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 the First Statement

Acetylene, which is ethyne (C₂H₂), is used in the commercial synthesis of 1,3-butadiene. This is a key step in the production of synthetic rubber. Therefore, statement (a) is true.

Step 2: Analyze the Second Statement

Butyne-1 ( ext{CH}_3 ext{C} ext{CH}) contains a terminal alkyne group which reacts with ammoniacal cuprous chloride to form a red precipitate of cuprous acetylide. However, butyne-2 ( ext{CH}_3 ext{C} ext{C} ext{CH}_3) has no terminal hydrogen and does not react with ammoniacal cuprous chloride. Therefore, statement (b) is true.

Step 3: Analyze the Third Statement

1-butene-3-yne ( ext{CH}_2= ext{CHC} ext{CH}) is an unsaturated hydrocarbon containing both double and triple bonds. It has 8 sigma bonds and 4 pi bonds. Thus, this statement is true. Sigma bonds occur between all connections, and the pi bonds occur in double and triple bonds.

Step 4: Analyze the Fourth Statement

For statement (d), propene ( ext{CH}_3 ext{CH}= ext{CH}_2) when reacted with chlorine at high temperatures, like 500°C, results in substitution rather than addition due to the breaking of the weakest C-H bond. This statement states that propene forms ext{CH}_2 ext{ClCH}= ext{CH}_2, which means the chlorine is added across the double bond, something that requires lower temperatures. Therefore, statement (d) is not true.

Key Concepts

Alkynes

Alkynes are fascinating hydrocarbons characterized by having at least one carbon-to-carbon triple bond. These triple bonds are strong and influence the molecule's reactivity.
They can participate in a variety of chemical reactions including polymerization, hydration, and oxidation. Alkynes are notable for their high reactivity due to the presence of the triple bond, consisting of one sigma bond and two pi bonds.
This makes them significantly more reactive than their alkene and alkane counterparts. The simplest alkyne, acetylene, is crucial in synthetic chemistry applications, such as the production of plastics and synthetic rubber.
It's worth remembering that terminal alkynes — those with the triple bond at the end of the carbon chain — can form precipitates, such as with cuprous chloride, making them distinguishable from internal alkynes.

Sigma and pi bonds

In organic chemistry, understanding the distinction between sigma and pi bonds is essential to grasp how molecules hold together and react.
Sigma bonds (-) are the strongest type of covalent bond and are formed by the head-on overlap of two atomic orbitals. They allow for free rotation about the bond axis, which is not possible in pi bonds. On the other hand, pi bonds are formed by the lateral overlap of two p-orbitals.
Pi bonds (=) are weaker than sigma bonds and prevent the rotation of bonded atoms.
Importantly, pi bonds are what allow multiple bonds (i.e., double and triple bonds) to exist beyond a single sigma bond. In double bonds, one pi bond accompanies the sigma bond, whereas in triple bonds, there are two pi bonds in conjunction with the sigma bond.
Recognizing the number of sigma and pi bonds helps in determining the structure, shape, and reactivity of organic molecules.

Reaction mechanisms

Studying reaction mechanisms is crucial in understanding how changes in molecules occur step by step during chemical reactions.
This allows chemists to predict reaction outcomes and develop new synthetic pathways. A reaction mechanism describes the specific series of steps and intermediates that provide the detailed pathway of a reaction.
For example, in the high-temperature reaction of propene with chlorine, the mechanism involves breaking the weakest C-H bond to form substitution rather than addition, resulting in different products than would be expected at lower temperatures.
Each step involves bond breaking and forming, along with the generation of intermediates. Identifying these steps aids in understanding how factors like temperature, catalysts, or solvents can influence the course of a reaction.
As these concepts bridge the gap between theoretical chemistry and practical laboratory applications, they are foundational in both academic research and industrial chemical processes.