Question

Hydroboration of 2 -methyl-2-pentene at \(25^{\circ} \mathrm{C}\), followed by oxidation with alkaline \(\mathrm{H}_{2} \mathrm{O}_{2}\), yields 2 -methyl-3-pentanol, but hydroboration at \(160^{\circ} \mathrm{C}\) followed by oxidation yields 4 -methyl-1-pentanol. Suggest a mechanism.

Short answer

At 25°C, it forms 2-methyl-3-pentanol; at 160°C, rearrangement forms 4-methyl-1-pentanol.

Step-by-step solution

Understand Hydroboration-Oxidation

Hydroboration-oxidation of alkenes is a two-step reaction that converts alkenes into alcohols. The hydroboration step adds a boron atom across the double bond, while the oxidation step replaces the boron with a hydroxyl group.

Identify the Structure of 2-Methyl-2-Pentene

2-Methyl-2-pentene is a five-carbon chain with a double bond between the second and third carbons, and a methyl group attached to the second carbon.

Hydroboration at 25°C

At 25°C, hydroboration of 2-methyl-2-pentene occurs via an anti-Markovnikov addition, where boron adds to the less substituted carbon. This results in the addition of boron to the third carbon, forming the intermediate 2-methyl-3-pentylborane.

Oxidation with Alkaline H2O2 at 25°C

Oxidation replaces the boron with a hydroxyl group, resulting in the production of 2-methyl-3-pentanol. This occurs because the boron is on the less hindered carbon.

Hydroboration at 160°C

At 160°C, the hydroboration is more likely to involve rearrangements due to the increased thermal energy. Here, boron may initially add to the less substituted carbon, but the intermediate can rearrange to form 4-methyl-1-pentylborane due to more stable isomer formation.

Oxidation with Alkaline H2O2 at 160°C

In this case, oxidation replaces the boron with a hydroxyl group, resulting in 4-methyl-1-pentanol, since the intermediate rearranged to a more thermodynamically stable form during hydroboration.

Key concepts

Anti-Markovnikov Addition

In the world of organic chemistry, predicting how molecules will react is crucial. One key concept involving alkenes is the 'Anti-Markovnikov Addition.' This principle describes how certain reactions lead to the addition of atoms or groups to a double bond in an unexpected manner. Specifically, during hydroboration, boron adds to the less substituted carbon atom of an alkene, contrary to what Markovnikov's rule would predict.

When we consider the example of 2-methyl-2-pentene, the hydroboration reaction at low temperatures (such as 25°C) follows this anti-Markovnikov pathway. This means that the boron atom prefers attaching itself to the third carbon in the chain, which is less hindered than the second carbon already burdened with a methyl group. Consequently, the initial product is 2-methyl-3-pentylborane.

This selectivity is influenced by a combination of steric and electronic factors which favor the less crowded carbon during the addition. It's a fascinating departure from conventional expectations in chemistry and serves as a basis for diverse synthetic applications.

Alkene to Alcohol Conversion

Transforming alkenes like 2-methyl-2-pentene to alcohols involves multiple steps, critical for creating various compounds in both laboratory and industrial settings. Through the process of hydroboration-oxidation, an alkene's double bond is efficiently turned into an alcohol group.

The course of this transformation begins with the hydroboration, where a boron-containing compound adds to the alkene. In the standard condition (25°C in our example), boron attaches anti-Markovnikov style to give an alkylborane, specifically 2-methyl-3-pentylborane.

Following this, the crucial oxidation step uses hydrogen peroxide in an alkaline medium to swap the boron atom out for a hydroxyl group. This feat allows chemists to generate 2-methyl-3-pentanol as a final product. The overall sequence avoids carbocation rearrangements and stereo complications, favoring a straightforward conversion from alkenes to their respective alcohols.

• Boron attaches to less substituted carbon (anti-Markovnikov)
• Oxidation exchanges boron with OH group
• Resulting in alcohol formation

Temperature Effects in Reactions

Temperature can drastically influence the course of a chemical reaction. It dictates not only the rate but also the pathway and outcome of reactions due to the energy available to the reacting species.

In the mechanism of hydroboration-oxidation, the example with 2-methyl-2-pentene showcases how temperature plays a pivotal role. At a lower temperature of 25°C, the addition of boron to the alkene follows the anti-Markovnikov rule, leading to the expected product: 2-methyl-3-pentanol.

However, increasing the temperature to 160°C affords the reaction more energy. This additional thermal energy can facilitate unexpected rearrangements during hydroboration. At such higher temperatures, the intermediate may shift its structure to form a more stable isomeric form, resulting in 4-methyl-1-pentanol instead.

• Lower temperatures favor predictable reactions
• Higher temperatures can lead to rearrangements
• Temperature influences both rate and selectivity

Understanding these temperature effects is crucial for organic synthesis, enabling chemists to tailor conditions for desired outcomes efficiently.