Relative Rates of Radical Bromination Reactions

Objective

To perform a series of radical brominations, correlate the rate of the reactions with structural features of the reactions, and provide an explanation for the differing rates of the reactions.

Background

Alkanes are not a particularly reactive class of molecules; thus, to perform a reaction with an alkane, highly reactive reagents are required. Reagents like Cl2 and Br2 are reactive enough to cause the substitution of one of the alkane’s hydrogen atoms for a halogen atom.

H H H Br H H C C Br Br C C H Br eq 1 H H H H H H

While Br2 is an electrophile, this reaction proceeds due to the relative ease with which Br2 undergoes homolytic cleavage, as drawn below.

Br Br Br Br eq 2

Both light and heat can initiate this Br to Br bond breaking, and once the radical forms, it will seek out an electron to fill its valence shell. Typically, this means the radical will abstract a hydrogen atom from the nearest alkane. Of course, when the bromine radical, an odd electron species, reacts with an even electron species, the alkane, another radical must form.

H H H H Br H H C C C C H Br eq 3 H H H H H

The reactive, electron deficient alkyl radical will now react with, essentially, the first molecule that it bumps into. If that molecule happens to be a Br2 then a new bromine radical is formed, and the cycle repeats itself. In other words, the formation of a bromine radical at the onset of the reaction has initiated a radical chain reaction as seen in Scheme 1.

Scheme 1 Br Br

Br H Br H H H H C C C C Br H H H H H H

H H H H Br C C H Br Br H

Radical carbon atoms are carbon atoms that do not have a complete valance shell. As such, they similar to carbocations. That is, both carbocations and carbon radicals are electron deficient, and it would be safe to hypothesize that the structural traits that stabilize carbocations could stabilize radical carbon atoms too.

In this experiment, you will perform five reactions. Your substrates are cyclohexane, ethylbenzene, methylbenzene (toluene), methylcyclohexane, and 1-methylethylbenzene (isopropylbenzene). You will perform a radical substitution using bromine, and you will determine the relative rates of the reactions. From this information, you will determine what structural features stabilize radical carbon atoms.

Procedure1

The following experiment should be conducted in a fume hood. 1. Obtain a lamp, a PowerStat power supply, and a lab jack and place them in a fume hood. 2. Place a beaker filled with cold (tap) water on the lab jack and position the lab jack next to your lamp. 3. Plug the lamp into the PowerStat and set the PowerStat to 80. 4. Obtain five test tubes. Carefully fill each test tube with 1 mL of the bromine–methylene chloride solution. Caution: even though the bromine has been diluted in methylene chloride it is still hazardous. 5. The bromine solutions should remain in the hood and proper precautions should be observed when handling the bromine solutions (gloves, eye protection, proper attire) 6. Label these test tubes with the names of the five substrates and place these test tubes in the cold water bath. 7. Obtain five more test tubes, and label each one with the name of one of the five substrates (one name for each tube).

1 Adapted, with permission, from “Halogenation of Alkanes: Relative Rates of Free Radical Bromination”, University of Colorado, Boulder, Department of Chemistry and Biochemistry.

8. Fill the test tubes with 1 mL of the methylene chloride solutions of cyclohexane, ethylbenzene, methylbenzene, methylcyclohexane, and 1-methylethylbenzene (one substrate per test tube). 9. Turn on the lamp and add these solutions quickly but carefully to the test tubes that contain the methylene chloride–bromine solutions.

10. Note the order that the reactions decolorize, and after fifteen minutes, transfer the still colored reactions to a warm water bath (50 °C). 11. Continue to note the order that the reactions decolorize. Discontinue the experiment when all but one of the test tubes has decolorized.

Wash out your test tubes with methylene chloride and repeat the experiment. During the second trial, you may wish to time the reactions especially for those reactions where it was difficult to determine the order in which the solutions decolorized.

Experimental Report

Complete the activity on the following page. Name ______Organic Lab

Draw structures for the substrates used in this experiment, and rank them in order of increasing reaction rate (1 for the fastest through 5 for the slowest).

(1-methylethyl)- methylbenzene methylcyclohexan benzene cyclohexane ethylbenzene (toluene) e (isopropylbenzene )

1. Which molecule was unreactive under the conditions employed.

2. Considering that benzene rings are unreactive toward the conditions used in this experiment, predict the product of the reaction of Br2 with toluene.

3. Explain the reactivity order for the substituted benzenes. Remember, radicals are electron deficient like carbocations are electron deficient.

4. Draw the hydrocarbon radical intermediates for the three reactions involving the three alkyl substituted benzenes.

5. Based on the reactivity order for substituted benzenes and your answer to question 1, predict the product of the reaction of Br2 with methylcyclohexane.

6. For radical substitution reactions, which process appears to be more important, resonance or degree of substitution? Support your response with the evidence collected in this lab.

7. Draw the radical hydrocarbon intermediate that is formed during the bromination of ethylbenzene. Also, draw all of resonance structures that help to stabilize the radical intermediate.

8. If a radical reacts with a non-radical, since one of the products of the reaction will be a radical, the radical chain reaction will continue. Starting with the intermediate drawn in question 7, suggest a reaction that would terminate the chain reaction.