Problem 19 - Chemistry and Chemical Reactivity

Question

Draw the structure and give the systematic name for the products of the following reactions: (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{Br}_{2} \longrightarrow\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}+\mathrm{H}_{2} \longrightarrow\)

Step-by-Step Solution

Answer

(a) 1,2-dibromopropane; (b) pentane.

Step 1: Understand Reaction (a)

In reaction (a), propene \( \mathrm{CH}_3 \mathrm{CH} = \mathrm{CH}_2 \) is reacting with bromine \( \mathrm{Br}_2 \). This is an addition reaction where bromine will add across the double bond.

Step 2: Predict the Product for (a)

The double bond between the second and third carbon will break and each carbon involved in the double bond will bond with a bromine atom. The structure becomes \( \mathrm{CH}_3 \mathrm{CHBrCH}_2\mathrm{Br} \), which is called 1,2-dibromopropane.

Step 3: Understand Reaction (b)

In reaction (b), 2-pentene \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH} = \mathrm{CHCH}_3 \) is reacting with hydrogen \( \mathrm{H}_2 \). This is a hydrogenation reaction which will saturate the molecule by converting the double bond into a single bond, adding hydrogen atoms.

Step 4: Predict the Product for (b)

During hydrogenation, one hydrogen atom will add to each carbon that was previously double-bonded. The structure becomes \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3 \), which is called pentane.

Key Concepts

Addition Reactions

Addition reactions are a fundamental type of reaction in organic chemistry. These reactions involve the addition of atoms or groups of atoms to a molecule, typically across a double bond or triple bond. In an addition reaction:

• The pi bond (double or triple bond) between two carbon atoms is broken, resulting in two new sigma bonds.
• This usually increases the saturation of the molecule, transforming unsaturated compounds into saturated ones.
• The two new atoms or groups are added to each carbon atom that was previously part of the double or triple bond.

Addition reactions are important because they allow chemists to systematically modify carbon skeletons, introducing new functional groups and building complexity in organic molecules.

Bromination

Bromination is a specific type of addition reaction where bromine (Br₂) is added to compounds containing double or triple bonds. This typically involves:

• The bromine molecule approaching the carbon-carbon double bond.
• Breaking of the pi bond upon interaction, which allows each carbon atom to form a new bond with a bromine atom.
• Formation of a colorless dibromide product from the initially colored bromine solution, indicating the reaction has occurred.

In reaction (a) from our exercise, bromination was used on propene (an alkene), resulting in the formation of 1,2-dibromopropane. Bromination reactions serve as a simple test for unsaturation, as alkenes and alkynes decolorize bromine water.

Hydrogenation

Hydrogenation is a vital reaction in organic chemistry, transforming unsaturated molecules, such as alkenes and alkynes, into saturated alkanes. The key aspects of hydrogenation include:

• Addition of hydrogen (H₂) across a double or triple bond.
• Use of a catalyst, often platinum, palladium, or nickel, to facilitate the reaction by lowering the activation energy.
• Saturation of the molecule, as hydrogen atoms break the double bonds, converting them into single bonds.

In the context of our exercise, hydrogenation was applied to 2-pentene, converting it into pentane. Hydrogenation is practical in industries for processes such as the hardening of fats and oils.

Propene

Propene, also known as propylene, is a simple alkene with the molecular formula C₃H₆. It features:

• A three-carbon chain with a single double bond between the first and second carbon atoms.
• Being a typical example of a small industrial alkene used for making plastics and chemicals.
• A reactive nature due to its double bond, making it a major candidate for addition reactions like bromination and hydrogenation.

In the given exercise, propene undergoes bromination, illustrating its role as a reactant that can easily participate in addition reactions to form more complex compounds.

2-Pentene

2-Pentene is a type of alkene with the formula C₅H₁₀. It is characterized by:

• A five-carbon chain with a double bond between the second and third carbon atoms, distinguishing it from its isomer 1-pentene.
• Being part of the pentene family, which are used as intermediates in chemical syntheses.
• Having geometric isomers (cis and trans forms) due to the restricted rotation around the double bond.

In our exercise, 2-pentene is the compound subject to hydrogenation, resulting in the fully saturated alkane, pentane. Understanding its structure and reactivity assists with grasping how hydrogenation eliminates unsaturation in alkenes.