Problem 39 - Chemistry Principles and Practice

Question

Write the structural formula of propyne, and name the product of its reaction with two molecules of \(\mathrm{Cl}_{2}\).

Step-by-Step Solution

Answer

The product is 1,1,2,2-tetrachloropropane.

Step 1: Determine the structure of propyne

Propyne is an alkyne with three carbon atoms. Alkynes have a carbon-carbon triple bond. Therefore, the structural formula of propyne is: \( \text{CH}_3-\text{C} \equiv \text{C}-\text{H} \).

Step 2: Understand the reaction with chlorine

Propyne reacts with chlorine through an addition reaction. Since propyne has a triple bond, the addition reaction occurs at this bond. Each \( \text{Cl}_2 \) molecule adds across the \( \equiv \) bond, first converting it to a double bond, and then to a single bond after the second addition of \( \text{Cl}_2 \).

Step 3: Determine the structural formula after the first addition of Cl2

With the addition of the first \( \text{Cl}_2 \), the triple bond in propyne becomes a double bond, forming 1,2-dichloropropene: \( \text{CH}_3-\text{C}(\text{Cl})=\text{C}(\text{H})-\text{Cl} \).

Step 4: Determine the structural formula after the second addition of Cl2

With the addition of the second \( \text{Cl}_2 \), the double bond is converted to a single bond, resulting in the product 1,1,2,2-tetrachloropropane: \( \text{CH}_3-\text{C}(\text{Cl}_2)-\text{C}(\text{Cl}_2)-\text{H} \).

Step 5: Name the product

The final product, after the reaction with two molecules of chlorine, is named 1,1,2,2-tetrachloropropane due to the four chlorine atoms now bonded to the propane chain.

Key Concepts

Alkyne Reaction

Alkynes are hydrocarbons characterized by at least one carbon-carbon triple bond. These bonds are notably reactive due to their high energy and linear structure, making alkynes eager participants in chemical reactions. One fascinating feature of alkynes is their ability to undergo addition reactions, where the triple bond is gradually converted into a single bond.

In an alkyne reaction, the triple bond provides a site for reagents to add across the bond, forming a more stable product. Propyne (\( \text{CH}_3-\text{C} \equiv \text{C}-\text{H} \)) is a simple alkyne, and when it reacts, it exemplifies typical behaviors of this class of compounds.

• The triple bond in propyne allows for sequential addition of atoms or groups.
• This changes the unsaturated alkyne into a fully saturated product.

Addition Reaction

Addition reactions are a core concept in organic chemistry, particularly relevant for compounds with multiple bonds, like alkynes. In these reactions, atoms are added to the compound, breaking the multiple bonds and forming new single bonds, which results in a decrease in a molecule's degree of unsaturation.

For propyne reacting with chlorine (\( \text{Cl}_2 \)), the mechanism occurs in two steps:

• First, one molecule of chlorine adds across the triple bond, forming a double bond and a compound called 1,2-dichloropropene.
• Next, the second molecule of chlorine adds across the existing double bond, resulting in the final product, 1,1,2,2-tetrachloropropane.

This stepwise addition profoundly changes the chemical structure, turning a reactive alkyne into a more stable alkane.

Structural Formula

The structural formula in organic chemistry represents the arrangement of atoms within a molecule. It's like a map showing how atoms are connected, providing insight into the molecule's geometry and reactivity.

For propyne, its structural formula is \( \text{CH}_3-\text{C} \equiv \text{C}-\text{H} \). This format highlights the alkyne's carbon-carbon triple bond, a key feature.

When propyne reacts with two molecules of chlorine, the structural formula transforms successively:

• After the first addition: \( \text{CH}_3-\text{C}( ext{Cl})=\text{C}( ext{H})-\text{Cl} \). This indicates the conversion of the triple bond to a double bond.
• After the second addition: \( \text{CH}_3-\text{C}( ext{Cl}_2)-\text{C}( ext{Cl}_2)-\text{H} \), showing the complete saturation of the molecule with chlorine atoms and the formation of a single-bonded alkane.

Such transformations emphasize the dynamic nature of organic molecules and their ability to change structure through reactions.