RELATIVE RATES OF COMPETING REACTIONS

By Dr. Robert D. Craig, Ph.D.• Analysis of 7 alkyl Halides-Orgo 1 lab

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

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 Acetonefavors 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 Acetonefavors the Sn2 reaction

Electrophile

Leaving Group

nucleophile

Rate = k[ROTs][OAc]

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 solutionFavors 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 carbocationrate = k[RX]

SN1 in Reality

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

Lets go to the Video tape!!!

• http://www.youtube.com/watch?v=g5_GEzr91BQ

• • http://www.youtube.com/watch?v=hXIQUChlJ

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