Problem 119 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The structure of alkyne is linear. Terminal alkyne is acidic in character. It reacts with base to give acid-base reaction. Alkyne is nucleophile and gives electrophilic as well as nucleophilic addition reaction. Which one of the following compounds forms carbonyl production reaction with \(1 \%\) \(\mathrm{HgSO}_{4}+\operatorname{dil.} \mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (b) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{C}_{6} \mathrm{H}_{5}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (d) Cyclohexene

Step-by-Step Solution

Answer

Compound (b) CH3-CCC-C6H5 gives a carbonyl product.

Step 1: Identify the compound type

To form a carbonyl compound through hydration with mercury(II) sulfate and dilute sulfuric acid, the compound needs to be an alkyne. Alkenes and cycloalkenes will not undergo this reaction. Therefore, compounds (a) and (d) are eliminated as they are an alkene and a cycloalkene respectively.

Step 2: Choose the terminal alkyne

Mercury(II) sulfate-assisted hydration specifically applies to terminal alkynes, which have a triple bond at the end of the carbon chain. Testing (b) CH3-C ==-C6H5, we see it's a terminal alkyne; whereas, (c) CH3-C -CCCH3 is an internal alkyne. Thus, compound (b) is most likely to undergo the carbonyl formation reaction. (In representation, CH3-C(b)-$CC)

Step 3: Recognize the probable product

In terminal alkynes, the addition of water under the presence of  ext{HgSO}_{4} gives a ketone. Specifically, for (b) CH3-C-CC-C6H5, the result of this reaction leads to the generation of acetophenone.

Key Concepts

Terminal Alkyne

A terminal alkyne is a type of alkyne where the triple bond exists at the end of a carbon chain. This configuration means that one of the carbons involved in the triple bond is bound to only one other carbon atom, with the rest of its valency saturated by hydrogen.
Terminal alkynes are important in organic chemistry due to their reactivity and acidic properties. When considering reactions involving terminal alkynes, it's key to note that their placement at the end of a carbon chain makes them particularly suitable for specific reactions such as hydration.
In the context of the chosen exercise, compound (b), \( ext{CH}_3- ext{C} \- ext{C}- ext{C}_6 ext{H}_5\), is identified as a terminal alkyne. This is important because the presence of the terminal alkyne allows it to participate in the carbonyl formation reaction in the presence of \(1 \% \ ext{HgSO}_4 + \ ext{dil.} \ ext{H}_2 ext{SO}_4\), achieving further functional group transformations.

Acidity of Alkynes

Alkynes exhibit acidic properties, specifically at the hydrogen attached to a terminal carbon in a terminal alkyne. This is due to the sp-hybridized nature of the carbon, which holds the hydrogen atom. Sp-hybridized carbons possess a significant amount of s-character, resulting in a stronger hold of electrons, thus making the bonded hydrogen more acidic.
This acidic behavior can be observed through reactions with strong bases, which can deprotonate the alkyne. This property is crucial in many synthetic applications where the formation of carbon-carbon bonds is desired.

• The acidity of terminal alkynes means they can form intermediates that are good nucleophiles.
• The reaction with bases makes them unique participants in organic synthesis routes.

In our exercise, understanding the acidity of terminal alkynes helps elucidate why these compounds react well in environments conducive to carbonyl formation, reflecting the functional manipulation possible with these substrates.

Hydration of Alkynes

Hydration of alkynes is a well-known reaction in organic chemistry where an alkyne is converted into a carbonyl compound through the addition of water. For this reaction to occur effectively, especially for terminal alkynes, an acid catalyst like mercury(II) sulfate in dilute sulfuric acid is used. This catalyst facilitates the addition of water to the triple bond.
The process of hydration can lead to different products depending on the type of alkyne. Specifically, terminal alkynes typically form ketones upon hydration. This is because the addition of water to the terminal alkyne first yields an enol, which then rearranges to a ketone.

• The position of the triple bond significantly impacts the outcome of the reaction.
• Terminal alkynes form ketones, while internal alkynes might yield a mixture of products.

In the given solution, the compound \( ext{CH}_3- ext{C} \ ext{C}- ext{C}_6 ext{H}_5\) undergoes hydration to produce acetophenone, clearly demonstrating how terminal alkynes can be efficiently transformed through catalytic processes.