Problem 36 - Chemistry Principles and Practice

Question

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

Step-by-Step Solution

Answer

(a) 1,2-dibromopropane, (b) 3-methylpentane.

Step 1: Understand the First Reaction

The first reaction involves the addition of bromine (\( \mathrm{Br}_2 \)) to a propene (\( \mathrm{CH}_3\mathrm{CH}=\mathrm{CH}_2 \)). This type of reaction is called a halogenation of an alkene.

Step 2: Predict the Product for Part (a)

In this reaction, bromine will add across the double bond in the propene. Each carbon involved in the double bond will bond to a bromine atom, resulting in the formation of 1,2-dibromopropane.

Step 3: Draw the Structure for Reaction (a)

The structure for 1,2-dibromopropane is: \( \mathrm{CH}_3\mathrm{CHBrCH}_2\mathrm{Br} \). The two bromine atoms are added to what was the double bond in the original propene molecule.

Step 4: Understand the Second Reaction

The second reaction involves the hydrogenation of an alkene, using hydrogen gas (\( \mathrm{H}_2 \)). This process converts an alkene into an alkane.

Step 5: Predict the Product for Part (b)

For \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}=\mathrm{CHCH}_3 + \mathrm{H}_2 \), hydrogen will add across the double bond, resulting in 3-methylpentane where each carbon of the double bond gains a hydrogen atom.

Step 6: Draw the Structure for Reaction (b)

The structure for 3-methylpentane is: \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3 \). The double bond is replaced with hydrogen atoms, saturating the hydrocarbon.

Step 7: Naming of Products

The product from the first reaction is named 1,2-dibromopropane, and the product from the second reaction is named 3-methylpentane, according to IUPAC nomenclature.

Key Concepts

Halogenation of Alkenes

Halogenation is an essential reaction in organic chemistry that involves the addition of halogens, like bromine (\(\text{Br}_2\)), to alkenes. In the reaction between \(\text{CH}_3\text{CH}=\text{CH}_2\) and bromine, an "addition" reaction occurs. This means the \(\text{Br}_2\) molecule is added across the double bond.
This transforms propene into 1,2-dibromopropane. Here's how it happens:

• The double bond in the alkene breaks, providing sites for new bonds to form.
• Each carbon atom that was originally part of the double bond now bonds with a bromine atom.
• The result is a saturated product where propene is converted into a dibromo compound, specifically 1,2-dibromopropane.

The bond formation with halogens like bromine generally occurs on opposite sides of the former double bond (anti addition), although syn addition can also occur. Such reactions increase the reactivity and polarity of the molecule by introducing more electronegative atoms.

Hydrogenation of Alkenes

Hydrogenation refers to the addition of hydrogen (\(\text{H}_2\)) to alkenes, turning them into alkanes by eliminating the double bond. This reaction typically requires a catalyst, like platinum, palladium, or nickel, although this step wasn't specified here. In part (b) of the exercise, \(\text{CH}_3\text{CH}_2\text{CH}=\text{CHCH}_3\) meets hydrogen gas resulting in hydrogenation.

• Here, the double bond in the alkene is broken and each carbon atom involved in the double bond forms a new bond by adding a hydrogen atom.
• This process turns the alkene, 3-methyl-1-butene, into an alkane, 3-methylpentane.
• The new structure is fully saturated since it now contains only single bonds, making it more stable.

Hydrogenation is a common industrial process used for making saturated fats from oils, providing more stable compounds and changing the properties of the molecules involved.

IUPAC Nomenclature

IUPAC nomenclature stands for the International Union of Pure and Applied Chemistry rules for naming chemical compounds. These rules ensure that each chemical structure has a unique and universally accepted name. Let’s look at the names involved in this exercise.

• In part (a), the product is named 1,2-dibromopropane. The numbering "1,2-" indicates the positions of the bromine atoms on the propane chain.
• The base name, "propane," suggests a three-carbon backbone, and the "di-" prefix indicates there are two bromine atoms.
• In part (b), the product is named 3-methylpentane. "Methylpentane" indicates a five-carbon alkane chain with a methyl group.
• The "3-" specifies the location of the methyl group on the third carbon of the main chain.

Use of such systematic naming helps in clearly identifying the chemical structure, minimizing confusion and allowing chemists worldwide to effectively communicate and understand molecular structures.