Question

Which of the following alkynes can be identified and distinguished from the rest of the alkynes on reaction with ammoniacal silver nitrate to gave a white precipitate? (a) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{3}\). (d) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

Alkyne (b) \boxed{\textbf{CH}_{3}\textbf{CH}_{2}\textbf{C} \triangleright \textbf{CH}} can be distinguished by its reaction with ammoniacal silver nitrate, forming a white precipitate.

Step-by-step solution

Understand the Reaction

Ammoniacal silver nitrate, also known as Tollen's reagent, reacts with terminal alkynes to give a white precipitate of silver acetylide. Terminal alkynes have a triple bond at the end of the carbon chain, with one of the carbons in the triple bond having a hydrogen atom.

Identify Terminal Alkynes

Examine each alkyne given in the options to determine if it is a terminal alkyne. Look for the presence of at least one hydrogen atom bonded directly to a carbon atom that is part of a triple bond.

Verify the Answer

Confirm that the identified alkyne(s) with the structural features of a terminal alkyne will indeed react with ammoniacal silver nitrate and form a white precipitate.

Key concepts

Ammoniacal Silver Nitrate Reaction

A fundamental test for identifying terminal alkynes in organic chemistry involves the ammoniacal silver nitrate reaction. This specific reaction is distinct for terminal alkynes – hydrocarbons with a triple bond at the end of their carbon chain. When a terminal alkyne is introduced to ammoniacal silver nitrate, also recognized by chemists as Tollen's reagent, a fascinating transformation occurs.

The terminal alkyne, due to the presence of a slightly acidic hydrogen atom attached to the carbon with a triple bond, readily donates this hydrogen to form what is known as an acetylide ion. This ion then interacts with the silver ions present in the reagent to form a silver acetylide complex. It's this reaction that results in the formation of a white precipitate, which serves as a visual confirmation of a terminal alkyne.

Silver Acetylide Precipitate

Upon reaction of a terminal alkyne with ammoniacal silver nitrate, the observable outcome is the formation of a silver acetylide precipitate - a white solid that appears at the bottom of the testing vessel. This solid is insoluble in the reaction mixture and stands out as an incontrovertible sign that a terminal alkyne was present.

The creation of the silver acetylide precipitate is crucial for its implications in identifying chemical structure. More than just a visual aid, the precipitate embodies the acetylide ion's capability to bond with metallic silver, a unique trait among hydrocarbons. In a classroom or laboratory setting, the emergence of this precipitate becomes a teachable moment, underscoring the interplay between chemical reactivity and structural properties.

Chemistry Problem Solving

In solving chemistry problems, especially those involving identifying chemical compounds, it's vital to apply a methodical approach. The journey begins with an understanding of the underlying chemical principles and the specific characteristics of the reactions involved.

For example, when tasked with distinguishing different alkynes using the ammoniacal silver nitrate test, one must first grasp the unique behavior of terminal alkynes in this reaction. Next, scrutinize the structure of each alkyne provided. This involves searching for the presence of a hydrogen atom bonded to a terminal carbon with a triple bond - the quintessential hallmark of the terminal alkyne. Finally, matching theory with practice, confirming through experimentation and observation (such as the formation of a white precipitate) fulfills the problem-solving process. This stepwise approach not only spotlights the solutions but also reinforces fundamental concepts integral to the discipline of chemistry.