solid silver halide is formed. We will monitor the reaction by looking for the formation of the solid.
R X Ag R+ + AgX
Assemble and label six clean, dry test tubes. To follow the rates, it will be easier to react only four test tubes at a time. To each add a different alkyl halide (0.1 mL). To each test tube add 1 mL of a 1 M solution of NaI in acetone and mix thoroughly. For each tube, note how long after the addition of the NaI solution the first trace of precipitate appears. Watch the tubes to a maximum time of 6 minutes after the addition. If no reaction occurs, place the tubes in a 50q water bath and record the time to reaction at the higher temperature. (Record the temperature of the bath.) Use Aluminum foil to cover the test tubes to discourage acetone from evaporating.
Assemble and label six clean, dry test tubes. To each add a different alkyl halide (0.1 mL). (Again, to follow the rates, it may be easier to react only four test tubes at a time.) To each test tube add 1 mL of a 0.1 M solution of AgNO3 in ethanol and mix thoroughly. (Avoid skin contact! Silver salts + skin -> long-lived brown stain). For each tube, note how long after the addition of the...
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