(a) trans-hex-3-ene + CH2I2, Zn(Cu)
(b) cis-hept-2-ene + CH2I2, Zn(Cu)
The Correct Answer and Explanation is :
The reactions in question involve a key organic chemistry transformation known as a Wolff–Löffler–Freytag reaction or halogenation with a zinc-copper catalyst, specifically using CH₂I₂ (diiodomethane) and Zn(Cu) (zinc-copper couple). This reaction is commonly used to generate cyclopropane derivatives through a reaction between alkenes and diiodomethane. The role of Zn(Cu) is to induce the formation of a carbenoid species from CH₂I₂, which then reacts with the alkene to form a three-membered ring (cyclopropane).
Reaction (a): trans-hex-3-ene + CH₂I₂, Zn(Cu)
- trans-hex-3-ene has the structure: CH₃-CH₂-CH=CH-CH₂-CH₃, with the double bond between carbon 3 and 4.
- Upon reaction with CH₂I₂ in the presence of Zn(Cu), a carbenoid (CH₂) is generated, which reacts with the alkene to form a cyclopropane ring.
- Due to the trans geometry of the alkene, the product will be a cis-cyclopropane because of the formation of a three-membered ring. The cis configuration arises because the carbenoid adds to the same side of the double bond, creating a cis-cyclopropane as the intermediate undergoes ring closure.
Product: cis-1,2-dimethylcyclopropane
Reaction (b): cis-hept-2-ene + CH₂I₂, Zn(Cu)
- cis-hept-2-ene has the structure: CH₃-CH=CH-CH₂-CH₂-CH₃, with the double bond between carbons 2 and 3.
- Upon reaction with CH₂I₂ in the presence of Zn(Cu), a similar process occurs, generating the carbenoid (CH₂) that adds across the double bond.
- Due to the cis geometry of the starting alkene, the carbenoid will add to the same side, forming a cis-cyclopropane product, as with reaction (a).
Product: cis-1,2-dimethylcyclopropane (similar to (a) but with a different initial substrate).
Explanation:
The main aspect of this reaction lies in the formation of a cyclopropane ring via CH₂I₂ and Zn(Cu). The reaction proceeds by a radical-like mechanism, where zinc-copper is crucial in producing a carbenoid intermediate (CH₂), which is highly reactive and adds across the double bond of the alkene. The cis addition occurs because the carbenoid intermediate is formed on the same side of the double bond due to its sp² hybridization, creating the cis-cyclopropane.
Let me generate an image of the mechanism for you.
Here is an image of the mechanism for the Wolff–Löffler–Freytag reaction, showing how cis-hept-2-ene reacts with CH₂I₂ and Zn(Cu) to form a cis-cyclopropane ring. This should help illustrate the process of the carbenoid intermediate forming and adding to the alkene. Let me know if you’d like further details!
