Radical Halogenation PDF

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Summary

Detailed notes relating to radical halogenation in organic chemistry....

Description

1. Within each of the following sets of alkyl radicals: identify the type of each radical (primary, secondary, or tertiary); and order each set from most to least stable.

Structure:

H C

C H

C

H

Type: Stability:

H C

Structure:

H H

C

C

Type: Stability:

Structure:

F3C

C H

H C F3C

H C F 3C

Type: Stability: 2. Using structures, explain the difference in the stability of primary and tertiary radicals.

3. For each reaction below, identify how many different constitutional isomers can form upon reaction with a generic halogen (X2). a. In the boxes to the right, draw the product(s) you would expect to be major if the reaction was fluorination versus bromination.

Structures

F F

# of isomers

Fluorination product(s)

Bromination product(s)

4. Experiments show that reaction A is much faster than reaction B, despite both involving secondary radicals of nearly equivalent energy. Explain this difference. Hint: It may be helpful to use energy diagrams to support your argument. Reaction A Br2 h

Reaction B Br

Br +

HBr

Br2 h

+

HBr

5. Consider the Boltzmann distributions below. These distributions correspond to radical halogenations conducted with three different halogens, all conducted at the same temperature. The activation energies for the formation of primary, secondary, and tertiary radicals are shown on each diagram.

3º

2º

1º

3º

2º

1º

3º

2º

1º

a. Identify the regions on the diagrams where you would expect ONLY tertiary radicals to form. b. Label each diagram as fluorination, chlorination, or bromination. c. Justify your choice, being sure to cite Hammond's postulate as part of your explanation. You may want refer to the energy values for this mechanism in your textbook. 6. Consider the Boltzmann distributions below. These distributions correspond to the same radical halogenation reaction, conducted at three different temperatures. Again, the activation energies for the formation of primary, secondary, and tertiary radicals are shown on each diagram.

3º

2º

1º

3º

2º

1º

3º

2º

1º

a. Identify the regions on the diagrams where you would expect primary, secondary, or tertiary radicals to form. b. Which halogenation was conducted at the highest temperature? c. Does a lower or a higher temperature give you a more selective reaction? Explain your choice.

7. Draw a complete arrow-pushing mechanism for the bromination of 1-methylcyclohexane. Show all possible termination steps. Br

h

+ HBr

+ Br 2

8. Iodomethane and hydroiodic acid react to form methane and molecular iodine upon irradiation with light. Propose a mechanism for this reaction. CH3I

+

HI

→

CH4

+

I2

9. “Direct” iodination of an alkane with iodine (I2) is energetically unfavorable, and has no synthetic utility. However, a different reagent (N-iodosuccinimide, below) can be used to introduce iodine via a radical mechanism. Propose a reasonable mechanism for this reaction. Hint: Bonds between electronegative atoms are typically weak... O N I O

O +

I

+

N H O...