Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is not true ? (a) Acetylene is used in the preparation of butadiene. (b) Butyne \(-1\) gives red precipitate with ammoniacal cuprous chloride but Butyne \(-2\) will not (c) The number of sigma and pi- bonds in 1 - butene \(-3\) - yne are 8 and 4 respectively (d) Propene when heated with chlorine at \(500^{\circ} \mathrm{C}\) forms \(\mathrm{CH}_{2} \mathrm{ClCH}=\mathrm{CH}_{2}\)

Step-by-Step Solution

Answer

Statement (d) is not true.

Step 1: Analyze Statement (a)

Acetylene, also known as ethyne, is commonly used in the industrial synthesis of butadiene, among other chemicals. Butadiene can be prepared by polymerizing acetylene with catalysts such as nickel or by other methods that involve acetylene as a starting material. Thus, statement (a) is true.

Step 2: Analyze Statement (b)

1-Butyne reacts with ammoniacal cuprous chloride to form a red precipitate because it has a terminal triple bond (C≡C-H), which reacts with the cuprous chloride. On the other hand, 2-butyne does not form this precipitate since it has an internal triple bond (C≡C) not exposed hydrogen. Thus, statement (b) is true.

Step 3: Analyze Statement (c)

1-Butene-3-yne, also known as vinylacetylene, has a molecular formula of C₄H₄. It has a structure consisting of a double bond and a triple bond. There are three σ-bonds in each of the two C-H bonds, C=C bond has one σ and one π bond, C≡C has one σ and two π bonds, totaling 8 σ-bonds and 4 π-bonds. Thus, statement (c) is true.

Step 4: Analyze Statement (d)

Propene, when heated with chlorine at 500°C, typically undergoes an allylic substitution rather than forming the compound listed as CH₂ClCH=CH₂. This would result in a substitution that doesn't directly match the product given in the statement due to the conditions. Thus, statement (d) is not generally true.

Key Concepts

Acetylene

Acetylene, commonly known by its chemical name ethyne, is a simple alkyne characterized by its molecular composition of two carbon atoms connected by a triple bond (C≡C), with a molecular formula of C₂H₂. This compound is famously used as a fuel for oxy-acetylene welding and cutting due to its high flame temperature when burned with oxygen.
In the realm of organic synthesis, acetylene serves as a versatile building block. It is a key precursor in the industrial synthesis of various organic compounds, one of which is butadiene. Butadiene is a monomer used in the production of polymers like synthetic rubber. The synthesis of butadiene from acetylene can be achieved through catalytic processes often involving metal catalysts such as nickel. The versatility and reactivity of the triple bond in acetylene make it an essential component for chemists looking to construct complex organic molecules.

Butadiene synthesis

Butadiene, with the chemical formula C₄H₆, is a simple conjugated diene used extensively in the production of synthetic rubber. The synthesis of butadiene can be accomplished in various ways, with acetylene serving as a critical starting material in some methods.

• One common method is the acetylene dimerization, where acetylene is converted to butadiene using catalysts such as nickel or palladium.
• Another approach is oxidative coupling, which involves reacting acetylene under controlled conditions to form the butadiene structure.

The production of butadiene from acetylene highlights the importance of catalytic systems in facilitating these types of organic synthesis reactions. The presence of a conjugated diene system in butadiene is crucial for its polymerization properties, enabling the formation of durable synthetic materials.

Ammoniacal cuprous chloride

Ammoniacal cuprous chloride is a reagent used for detecting and reacting with certain alkynes, primarily those with terminal alkyne groups. A terminal alkyne, like 1-butyne, possesses a triple bond at the end of the carbon chain ( C≡C-H), which can react with ammoniacal cuprous chloride, resulting in a characteristic red precipitate.
This reaction occurs because the terminal hydrogen is slightly acidic, allowing it to react with the ammoniacal cuprous chloride. On the contrary, internal alkynes, such as 2-butyne, do not have this terminal hydrogen, making them unreactive to this reagent. This property is useful in distinguishing between terminal and non-terminal alkynes in a laboratory setting. The formation of the precipitate can be a key indication of the presence of a terminal alkyne in an unknown sample.

Propene chlorination

Propene chlorination involves the reaction of propene, an alkene with the formula C₃H₆, with chlorine gas. This process can lead to different products depending on the reaction conditions, such as temperature and the presence of light or catalysts.
Under high-temperature conditions, such as 500°C, propene primarily undergoes allylic chlorination. This type of reaction targets the hydrogen atoms adjacent to the double bond, resulting in allylic substitution products. However, simply heating propene with chlorine does not generally lead to the formation of compounds like CH₂ClCH=CH₂ as mentioned in some predictions, since the conditions generally promote formation of allylic products.

• This reaction, under specific conditions, might yield dichlorinated products if excess chlorine is present.
• The reaction is highly dependent on the reaction conditions, showcasing the importance of understanding mechanistic pathways in organic reactions.

Overall, propene chlorination demonstrates the nuanced behavior of organic molecules under varying reaction conditions, making it an interesting example of selective organic transformations.