Question

Which reagent is used to distinguish 1-butyne from 2-butyne? (1) \(\mathrm{HC} \mathrm{Cl}\) (2) \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) (3) Baeyer's reagent (4) Ammonical cuprous chloride

Short answer

(4) Ammoniacal cuprous chloride

Step-by-step solution

Identify the Molecular Structure

Understand that 1-butyne and 2-butyne are isomers. 1-Butyne has a triple bond at the end of the chain whereas 2-butyne has the triple bond between the second and third carbon atoms in the chain.

Understand the Nature of Triple Bonds

Recognize that terminal alkynes (like 1-butyne) have a triple bond at the end of the carbon chain with a hydrogen attached directly to the carbon of the triple bond.

Consider the Reagents

Analyze the given reagents. (1) \(\text{HCl}\) is a strong acid and won't preferentially react with either alkyne under mild conditions.(2) \(\text{Br}_2 / \text{CCl}_4\) will add across the triple bond in both 1-butyne and 2-butyne, making it non-selective.(3) Baeyer's reagent (an alkaline solution of potassium permanganate) oxidizes alkynes but doesn't distinguish between terminal and internal alkynes.

Select the Correct Reagent

Identify that ammoniacal cuprous chloride reacts specifically with terminal alkynes (like 1-butyne) to form a precipitate. Internal alkynes (like 2-butyne) do not form this precipitate.

Conclude the Answer

The reagent used to distinguish 1-butyne from 2-butyne is ammoniacal cuprous chloride.

Key concepts

1-Butyne

1-Butyne is a type of alkyne. Alkynes are hydrocarbons with at least one carbon-carbon triple bond. In 1-butyne, the triple bond is located between the first and second carbon atoms in a four-carbon chain. This specific position of the triple bond classifies it as a terminal alkyne.
Because the triple bond is at the end of the chain, it is termed 'terminal'. This means it has a hydrogen atom directly attached to one of the carbons in the triple bond. Terminal alkynes can be easily distinguished by certain reagents, specifically ones that interact with the hydrogen bonded to the carbon of the triple bond.

2-Butyne

2-Butyne is also an alkyne, but unlike 1-butyne, its triple bond is between the second and third carbon atoms in the chain. This classification makes 2-butyne an internal alkyne.
Internal alkynes have the triple bond located within the carbon chain, rather than at the end. This means there are no hydrogen atoms directly attached to the carbon atoms of the triple bond. The absence of this hydrogen atom makes 2-butyne react differently to certain chemical reagents compared to 1-butyne.

Ammoniacal Cuprous Chloride

Ammoniacal cuprous chloride is a reagent specifically used to identify terminal alkynes. It consists of cuprous chloride dissolved in ammonia.
The special property of this reagent is its ability to form a red precipitate when it reacts with terminal alkynes (like 1-butyne). This reaction does not occur with internal alkynes (like 2-butyne). Therefore, this reagent is particularly useful in distinguishing between terminal and internal alkynes in a mixture.

Terminal Alkynes

Terminal alkynes are a subtype of alkynes with a distinctive feature: a triple bond at the end of a carbon chain along with a hydrogen atom directly bonded to the terminal carbon.
This terminal carbon-hydrogen bond allows specific reactions, especially with reagents like ammoniacal cuprous chloride, to form distinct precipitates that help identify them. Examples of terminal alkynes include compounds like 1-butyne and ethyne.

Internal Alkynes

Internal alkynes have their triple bond located between carbon atoms that are not at the ends of the carbon chain. In these alkynes, there are no hydrogen atoms directly attached to the carbons involved in the triple bond.
Due to this structure, internal alkynes, like 2-butyne, don't react with certain reagents that specifically target terminal alkynes. Internal alkynes are generally more stable and less reactive compared to terminal alkynes.