Question

Terminal alkynes react with 9-borabicyclononane \((9-B B N)\) and then aq. alkaline hydrogen peroxide to give aldehydes because (a) Acid catalysed electrophilic addition of water follows Markovnikov's rule. (b) Acid catalysed electrophilic addition of water is anti-Markovnikov. (c) The radical addition is anti-Markovnikov. (d) Steric and electronic effects cause the boron to the terminal carbon.

Short answer

The correct answer is (d): Steric and electronic effects cause the boron to add to the terminal carbon.

Step-by-step solution

Analyze the Reaction

Terminal alkynes react with 9-borabicyclononane (9-BBN), followed by aqueous alkaline hydrogen peroxide, to yield aldehydes. This type of reaction is known as hydroboration-oxidation when applied to terminal alkynes.

Understand Anti-Markovnikov Addition

In hydroboration reactions, boron adds to the least substituted carbon (terminal carbon for terminal alkynes) due to steric and electronic effects. This is characteristic of anti-Markovnikov addition, which means boron, and subsequently, the hydroxyl group will add to the end of the carbon chain.

Role of 9-BBN

9-BBN is used to facilitate the hydroboration by adding across the triple bond. Its bulky structure results in straightforward regioselectivity, ensuring boron attaches to the terminal carbon atom.

Oxidation Step

In the next step, the organoborane compound is oxidized with hydrogen peroxide in an alkaline environment. The boron atom is replaced by a hydroxyl group, leading to the formation of an aldehyde.

Determine the Reason

While several options are presented, the primary reason for forming aldehydes with terminal alkynes in this reaction is because the 9-BBN adds boron to the terminal carbon due to steric and electronic effects. This aligns with option (d).

Key concepts

Terminal Alkynes

In organic chemistry, terminal alkynes are hydrocarbons that contain a carbon-carbon triple bond positioned at the end of the carbon chain. The triple bond is characterized by two carbon atoms sharing three pairs of electrons, resulting in linear geometry around the carbon atoms involved in the bond. This spatial configuration is responsible for the unique reactivity patterns of alkynes.

Terminal alkynes are significant because the carbon atom at the terminal position is slightly more acidic compared to internal alkynes. This makes it reactive towards specific reagents, leading to interesting transformations.

• They undergo reactions where the terminal hydrogen is replaced or reacted with other atoms or groups.
• Terminal alkynes can form organoborane compounds when subjected to hydroboration-oxidation.

Understanding the behavior and characteristics of terminal alkynes is essential for grasping advanced organic reactions like hydroboration-oxidation with reagents such as 9-BBN.

Anti-Markovnikov Addition

Anti-Markovnikov addition is a fascinating concept in chemistry where the addition reaction takes place in a manner contrary to Markovnikov's rule. Markovnikov's rule generally predicts that the hydrogen atom is added to the carbon with the greater number of hydrogen atoms ( rich get richer" principle), while the other atom or group adds to the carbon with fewer hydrogen atoms.

In contrast, anti-Markovnikov addition occurs such that the less substituted carbon atom (often the terminal carbon in alkynes) receives the new group or atom. This approach is key in modifying specific sites within organic compounds, offering more control over the products.

• Anti-Markovnikov additions are often facilitated by specific chemical environments and reagents.
• In the case of hydroboration-oxidation, boron adds to the less substituted carbon.
• This is due to steric and electronic considerations that favor the less crowded position.

This type of addition is pivotal in forming different functional groups on organic compounds, opening pathways to compounds like aldehydes and alcohols.

9-Borabicyclononane (9-BBN)

9-Borabicyclononane, commonly shortened to 9-BBN, is a reagent used prominently in the organic synthesis realm, particularly for hydroboration reactions. Known for its bulky and unique structure, 9-BBN offers distinct advantages in reactivity and selectivity when used with terminal alkynes.

The structure of 9-BBN involves a bicyclic compound with a boron atom, making it highly influential in guiding the boron atom to the desired position in a compound. This is mainly due to the steric effects that arise from its bulkiness:

• 9-BBN has a preference for adding boron to the terminal carbon of alkynes.
• The bulky structure ensures high regioselectivity, preventing boron from adding to other positions.
• Its use leads to efficient transformation of alkynes into useful functional groups like aldehydes.

The prowess of 9-BBN lies in its ability to make reactions proceed smoothly and with high preference for the terminal carbon, aligning perfectly with anti-Markovnikov principles.