Problem 58 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

All options (a), (b), and (c) produce aldehydes upon hydroboration-oxidation.

Step 1: Understand Hydroboration-Oxidation

Hydroboration-oxidation of alkynes involves a two-step process, where an alkyne is first converted into an alkene via hydroboration and then further oxidized to a carbonyl compound, either an aldehyde or a ketone, depending on the attachment of the alkyne in the molecule.

Step 2: Identify Terminal Alkynes

Hydroboration-oxidation of terminal alkynes yields aldehydes, while internal alkynes yield ketones. Terminal alkynes are those with a hydrogen atom attached to one of the sp hybridized carbon atoms at the end of the alkyne. Option (a) and (b) both appear to be terminal alkynes, as they have the structure of \(\;R-C\equivC-H\;\), where \(R\) is hydrogen or a hydrocarbon group.

Step 3: Evaluate Hydrocarbon Chains

Check each option to see if the given alkyne is a terminal alkyne. - Option (a) \(\;\mathrm{CH}\equiv\mathrm{CH}\;\): This is ethyne, a terminal alkyne.- Option (b) \(\;\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C}\equiv\mathrm{C}-\mathrm{H}\;\): Propyne, also a terminal alkyne.- Option (c) \(\;\mathrm{CH}_{3}-\mathrm{C}\equiv\mathrm{C}-\mathrm{H}\;\): 1-butyne, another terminal alkyne.Since all these are terminal alkynes, they all will produce aldehydes upon hydroboration-oxidation.

Key Concepts

Terminal Alkynes

Terminal alkynes are special types of alkynes characterized by their structure. They possess a carbon-carbon triple bond with one of the carbons bonded to a hydrogen atom. This means that in terminal alkynes, the triple bond is at the very end of the carbon chain. You can identify these alkynes by their general formula:

• The presence of \[ - ext{C} \equiv ext{C}- ext{H} \] within their structure.

The hydrogen atom attached to the sp hybridized carbon atom is what differentiates terminal alkynes from internal ones. Internal alkynes, by contrast, have two hydrocarbon groups attached to the carbon atoms of the triple bond. Terminal alkynes play a distinct role in reactions like hydroboration-oxidation due to this unique configuration, which often results in the formation of aldehydes. Remember, while dealing with chemical reactions, knowing the location of the triple bond is crucial for predicting the outcome.

Aldehyde Formation

Aldehydes are important organic compounds characterized by the presence of a carbonyl group, \( \text{C} = \text{O} \), at the terminal carbon. During the hydroboration-oxidation process of terminal alkynes, an aldehyde is typically formed. This process occurs in two main steps:

• **Hydroboration**: Here, the alkyne reacts with borane, \( \text{BH}_3 \), leading to an addition across the triple bond. This step is essential because it is through this addition that the reactivity is directed towards the end carbon.
• **Oxidation**: In the subsequent step, hydrogen peroxide \( \text{H}_2\text{O}_2 \) performs the oxidative transformation of the borane intermediate into an aldehyde.

The reaction is regioselective, meaning it generally favors the formation of products with particular structural configurations. In this case, it favors the formation of aldehydes when the alkyne is terminal. This specificity is incredibly valuable, particularly if you're aiming to selectively synthesize aldehydes from alkynes.

JEE Main Chemistry

The JEE Main Chemistry exam evaluates students on a comprehensive range of chemistry concepts, including important reactions like hydroboration-oxidation. Understanding these reactions is crucial for success, as they form the basis for several questions involving functional group transformations and synthesis in organic chemistry. When preparing for JEE Main, keep in mind:

• **Conceptual understanding**: It's important to grasp the core principles underlying each reaction, including why aldehydes are formed from terminal alkynes.
• **Application**: Expect questions that test your understanding by requiring you to predict the products of reactions, especially with hydroboration-oxidation.
• **Practice**: Regular practice with past paper questions and mock exams will help solidify your knowledge and build confidence.

Mastering these concepts will not only help with JEE Main but also prepare you for further studies in chemistry and related fields. Always pay attention to reaction mechanisms and the structural nuances of reactants, as these will guide your understanding and ability to tackle complex problems.