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 the given alkynes will form aldehydes: the answer is (d) all of these.

Step 1: Understand Hydroboration-Oxidation of Alkynes

Hydroboration-oxidation of alkynes converts the alkyne into a carbonyl compound. It mainly produces aldehydes from terminal alkynes (where the triple bond is at the end of the molecule) and ketones from internal alkynes (where the triple bond is between carbon atoms that are not at the end).

Step 2: Analyze Option (a)

Option (a) is acetylene, \(\mathrm{CH} \equiv \mathrm{CH}\). It's a terminal alkyne, and upon hydroboration-oxidation, it can form an aldehyde.

Step 3: Analyze Option (b)

Option (b) is \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\). This is a terminal alkyne. Hydroboration-oxidation will also yield an aldehyde here.

Step 4: Analyze Option (c)

Option (c) is \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\). This is another example of a terminal alkyne, thus it too will form an aldehyde upon undergoing hydroboration-oxidation.

Step 5: Conclusion

Since all options (a), (b), and (c) are terminal alkynes, they will all produce aldehydes when they undergo hydroboration-oxidation reactions.

Key Concepts

Alkynes

Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. This triple bond is a defining feature that gives alkynes unique reactions compared to other hydrocarbons. Because the triple bond is composed of one sigma and two pi bonds, it often serves as a reactive site in chemical transformations.

Reactivity of alkynes is typically high due to these pi bonds, which can be broken with relative ease during chemical reactions, such as additions. One important reaction that involves the triple bond in alkynes is the hydroboration-oxidation reaction, where the alkyne is eventually transformed into a carbonyl compound such as an aldehyde or ketone.

Aldehyde Formation

The formation of an aldehyde from an alkyne involves a two-step reaction known as hydroboration-oxidation. This reaction mechanism is particularly useful for converting terminal alkynes into aldehydes.

**Step 1: Hydroboration**
During the hydroboration stage, the alkyne undergoes an addition reaction with a borane reagent, such as disiamylborane or 9-BBN. This step adds the boron atom and a hydrogen across the triple bond, creating a trialkylborane intermediate. Thanks to the stereochemistry of boron, this step is syn-addition, which means the boron and hydrogen add to the same side of the alkyne, preserving specific stereostructures.

**Step 2: Oxidation**
Following hydroboration, the trialkylborane intermediate is oxidized using hydrogen peroxide ( H_2O_2 ) with a base. This oxidation step replaces the boron group with a hydroxyl group by reshuffling the electrons, ultimately forming an enol. The enol isomerizes spontaneously to form a carbonyl group, resulting in the formation of an aldehyde if the initial alkyne was terminal.

Terminal Alkynes

Terminal alkynes are a special class of alkynes where the triple bond is located at the end of the carbon chain. This positioning allows them to undergo specific chemical transformations, where they are particularly noted for their ability to form aldehydes in hydroboration-oxidation reactions.

Due to the unique terminal position of the triple bond, terminal alkynes are more exposed and ready for reactions compared to internal alkynes (where the triple bond is nestled between carbon atoms). During hydroboration-oxidation of terminal alkynes, the addition of boron and the subsequent oxidation steps are more predictable. This specificity makes terminal alkynes an ideal choice for producing aldehydes as opposed to internal alkynes, which tend to favor ketone formation.

• Terminal alkynes undergo **syn-addition** during hydroboration, facilitating the formation of an aldehyde.
• They provide a straightforward route to make aldehydes due to fewer steric hindrances, thanks to their terminal nature.

Understanding this concept helps in predicting outcomes in synthetic organic chemistry, especially in the design of pathways for synthesizing complex molecules.