Photobromination Virtual Lab

Photobromination Virtual Lab (20 points)

Alkanes can react with bromine or chlorine to produce halogenation products where one of the hydrogens is replaced with a halogen. This is the only reaction that is taught in organic chemistry that begins with an alkane. This is a radical reaction which has a mechanism involving the movement of single electrons instead of pairs like in most mechanisms. Radical intermediates have the same trend in stability as carbocations 3 > 2 > 1 > methyl. They are also stabilized by resonance. Every radical reaction must begin with initiation and product the product during propagation. Termination evets are possible in radical reactions and they are not product for the reaction even if they form product.

Part 1: Follow the link, click on the Add Bromine buttons to see what happens and answer the questions below.

https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtjml/virtualdemo/virtdemo.htm

  1. What happens to cyclohexane when bromine is added WITHOUT sunlight? Why does this happen? (0.5 points)
  2. What happens to cyclohexane when bromine is added WITH sunlight? Why does this happen? (1 point)
  3. What happens to cyclohexENE when bromine is added WITHOUT sunlight? Why does this happen without sunlight? (1 point)
  4. What happens when benzene is reacted with bromine? The pi bonds in benzene are special and they are difficult to break without a catalyst or groups that activate them. (0.5 point)

Part 2

The following chemicals listed in the table were dissolved in CH2Cl2 (reaction solvent) and placed in test tubes. A solution of bromine in CH2Cl2 (solvent) was added to each test tube and the tubes were placed under a heat lamp. Bromine is a reddish solution and when it reacts with a molecule the reddish color disappears or fades. Some reactions may not turn clear because some bromine remains and the products or minor impurities may have a slight color. Use the lab data table to determine when each reaction was complete. CH2Cl2 was used as a blank and it never reacts because the solvent cannot react with bromine. The blank is used to compare to the other test tubes to see if they change color.

Lab Data Table

Name

Color t = 0

Color t = 1 min

T = 5 min

T = 15 min

T= 30 min

T = 150 min

cyclohexane

Red

Red

Red

Red

clear

clear

cumene

Red

clear

clear

clear

clear

clear

Methyl cyclohexane

Red

Red

Red

clear

clear

clear

tertbutyl benzene

Red

Red

Red

Red

Red

Yellow

CH2Cl2 (blank)

Red

Red

Red

Red

Red

Red

Table (3 pts)

Chemical Name

Line Angle Structure

Bromination Reaction Product Structure

Time Reaction Was Completed in minutes

cyclohexane

   

cumene

   

Methyl cyclohexane

   

tertbutyl benzene

   

CH2Cl2 (blank)

NA

NA

Never reacts

  1. Which compound reacts the fastest? Explain all of the reasons why it is the fastest. (2 points)
  2. What were the 2nd fastest and the 3rd fastest compounds to react? Explain the different in reactivities between these two compounds. (2 points)
  3. Which compound takes the longest to react? Explain why it is the slowest. (1.5 points)
  4. If the following compound was reacted with Bromine and light under the same reaction conditions as the compounds from the experiment, approximately how long would it take to react? Explain your prediction. Draw the major product of the reaction (3 points)
  5. Draw the product from the following reaction. Assume only one bromine is added. (1.5 point)
  6. Draw the product and complete mechanism for the following reaction including initiation, propagation and 1 example of termination. (4 points)
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