Sn1 and Sn2 lab-Orgo 1 lab• 7 alkyl halides are treated with sodium iodide in acetone and with ethanolic solution of silver nitrate• Author: Dr. Robert D. Craig, Ph.D
The procedure for today!!• In the experiments that follow, 7 alkyl halides are treated with sodium iodide in acetone and with ethanolic solution of silver nitrate.•• 1. Sodium Iodide in Acetone• 2. ethanolic silver nitrate solution
7 alkyl halides• The seven alkyl halides are 1-chlorobutane, 1- butane, 2-chlorobutane, 2-chlorobutane, 2- chloro-2-mehtylpropane, 2-brombutane, 1- chloro-2-methylpropane and bromo benzene.••• You can just follow pg 338 to 340 exactly! Th Microscale Run. Low level Stress!!!•
1. Sodium Iodide in Acetone favors the Sn2 reaction• Acetone with a dielectric constant of 21 is a relatively nonpolar solvent that will readily dissolve sodium iodide. The iodide ion is an excellent nucleophile, and the nonpolar solvent (acetone) favors the Sn2 reaction; it does not favor ionization of the alkyl halide.
Sodium Iodide in Acetone favors the Sn2 reaction nucleophileElectrophile Rate = k[ROTs][OAc] Leaving Group
favors the Sn2 reaction• The extent of reaction can be observed because sodium bromide and sodium chloride are not soluble is acetone and precipitate from solution if a reaction occurs.
A precipitation reaction!!Na+ I- + R-Cl R-I + NaCl (s)Na+I- + R-Br R-I + NaBr (s)
2. ethanolic silver nitrate solution Favors an Sn1• When an alkyl halide is treated with an ethanolic solution of silver nitrate, the silver ion coordinates with an electron pair of the halogen. This weakens the carbon-halogen bond because a molecule of insoluble silver halide is formed, the promoting an Sn1 reaction of the alkyl halide. The solvent, ethanol, favors ionization of the halide, and nitrate ion is a very poor nucleophile, so alkyl nitrates do not form by an Sn2 reaction.
Rate-determining step is formation of carbocation rate = k[RX]
Carbocation is wants to do it own thing!• Carbocation is more likely to react on side opposite leaving group• Suggests reaction occurs with carbocation loosely associated with leaving group during nucleophilic addition (Ion Pair)
lets make it happen!R-X Ag+-> R-X -> R+ + AgX (s)•On the basis of the foregoing discussion,• Let’s “make it happen!”•• The Microscale run!
The Microscale run• Label 7 small containers (reaction tubes, 3 mL, centrifuge tubes, 10 x 75 mm test tubes, or 1 mL vials), and place 0.1 mL of 100 mg of each of the following halides in the tubes.•
The Microscale run• Youre alkyl halides!• 1-chlorobutane, 1-butane, 2-chlorobutane, 2- chlorobutane, 2-chloro-2-mehtylpropane, 2- brombutane, 1-chloro-2-methylpropane and bromo benzene
The Magic !• To each tube then rapidly add 1 ml of 18% solution of sodium iodide in acetone, stopper each tube, mix the contents thoroughly, and note the time.
The precipitate!!!• Note the time of the first appearance of any precipitate. If no reaction occurs within about 5 min, place those tube in a 50 oC water bath and watch for any reaction over the next 5 to 6 min.
The precipitate!!!• Empty the tubes, rinse them with ethanol, place the same amount of each of the alkyl halides in each tube as in the first part of the experiment, add 1 ml of 1% ethanolic silver nitrate to each tube,• And note the time of addition of as well as the time of appearance of the first traces of any precipitate.•
unreactive halides• If a precipitate does not occur in 5 min, heat the tubes containing these unreactive halides in a 50 oC water bath for 5 to 6 min and watch for any reaction
effect of the solvent• To test the effect of the solvent on the rate of Sn1 reactivity. Compare the time needed for a precipitate to appear when 2-chlorobutane is treated with 1% ethanolic silver nitrate solution and when treated with 1% silver nitrate in a mixture of 50% ethanol and 50% water .
Tertiary alkyl halides react rapidly in protic solvents in Sn1
your analysis• In your analysis of these experiments consider the :• Nature of the leaving group• Effect of the structure of the alkyl halide• Effect of the solvent• Effect of the temperature
Lets go to the Video tape!!!• http://www.youtube.com/watch?v=g5_GEzr9 1BQ•• http://www.youtube.com/watch?v=hXIQUChlJ q0&feature=related