Question

Which among the following alkyne will give an aldehyde on hydroboration oxidation reaction? (a) \(\mathrm{CH} \equiv \mathrm{CH}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\) (d) all of these

Short answer

All options (a), (b), and (c) give aldehydes on hydroboration-oxidation.

Step-by-step solution

Understanding Hydroboration-Oxidation

The hydroboration-oxidation reaction of alkynes typically transforms internal alkynes into ketones and terminal alkynes into aldehydes. The terminal alkyne is crucial here since it is converted into an aldehyde.

Identify Terminal Alkyne

Examine the given alkynes to identify which are terminal. Alkynes (b) and (c) are terminal because they each have a hydrogen atom attached directly to the carbon of the triple bond, while alkyne (a) is acetylene, the simplest alkyne, which also behaves as a terminal alkyne.

Choosing the Correct Option

Given that both terminal alkynes and acetylene can produce aldehydes upon hydroboration-oxidation, we need to ascertain which options would appropriately participate in such reactions.

Analyzing Each Option

Option (a), ethyne (acetylene), on hydroboration-oxidation forms acetaldehyde. Option (b), a terminal alkyne, forms an aldehyde, specifically butyraldehyde. Option (c), also a terminal alkyne, forms propionaldehyde. Therefore, all three given options can form an aldehyde.

Key concepts

Alkynes in Organic Chemistry

Alkynes are a fascinating group of hydrocarbons characterized by triple bonds. This unique triple bond is what sets alkynes apart from alkenes and alkanes, giving them distinct reactivity and chemical behaviors. Alkynes have the general formula \( \text{C}_n\text{H}_{2n-2} \), which reflects their high hydrogen deficiency compared to other types of hydrocarbons. Because of the triple bond, alkynes are linear, creating a 180-degree angle between atoms, which greatly influences their properties.

• Triple bonds are stronger and shorter than double bonds.
• Alkynes undergo addition reactions due to the multiple bonds between carbon atoms.

This reactive nature of alkynes, especially the terminal ones, makes them especially important in organic synthesis, where they can be transformed into other functional groups through specific reactions like hydroboration-oxidation.

Transforming Alkynes into Aldehydes

Aldehydes have a carbonyl group (\(\text{C=O}\)) bonded to at least one hydrogen atom, making them key intermediates in organic chemistry. They are typically formed from alcohols or through oxidation reactions. However, an interesting pathway to forming aldehydes involves the transformation of alkynes through hydroboration-oxidation.
The hydroboration-oxidation reaction is a two-step process invented to transform terminal alkynes into aldehydes:

• Hydroboration adds boron to the alkyne.
• Oxidation then replaces the boron with hydroxyl groups.

This results in the formation of an aldehyde, showcasing the versatility and transformation potential of alkynes.

The Significance of Terminal Alkynes

Terminal alkynes are a subset of alkynes where the triple bond is at the end of a carbon chain, meaning they possess a hydrogen atom directly bonded to one of the carbon atoms in the triple bond. This structure distinguishes them from internal alkynes where the triple bonded carbons are sandwiched between other carbons.
Terminal alkynes are especially reactive and are used extensively in synthesis because:

• They can be selectively functionalized at the terminal carbon.
• They participate readily in addition reactions leading to various transformations.

In the context of hydroboration-oxidation, terminal alkynes specifically convert into aldehydes due to the orientation of reactive intermediates, making them very important in organic synthesis pathways.

Exploring Organic Chemistry Reactions

Organic chemistry includes various reactions that transform one functional group into another, leading to the creation of new compounds with different chemical properties. This branch of chemistry is pivotal in the development of pharmaceuticals, dyes, and polymers.
Reactions such as hydroboration-oxidation are fundamental:

• They provide specific outcomes, such as converting alkynes into aldehydes or ketones.
• They are used to direct the synthesis of highly functionalized small molecules.

Understanding the mechanisms of such reactions allows chemists to predict and manipulate the course of chemical processes effectively. This knowledge is essential for both academic research and industrial applications.

Mechanisms Behind the Reactions

A reaction mechanism in organic chemistry is a step-by-step description of how a reaction occurs at the molecular level. Knowing the sequence of bonds forming and breaking is crucial to understanding why certain products are formed.
For hydroboration-oxidation:

• The alkyne undergoes syn addition of boron, where both the hydrogen and boron add to the same side of the triple bond.
• The boron acts as a guide, ensuring the subsequent attachment of water to form a hydroxyl group during oxidation.

This well-orchestrated mechanism results in the unique ability of terminal alkynes to convert into aldehydes, as seen in the exercise. A clear understanding of these steps provides deeper insight into organic reactions, essential for mastering organic synthesis.