σ-complex

Definition

A σ-complex, also known as a sigma complex, is an intermediate species formed during electrophilic addition reactions to conjugated dienes. It is characterized by the formation of a three-membered ring structure that stabilizes the allylic carbocation, a key step in the mechanism of these reactions.

5 Must Know Facts For Your Next Test

1. The σ-complex is a key intermediate in the mechanism of electrophilic additions to conjugated dienes, allowing for the formation of allylic carbocations.
2. The three-membered ring structure of the σ-complex helps to delocalize and stabilize the positive charge of the allylic carbocation.
3. Nucleophilic attack on the σ-complex leads to the formation of the final addition product, with the nucleophile adding to the more substituted carbon of the diene.
4. The stability of the σ-complex, and thus the ease of formation of the allylic carbocation, is influenced by the substitution pattern of the conjugated diene.
5. The σ-complex intermediate is often a high-energy species, making it a critical transition state in the overall reaction mechanism.

Review Questions

• Explain the role of the σ-complex in the mechanism of electrophilic additions to conjugated dienes.
  • The σ-complex is a key intermediate in the mechanism of electrophilic additions to conjugated dienes. It forms when the electrophile adds to one of the carbon-carbon double bonds in the diene, creating a three-membered ring structure that helps to stabilize the resulting allylic carbocation. This stabilization of the allylic carbocation is a crucial step in the overall reaction mechanism, as it allows for the subsequent nucleophilic attack and formation of the final addition product.
• Describe how the stability of the σ-complex is influenced by the substitution pattern of the conjugated diene.
  • The stability of the σ-complex, and thus the ease of formation of the allylic carbocation, is influenced by the substitution pattern of the conjugated diene. More highly substituted dienes, which can better stabilize the positive charge of the allylic carbocation through resonance, will tend to form more stable σ-complexes. This, in turn, can affect the reactivity and product selectivity of the electrophilic addition reaction, as the more stable σ-complex will be more readily formed as an intermediate.
• Analyze the significance of the σ-complex intermediate in the context of electrophilic additions to conjugated dienes and allylic carbocations.
  • The σ-complex is a critical intermediate in the mechanism of electrophilic additions to conjugated dienes, as it allows for the formation and stabilization of the key allylic carbocation species. This stabilization is essential, as the allylic carbocation is a high-energy intermediate that must be formed in order for the overall reaction to proceed. The stability of the σ-complex, which is influenced by the substitution pattern of the diene, can have a significant impact on the reactivity and product selectivity of the electrophilic addition. Understanding the role and characteristics of the σ-complex is therefore crucial for predicting and interpreting the outcomes of these important organic reactions.

Related terms

Allylic Carbocation: An allylic carbocation is a positively charged intermediate that forms when an electrophile adds to a conjugated diene. It is stabilized by the delocalization of the positive charge across the π-system.

Electrophilic Addition: Electrophilic addition is a type of organic reaction where an electrophile adds to a π-bond, typically a carbon-carbon double or triple bond, to form a new σ-bond.

Conjugated Diene: A conjugated diene is a organic compound with two carbon-carbon double bonds separated by a single carbon-carbon bond, forming a continuous π-system.