Question

An organic compound (A) \(\mathrm{C}_{5} \mathrm{H}_{8}\) does not react with Tollens reagent but reacts with \(\mathrm{H}_{2}\) in the presence of Lindlar catalyst to give (B). Aqueous chlorine water reacts with (B) to give (C) which when reacted with hot alcoholic KOH gives (D). The compound (D) is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_{3}\) (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\) (d)

Short answer

Answer: None of the above.

Step-by-step solution

Reaction with Tollens Reagent

If compound (A) does not react with Tollens reagent, it is likely an alkene or alkyne, as aldehydes and ketones typically react with this reagent. Since the formula is C₅H₈, we know that there is a degree of unsaturation of 2. Compound (A) can be an alkyne as (5 * 2) - 2 = 8.

Reaction with H₂ in presence of Lindlar Catalyst

Since (A) reacts with H₂ in the presence of Lindlar catalyst to give (B), we know that this reaction involves the partial hydrogenation of the triple bond in (A) to form a cis alkene (B). If we assume that (A) is a linear alkyne, then (B) will be a linear cis-alkene.

Reaction with Aqueous Chlorine Water

Compound (B) reacts with aqueous chlorine water to form compound (C). Aqueous chlorine water typically does an addition reaction with alkenes, where two chlorine atoms add across the double bond. This reaction will form a 1,2-dichloroalkane compound (C).

Reaction with Hot Alcoholic KOH

When compound (C), a 1,2-dichloroalkane, reacts with hot alcoholic KOH, it will undergo dehydrohalogenation twice, which results in the formation of an alkyne. This reaction will lead to the formation of compound (D). Now, let's compare the structure of compound (D) with the given options: (a) CH₃CH₂CH₂C≡CH (b) CH₃CH=CHCH(OH)CH₃ (c) CH₃CH=CHCH=CH₂ None of these options match the formula of compound (D) that we found through the step-by-step process. Therefore, the correct option is: (d) None of the above

Key concepts

Tollens Reagent

Tollens' reagent, often referred to as the 'silver mirror' test, is used to determine the presence of aldehydes in a compound. It contains an aqueous solution of silver nitrate () and ammonia, which reacts to form silver ammonia complex . When the reagent is added to an aldehyde and heated gently, it gets reduced to metallic silver, creating a distinctive silver mirror on the inside of the test tube.

This test is highly specific for aldehydes, as ketones and other functional groups do not typically give a positive result. The fact that compound (A) in the original exercise does not react with Tollens' reagent suggests that it lacks the aldehyde functional group, leading us to deduce that it is likely an alkene or alkyne.

Lindlar Catalyst

The Lindlar catalyst is a finely divided palladium deposited on calcium carbonate and treated with various additives, including lead acetate. This catalyst is used specifically for the hydrogenation of alkynes to alkenes. Unlike a typical hydrogenation which fully saturates the molecule, Lindlar catalyst performs partial hydrogenation, typically leading to the formation of a cis alkene as the product.

In the context of our exercise, the compound (A) reacts with hydrogen in the presence of Lindlar catalyst to create compound (B), which is a cis-alkene. This demonstrates the usefulness of the Lindlar catalyst in selectively stopping the hydrogenation at the alkene stage, allowing for further chemical transformations without over-reducing to an alkane.

Alkyne Hydrogenation

Hydrogenation is a chemical reaction between molecular hydrogen () and another compound or element, usually in the presence of a catalyst like palladium, nickel, or platinum. For alkynes, hydrogenation can be either partial or complete depending on the catalyst used. Complete hydrogenation converts an alkyne to an alkane, while partial hydrogenation -- typically done with a Lindlar catalyst -- converts an alkyne to a cis-alkene.

Thus, alkyne hydrogenation is a critical step in the original exercise solution that transforms the alkyne (A) to the alkene (B). The selectivity of this process is vital for the control of the final product's geometry, which ultimately impacts its reactivity and properties.

Dehydrohalogenation

Dehydrohalogenation is an elimination reaction where a halogen atom and a hydrogen atom are removed from adjacent carbon atoms of a halogenated organic molecule, resulting in the formation of a double or triple bond. The reaction typically employs a strong base, such as sodium hydride or potassium hydroxide, and is often performed under heated conditions, especially with an alcohol solvent for better results.

The process is significant in organic chemistry synthesis because it provides a method to create alkenes or alkynes from more saturated compounds. In the exercise, hot alcoholic KOH is used to induce dehydrohalogenation of compound (C), a 1,2-dichloroalkane, to eventually yield the alkyne (D). This step exemplifies how dehydrohalogenation can effectively be applied to synthesize more unsaturated and complex organic structures from relatively simple starting materials.