Question

Identify the reagent from the following list which can easily distinguish between 1 -butyne and 2 -butyne. (a) Bromine water (b) Baeyer's reagent (c) Dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{HgSO}_{4}\) (d) Ammoniacal \(\mathrm{Cu}_{2} \mathrm{Cl}_{2}\)

Short answer

Ammoniacal \(\mathrm{Cu}_{2}\mathrm{Cl}_{2}\) can distinguish between 1-butyne and 2-butyne by forming a precipitate with the terminal alkyne (1-butyne) only.

Step-by-step solution

Understanding the Distinction

It is important to recognize that 1-butyne and 2-butyne are isomers of each other with different structures - 1-butyne has a terminal triple bond, while 2-butyne has an internal triple bond. A good distinguishing reagent would react differently with these two structural features.

Analyzing the Options

Bromine water (a) adds across double bonds but reacts slowly with triple bonds and cannot distinguish terminal from internal triple bonds. Baeyer's reagent (b) also tests for the presence of double bonds by oxidizing them to diols, but it is not specific for terminal alkynes. Dilute \(\mathrm{H}_{2}\mathrm{SO}_{4}+\mathrm{HgSO}_{4}\) (c) facilitates the hydration of alkynes to form ketones or aldehydes but does not easily distinguish between different types of alkynes. Ammoniacal \(\mathrm{Cu}_{2}\mathrm{Cl}_{2}\) (d) reacts specifically with terminal alkynes to precipitate a red or yellow copper acetylide compound, while internal alkynes do not react in a similar manner.

Selecting the Correct Reagent

Given the specific reaction of ammoniacal \(\mathrm{Cu}_{2}\mathrm{Cl}_{2}\) with terminal alkynes, we can conclude that this reagent will distinguish between 1-butyne (terminal alkyne) and 2-butyne (internal alkyne) by forming a precipitate with 1-butyne only.

Key concepts

Organic Chemistry

At the heart of understanding compounds like 1-butyne and 2-butyne lies the field of organic chemistry. Organic chemistry is a sub-discipline of chemistry that deals with the structure, properties, and reactions of organic compounds, which are carbon-based compounds. In this context, 1-butyne and 2-butyne are both organic molecules known as alkynes, a type of hydrocarbon characterized by a carbon-carbon triple bond.

Structural isomerism is a core concept in organic chemistry, where molecules share the same molecular formula but differ in the arrangement of their atoms. This is exactly the case with 1-butyne and 2-butyne; both have the same molecular formula, C₄H₆, but their structures and points of unsaturation (triple bonds) differ. The location of the triple bond in these molecules significantly affects their chemical reactivity, making it possible to distinguish between them using appropriate chemical reactions or reagents.

Alkynes

Alkynes are a fascinating group of hydrocarbons in organic chemistry, marked by their carbon-carbon triple bonds and general formula C_nH_2n-2. This triple bond grants alkynes their unique properties and reactivity patterns. For example, 1-butyne's triple bond is at the end of the carbon chain (terminal alkyne), while in 2-butyne it is between the second and third carbon atoms (internal alkyne).

Distinguishing between terminal and internal alkynes is often necessary in synthetic chemistry and analytical testing. A particularity of terminal alkynes, like 1-butyne, is their slightly acidic hydrogen atom attached to the carbon with a triple bond. Under certain conditions, they can react to form salts, and hence, are amenable to unique reactions that do not occur with internal alkynes like 2-butyne. This distinctive reactivity allows scientists to utilize specific chemical tests to differentiate between the two, which is not only relevant for academic exercises but also has practical implications in research and industry.

Chemical Reagents

Chemical reagents are substances used to cause a chemical reaction. They're essential tools for chemists to investigate and manipulate matter at a molecular level. In the context of distinguishing 1-butyne and 2-butyne, the choice of reagent must selectively react with one but not the other, offering a clear indicator of the molecule's structure. As the solution discussed, ammoniacal Cu₂Cl₂ is adept at doing so because it forms a precipitate only with terminal alkynes due to their accessible hydrogen atom at the triple bond.

Understanding the behavior of various reagents with different functional groups is a fundamental skill in organic chemistry. For instance, bromine water and Baeyer's reagent are common tests for unsaturation but not specific enough for our purpose in this exercise. They interact with compounds containing double and triple bonds indiscriminately. The knowledge of such specificity is crucial when conducting organic syntheses or in qualitative analysis to identify unknown compounds.