Question

A student obtained two products from the reaction of 1,3-cyclohexadiene with Br2 (disregarding stereoisomers). His lab partner was surprised when he obtained only one product from the reaction of 1,3-cyclohexadiene with HBr (disregarding stereoisomers). Account for these results.

Accepted Answer

Step 1: Recognize that 1,3-cyclohexadiene is a conjugated diene, meaning it has alternating double bonds, which allows for resonance stabilization of intermediates during reactions. Step 2: Analyze the reaction with Br₂. This reaction proceeds via an electrophilic addition mechanism. The Br₂ molecule acts as an electrophile, and the π-electrons of the diene attack Br₂, forming a bromonium ion intermediate. This intermediate can be attacked by a bromide ion (Br⁻) at two different positions, leading to two possible products: 1,2-addition (kinetic product) and 1,4-addition (thermodynamic product). Step 3: Analyze the reaction with HBr. In this case, the reaction also proceeds via an electrophilic addition mechanism. The H⁺ from HBr acts as the electrophile, and the π-electrons of the diene attack H⁺, forming a carbocation intermediate. Due to resonance stabilization, the carbocation intermediate is delocalized, but the addition of Br⁻ occurs at the most stable position, leading to only one major product. Step 4: Compare the two reactions. The reaction with Br₂ allows for two different pathways (1,2- and 1,4-addition) because the bromonium ion intermediate can be attacked at two distinct positions. In contrast, the reaction with HBr leads to a single product because the carbocation intermediate directs the nucleophile (Br⁻) to the most stable position, resulting in regioselectivity. Step 5: Conclude that the difference in the number of products arises from the nature of the intermediates formed in each reaction. The bromonium ion in the Br₂ reaction allows for multiple attack sites, while the carbocation in the HBr reaction leads to a single, regioselective product.

A student obtained two products from the reaction of 1,3-cyclohexadiene with Br₂ (disregarding stereoisomers). His lab partner was surprised when he obtained only one product from the reaction of 1,3-cyclohexadiene with HBr (disregarding stereoisomers). Account for these results.

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Key Concepts

Electrophilic Addition Reactions

Markovnikov's Rule

Regioselectivity