Question

Oxymercuration-demercuration reaction of 1 -methylcyclohexene gives (a) trans-2-methyl cyclohexanol (b) cis-2-methylcyclohexanol (c) 1 -methylcyclohexanol (d) mixture of cis-and trans-2-methylcyclohexanol

Short answer

The reaction produces a mixture of cis- and trans-2-methylcyclohexanol (option d).

Step-by-step solution

Understand the Reaction

In the oxymercuration-demercuration reaction, an alkene is converted into an alcohol by adding a mercury(II) acetate and then reducing the intermediate with sodium borohydride. It typically proceeds in a Markovnikov fashion, placing the -OH group at the more substituted carbon.

Analyze the Starting Material

The starting material is 1-methylcyclohexene. This is a cyclohexene ring with a methyl group attached to the carbon at position 1. The double bond is between this carbon and another carbon in the ring.

Apply Markovnikov's Rule

According to Markovnikov's rule, during the addition of the -OH group, it will attach to the more substituted carbon, which is carbon 2 here, because it leads to a more stable carbocation intermediate.

Reaction Specifics

For cycloalkenes like 1-methylcyclohexene, the addition of the -OH group and -Hg(OAc) group before reduction occurs across the double bond. Oxymercuration typically leads to anti-addition, but in this case, the sterics of the cyclohexane ring influence the stereochemistry.

Product Configuration

In cyclohexanes, the orientation can lead to one of two stereochemical outcomes. Oxymercuration usually leads to formation of an anti-product, but the reduction step can impact the stereochemistry, leading to a mixture of isomers in some cases.

Key concepts

Markovnikov's Rule

Markovnikov's rule is a guideline for predicting the outcome of certain types of addition reactions in organic chemistry. When an unsymmetrical reagent, like water or an alcohol, adds to an unsaturated compound such as an alkene, Markovnikov's rule helps determine where the new atoms will attach. According to this rule, the hydrogen from the reagent will bond to the carbon atom with more hydrogen atoms, while the other part of the reagent, usually the more electronegative component, attaches to the carbon with fewer hydrogen atoms. Therefore, during the oxymercuration-demercuration reaction, the hydroxyl group from water attaches to the more substituted carbon in the alkene. This ensures that the resulting carbocation intermediate is more stable.

This principle can be understood using the concept of carbocation stability—more highly substituted carbocations, like tertiary ones, are more stable than those with fewer substituents. Oxymercuration follows Markovnikov's rule because it proceeds through a mechanism that favors the formation of stable carbocations, even though the reaction itself does not create free carbocations.

Stereochemistry in Reactions

Stereochemistry is a crucial aspect of understanding organic reactions, as it deals with the spatial arrangement of atoms in molecules and how this influences the properties and reactions of these molecules. For reactions like oxymercuration-demercuration, stereochemistry becomes particularly important because it can determine the physical and chemical properties of the final product.

In the oxymercuration step, the addition of the mercuric acetate and water occurs through an anti-addition mechanism, where the attacking groups add to opposite sides of the double bond. However, because of the unique three-dimensional shape of cyclic compounds like cyclohexane, the resulting product can be affected by sterics, leading to different stereochemical outcomes.

The subsequent demercuration step involves the reduction of the mercury-containing intermediate by sodium borohydride, which can further influence the stereochemistry of the product. As a result, this reaction doesn't always produce a single stereoisomer but often a mixture, depending on the specific conditions and the starting material's structure.

Oxymercuration Mechanism

The oxymercuration mechanism involves the sequence of adding mercury acetate to an alkene and then reducing the intermediate to form an alcohol. The process starts with the formation of a three-membered mercurinium ion bridge after mercury(II) acetate reacts with the carbon-carbon double bond of the alkene.

This intermediate ion is crucial because it prevents rearrangements, stabilizing the carbocation as Markovnikov's addition occurs. Then, water—acting as a nucleophile—attacks the more substituted carbon within the mercurinium ion bridge, leading to the addition of an -OH group to form an organomercury alcohol intermediate.

The final step, demercuration, involves the reduction of this organomercury intermediate using sodium borohydride. This reduction replaces the mercury group with a hydrogen, converting the intermediate into the desired alcohol. This multistep reaction is well-regarded for its ability to produce Markovnikov products without carbocation rearrangement.

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is vital for deciphering how transformations occur at the molecular level. They describe each step of a chemical reaction, providing a detailed pathway of how reactants are converted into products. For oxymercuration-demercuration, this means detailing the sequence of interactions from the initial formation of the mercurinium ion to the final alcohol product.

Organic mechanisms illustrate the movement of electrons with arrows to show the progression from reactants to transition states and intermediates to final products. These mechanisms help chemists predict products and their stereochemistry, anticipate reaction conditions, and develop new synthetic strategies.

In the case of the oxymercuration-demercuration reaction, understanding the mechanism enables chemists to control outcomes, such as preferentially forming a product with particular stereochemistry or reducing unwanted isomers. It also helps in recognizing the role of catalyst-like reagents such as mercury acetate and sodium borohydride, which are crucial for driving the reaction forward efficiently.