Free Radical Substitution

Definition

Free radical substitution is a type of organic reaction where a free radical intermediate is involved in the replacement of one atom or group in a molecule with another. This process is particularly relevant in the context of preparing alkyl halides from alkanes through radical halogenation.

5 Must Know Facts For Your Next Test

1. Free radical substitution reactions occur when a free radical intermediate is formed, which then reacts with another molecule to replace a specific atom or group.
2. The initiation step in radical halogenation involves the homolytic fission of the halogen molecule (X2) to generate two halogen radicals (X•).
3. The halogen radicals then abstract a hydrogen atom from the alkane, forming an alkyl radical intermediate.
4. The alkyl radical can then combine with another halogen molecule, replacing the hydrogen atom with a halogen atom and producing the alkyl halide product.
5. Radical halogenation reactions are typically carried out in the presence of light or heat to facilitate the homolytic fission of the halogen molecule and initiate the radical chain reaction.

Review Questions

• Explain the role of free radicals in the radical halogenation of alkanes.
  • In the radical halogenation of alkanes, free radicals play a crucial role. The initiation step involves the homolytic fission of the halogen molecule (X2) to generate two highly reactive halogen radicals (X•). These halogen radicals then abstract a hydrogen atom from the alkane, forming an alkyl radical intermediate. The alkyl radical can then combine with another halogen molecule, replacing the hydrogen atom with a halogen atom and producing the desired alkyl halide product. The free radical intermediates are essential for the propagation of the radical chain reaction that ultimately leads to the formation of the alkyl halide.
• Describe the mechanism of free radical substitution in the context of preparing alkyl halides from alkanes.
  • The mechanism of free radical substitution in the preparation of alkyl halides from alkanes involves three key steps: initiation, propagation, and termination. The initiation step begins with the homolytic fission of the halogen molecule (X2) to generate two halogen radicals (X•). These halogen radicals then abstract a hydrogen atom from the alkane, forming an alkyl radical intermediate. In the propagation step, the alkyl radical combines with another halogen molecule, replacing the hydrogen atom with a halogen atom and producing the alkyl halide product. The termination step occurs when two radicals combine, forming a stable molecule and ending the radical chain reaction.
• Evaluate the factors that influence the efficiency and selectivity of free radical substitution reactions in the context of preparing alkyl halides from alkanes.
  • The efficiency and selectivity of free radical substitution reactions in the preparation of alkyl halides from alkanes depend on several factors. The choice of halogen (X2) can impact the ease of homolytic fission and the reactivity of the resulting radicals, affecting the rate and selectivity of the reaction. The presence and intensity of light or heat can also influence the initiation of the radical chain reaction, as well as the propagation and termination steps. Additionally, the structure of the alkane substrate can play a role, as the stability and reactivity of the resulting alkyl radicals can vary depending on the substitution pattern. Careful control of these factors is crucial for maximizing the yield and regioselectivity of the desired alkyl halide product.

Related terms

Free Radical:

A highly reactive species that contains an unpaired electron, often denoted by a dot (•).

Radical Halogenation:

A reaction where a halogen molecule (X2) is added to an alkane, resulting in the replacement of a hydrogen atom with a halogen atom.

Homolytic Fission: The breaking of a covalent bond in a way that each fragment retains one of the bonding electrons, forming two free radicals.