Propose a mechanism for the conjugate addition of a nucleophile (Nuc:^–) to acrylonitrile (H₂C=CHCN) and to nitroethylene. Use resonance forms to show how the cyano and nitro groups activate the double bond toward conjugate addition.

Verified step by step guidance

1. Identify the structure of acrylonitrile (H₂C=CHCN) and nitroethylene (H₂C=CHNO₂). Both molecules contain an electron-withdrawing group (cyano or nitro) attached to a carbon-carbon double bond, which makes the β-carbon electrophilic and susceptible to nucleophilic attack.
2. Explain the resonance stabilization of the electron-withdrawing groups. For acrylonitrile, the cyano group (-CN) can delocalize the negative charge through resonance: $H{C}_2=C_{1}CN\leftrightarrow C_2-C_1=CN$. For nitroethylene, the nitro group (-NO₂) can delocalize the negative charge through resonance: $H{C}_2=C_{1}NOO\leftrightarrow C_2-C_1=NOO$.
3. Describe the nucleophilic attack. The nucleophile (Nuc:-) attacks the β-carbon of the double bond (C2) because it is electrophilic due to the electron-withdrawing effects of the cyano or nitro group. This forms a new bond between the nucleophile and the β-carbon, while the π-bond electrons shift to the α-carbon (C1), creating a carbanion intermediate.
4. Show the resonance stabilization of the carbanion intermediate. For acrylonitrile, the negative charge on the α-carbon can delocalize into the cyano group: $C_1-C_{2}CN\leftrightarrow C_1=C_2-CN$. For nitroethylene, the negative charge on the α-carbon can delocalize into the nitro group: $C_1-C_{2}NOO\leftrightarrow C_1=C_2-NOO$.
5. Conclude the mechanism by protonating the carbanion intermediate. The carbanion formed after the nucleophilic attack is protonated by a suitable proton donor (e.g., water or an acid), resulting in the final product of the conjugate addition reaction. This step restores the stability of the molecule and completes the reaction.

Key Concepts

Conjugate Addition

Conjugate addition refers to the nucleophilic attack on the β-carbon of an α,β-unsaturated carbonyl compound, leading to the formation of a new bond. This reaction typically occurs when a nucleophile adds to the double bond, resulting in a product that retains the original carbonyl functionality. Understanding this mechanism is crucial for analyzing how nucleophiles interact with compounds like acrylonitrile and nitroethylene.

Resonance Stabilization

Resonance stabilization involves the delocalization of electrons across multiple structures, which can enhance the reactivity of a molecule. In the context of acrylonitrile and nitroethylene, the cyano and nitro groups can stabilize the negative charge that develops during nucleophilic attack through resonance, making the double bond more electrophilic. This concept is essential for understanding how these groups activate the double bond toward conjugate addition.

Electrophilicity of Double Bonds

Electrophilicity refers to the tendency of a species to attract electrons, making it susceptible to nucleophilic attack. In α,β-unsaturated systems like acrylonitrile and nitroethylene, the presence of electron-withdrawing groups (EWGs) such as cyano and nitro increases the electrophilicity of the double bond. This enhanced reactivity is a key factor in facilitating the conjugate addition mechanism, as it lowers the energy barrier for the nucleophile to attack.