Introduction:

    Chemists have studied the mechanism of substitution reactions in great detail.  The results of these studies have indicated that substitution reactions can be broadly divided into two mechanistic types, called S_N2 and S_N1.   S_N2 (Substitution,Nucleophilic, 2nd order) reactions proceed via a one step mechanism in which the incoming nucleophile attacks the electrophilic carbon center from the side opposite the leaving group.   This reaction mechanism implies that  the stereochemistry of a chiral center will be inverted.   Since the reaction occurs in one step without the formation of an intermediate, and both the nucleophile and the electrophile are involved in the formation of the transition state, the reaction is second order overall:  rate =[RX][:Nu]

    S_N1 reactions (Substitution,Nucleophilic, 1st order) proceed via two steps:  (1) slow dissociation of the C-X bond to form an intermediate carbocation and (2) a fast second step in which the C-Nu bond is formed.  Since only RX is involved in the slow step, the reaction is first order overall:  rate =k[RX].  Since the

intermediate carbocation is trigonal planar, the nucleophile can attack with equal probability from above or below.  This will result in racemization of a chiral center, since equal amounts of each enantiomer will result.

 

R—X  +  CH3CH2OH	AgNO3	R—OCH2CH3  +  AgX(ppt)  +  H+  + NO3-
R—X  +  NaI		R—I   + NaX

Kinetic studies have shown that the reaction of an alkyl halide with ethanol in the presence of silver nitrate proceeds through an S_N1 mechanism.  The silver coordinates with the leaving group and helps form a carbocation.  The ethanol solvent will act as the nucleophile, forming the ethyl ether.  Similar studies have shown that the reaction of an alkyl halide with sodium iodide proceeds through an S_N2 mechanism.  Given this, you will be investigating how the structure of the alkyl halide (substrate - the species being attacked), the structure of the nucleophile (the species attacking), the type of leaving group, and even the solvent affect the rates of these two reactions.

1. Add 2 mL of 15% sodium iodide (in acetone) into each of three clean, dry test tubes.  Add 2 drops of 1-bromobutane to the first test tube;  add 2 drops of 2-bromobutane to the second test tube; and add 2 drops of 2-bromo-2-methylpropane to the third test tube.  Stopper the test tubes with cork stoppers and shake to ensure good mixing.  Observe closely for 15-20 minutes for the appearance of cloudiness or precipitation.  Continue observing at intervals throughout the lab period.  Record your observations.
2. Add 2 mL of 0.1 M silver nitrate (in absolute ethanol) into each of  three clean, dry test tubes.  Add 1 drop of 1-bromobutane to the first test tube;  add 1 drop of 2-bromobutane to the second test tube; and add 1 drop of 2-bromo-2-methylpropane to the third test tube.  Stopper the test tubes with cork stoppers and shake to ensure good mixing.  Observe closely for 15-20 minutes for the appearance of cloudiness or precipitation.  Continue observing at intervals throughout the lab period.  Record your observations.

1. Add 1 mL of 15% sodium iodide (in acetone) into each of two clean, dry test tubes.  Add 2 drops of 1-bromobutane to the first test tube, and add 2 drops of 1-bromo-2,2-dimethylpropane to the second test tube.  Stopper the test tubes with cork stoppers and shake to ensure good mixing.  Record your observations.

1. Add 1 mL of 15% sodium iodide (in acetone) into each of two clean, dry test tubes.  Add 2 drops of 1-bromobutane to the first test tube, and add 2 drops of 1-chlorobutane to the second test tube.  Stopper the test tubes with cork stoppers and shake to ensure good mixing.  Record your observations.
2. Add 2 mL of 0.1 M silver nitrate (in absolute ethanol) solution into each of two clean, dry test tubes.  Add 1 drops of 2-bromo-2-methylpropane to the first test tube, and add 1 drop of 2-chloro-2-methylpropane to the second test tube.  Stopper the test tubes with cork stoppers and shake to ensure good mixing.  Record your observations.  
   

1. Add 1.0 mL of 15% sodium iodide (in acetone) into each of two clean, dry test tubes.  At the same time, add 0.1 mL of 1.0 M 1-bromobutane to the first test tube, and add 0.1 mL of 2.0 M 1-bromobutane to the other.  Record your observations.
2. Add 1.0 mL of 1.0 M 1-bromobutane into each of two clean, dry test tubes.  At the same time, add 0.1 mL of 7.5 % sodium iodide (in acetone) to the first test tube, and add 0.1 mL of 15% sodium iodide (in acetone) to the other.  Record your observations.
3. Add 0.5 mL of 0.1 M 2-chloro-2-methylpropane (in ethanol) into a clean, dry test tube.  Into a second clean, dry test tube, add 0.5 mL of 0.2 M 2-chloro-2-methylpropane (in ethanol).  At the same time, add 1.0 mL of 0.1 M silver nitrate (in ethanol) to each of the two test tubes.  Record your observations.
4. Add 1.0 mL of 0.1 M silver nitrate solution (in ethanol) into a clean, dry test tube.  Into a second clean, dry test tube, add 0.5 mL of 0.1 M silver nitrate solution (in ethanol) and 0.5 mL of ethanol.  At the same time, add 1.0 mL of 0.1 M 2-chloro-2-methylpropane to each of the two test tubes.  Record your observations.

• From your experiments, what role does the structure of the substrate play on the rate of S_N1 versus S_N2 reactions?
• From your experiments, what role does the steric hindrance play on the rate of S_N1 versus S_N2 reactions?
• From your experiments, what role does the leaving group play on the rate of S_N1 versus S_N2 reactions?.
• From your experiments, determine the expression for the rate law for the S_N1 reaction.  Does this fit with what you expected?
  
• From your experiments, determine the expression for the rate law for the S_N2 reaction.  Does this fit with what you expected?
• What is the advantage of using acetone as the solvent to study the S_N2 reactions?
  

 (Dreated by C. Snelling)
Updated 12/01/08 by J. Neilan)