The document discusses two reactions involving seven alkyl halides: 1) treatment with sodium iodide in acetone, which favors an Sn2 reaction, and 2) treatment with ethanolic silver nitrate solution, which favors an Sn1 reaction. For the Sn2 reaction, a non-polar solvent like acetone is used, favoring the reaction pathway. For the Sn1 reaction, a polar protic solvent like ethanol is used, promoting ionization of the alkyl halide. The rate of each reaction is observed by noting the time of appearance of any precipitate formed.

1. RELATIVE RATES OF COMPETING
REACTIONS
By Dr. Robert D. Craig, Ph.D.
• Analysis of 7 alkyl Halides-Orgo 1 lab

2. 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

3. 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

4. 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!!!
•

5. 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.

6. Sodium Iodide in Acetone
favors the Sn2 reaction
nucleophile
Electrophile Rate = k[ROTs][OAc]
Leaving Group

7. 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.

8. A precipitation reaction!!
Na+ I- + R-Cl  R-I + NaCl (s)
Na+I- + R-Br  R-I + NaBr (s)

9. 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.

10. Rate-determining step is formation of
carbocation
rate = k[RX]

11. SN1 in Reality

12. 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)

13. 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!

14. 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.
•

15. 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

16. 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.

17. 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.

18. 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.
•

19. 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

20. 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 .

21. Tertiary alkyl halides react rapidly in
protic solvents in Sn1

22. 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