Radical Reactions

Radical Reactions Study Notes

10.2 Identifying the Weakest C-H Bond in a Compound

• Step 1: Consider all possible radicals that would result from homolytic cleavage of a C-H bond.

  • Types of Radicals:

  • Primary Radicals

  • Tertiary Radicals

  • Allylic Radicals

  • Vinylic Radicals

• Step 2: Identify the most stable radical from the cleavage results.

• Step 3: The most stable radical indicates which bond is the weakest.

  • Weakest bond is determined by the stability of resulting radicals.

10.3 Drawing Fishhook Arrows for a Radical Process

• There are six characteristic patterns to recognize in radical processes.

• Homolytic cleavage is the initial step leading to the formation of radicals.

• Addition to a bond is a subsequent reaction pathway.

Practice Problems 493

• Engage in practice problems to reinforce understanding of radical processes.

10.5 Predicting the Selectivity of Radical Bromination

• Step 1: Identify all possible positions for radical formation.

  • Types of positions include:

  • Primary

  • Secondary

  • Tertiary

• Step 2: Determine the most stable location for a radical. The stability hierarchy impacts the selectivity in bromination.

  • Example:

  • Tertiary position is favored for bromination due to stability.

• Step 3: Bromination occurs at the location of the most stable radical.

  • Major Product: the outcome of the homogeneous reaction at the preferred site.

Try Problems 10.12, 10.13, 10.26, 10.29, 10.34a-c,f, 10.38, 10.40

10.6 Predicting the Stereochemical Outcome of Radical Bromination

• Step 1: Identify the specific location where bromination will occur.

• Step 2: If the position is a chiral center, or will become a chiral center, a loss of configuration is expected leading to the formation of both possible stereoisomers.

Steps in Bromination Process

1. Identify the type of process—halogen abstraction.

2. Determine the number of fishhook arrows required (usually three).

3. Identify any bonds being broken or formed during the reaction.

4. For a bond being formed, draw two fishhook arrows.

5. For a bond being broken, ensure to show both fishhook arrows.

Try Problems 10.5-10.7, 10.25, 10.45

10.4 Drawing a Mechanism for Radical Halogenation

1. Initiation:

   • Homolytic bond cleavage creates radicals, thereby initiating the radical process.

2. Propagation:

   • Hydrogen abstraction forms a radical on a carbon atom.

   • Halogen abstraction forms the final product and regenerates a halogen radical.

3. Termination:

   • Coupling can lead to producing the final product while destroying radicals.

Example of Radical Mechanism

• Hydrogen Abstraction: Abstraction means 'taking away'. In this step, a radical (often a halogen radical, $X$) removes a hydrogen atom from a molecule (R-C-H), forming a new bond with that hydrogen and creating a new carbon-centered radical ($R-C \bullet$ ). The original molecule loses a hydrogen, becoming a radical itself.

  • Reaction: $(R-C-H) + X \bullet \rightarrow (R-C \bullet) + (H-X)$

• Halogen Abstraction: In this step, a carbon-centered radical ($R-C \bullet$) removes a halogen atom (X) from another

• Halogen Abstraction:

  • Reaction: $(R-C) + (X:\text{R}) <br>ightarrow (R-C:X) + (R)$

10.7 Predicting the Products of Allylic Bromination

• Step 1: Identify the allylic position within the compound.

• Step 2: Remove a hydrogen atom and draw resonance structures which reflect the possible states of the resulting radical.

• Step 3: Place a bromine atom at each location that bears an unpaired electron, resulting in a mixture of products.

  • Example of products can lead to a racemic mixture.

Try Problems 10.14, 10.15, 10.30, 10.35, 10.41, 10.55

10.8 Predicting the Products for Radical Addition of HBr

• Step 1: Identify the two groups being added across the double bond.

• Step 2: Determine the expected regioselectivity for the addition.

• Step 3: Identify the expected stereospecificity of the addition.

  • Example: $(HBr)$ adds with bromine preferentially to the less substituted position.

Practice Problems

• Try Problems 10.21, 10.22, 10.47

Practice Problems

• Included in Answers Section:

  • 10.24: Draw all resonance structures for each of the following radicals.

  • 10.25: Consider all types of C-H bonds in cyclopentene and rank them in order of increasing bond strength, involving detailed evaluation of bond types.

  • For exercise:

  • (a)

  • (b)

  • (c)

  • (d)

  • (e)