Question

There are three different substances that contain a carbon-carbon double bond and have the formula \(\mathrm{C}_{4} \mathrm{H}_{8} .\) Draw them, and tell how they differ.

Short answer

The differences are in double bond position (1-butene) and geometric arrangement (cis/trans for 2-butene).

Step-by-step solution

Identify the Formula

We are asked to identify different substances with the molecular formula \( \mathrm{C}_{4} \mathrm{H}_{8} \) that contain a carbon-carbon double bond. This indicates these substances are alkenes, which are hydrocarbons with at least one carbon-carbon double bond.

Draw Butene Structures

The molecular formula \( \mathrm{C}_{4} \mathrm{H}_{8} \) corresponds to isomers of butene. Draw the following structures for the isomers:1. **1-Butene**: A linear chain with the double bond at the first carbon: \( \mathrm{CH}_2=\mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}_3 \).2. **2-Butene**: A linear chain with the double bond between the second and third carbon atoms, existing in two geometric isomers: - **cis-2-Butene**: \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3 \) with both methyl groups on the same side. - **trans-2-Butene**: \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_3 \) with methyl groups on opposite sides.

Identify Differences Among Isomers

The three substances differ in their structure and configuration: 1. **1-Butene** differs by the position of the double bond at the terminal end. 2. **cis-2-Butene** and **trans-2-Butene** differ in the spatial arrangement of the hydrogen (H) and methyl (CH3) groups around the double bond, leading to different physical properties like boiling point and density.

Key concepts

Chemical Structure

The chemical structure of a compound is a way of representing the arrangement of atoms within molecules. For alkenes, the focus is on the carbon atoms and their bonds because they define the core framework of the molecule. Let's take butene, a molecule with the formula \( \mathrm{C}_{4} \mathrm{H}_{8} \), as our example.

• **1-Butene** has a linear structure with the double bond starting at the first carbon atom. This results in the structure: \( \mathrm{CH}_2 = \mathrm{CH}-\mathrm{CH}_2-\mathrm{CH}_3 \).
• **2-Butene** features the double bond between the second and third carbon atoms, which can exist as isomers itself such as cis-2-butene and trans-2-butene.

Understanding chemical structures is vital since each structural variation can result in different chemical properties, even when the molecular formula remains the same.
This characteristic is particularly crucial with compounds like butenes, where the position and type of bonds influence the compound's properties and functions.

Geometric Isomers

Geometric isomers are fascinating because they have the same molecular formula but different spatial arrangements. In the case of butenes, we study the geometric isomers of 2-butene.

• **Cis-2-Butene**: Both methyl groups (\( \mathrm{CH}_3 \)) are on the same side of the double bond. This alignment results in distinct properties, such as a higher boiling point.
• **Trans-2-Butene**: Here, the methyl groups are on opposite sides of the double bond, which affects the molecular shape and reduces intermolecular forces, typically resulting in a lower boiling point.

Geometric isomerism arises from the restricted rotation around carbon-carbon double bonds. This characteristic means the same atoms bond differently in three-dimensional space, thus giving rise to isomers with unique physical properties.
This concept is essential in understanding the diversity of chemical behavior among molecules with the same constituents.

Carbon-Carbon Double Bond

The carbon-carbon double bond is a defining feature of alkenes. It greatly influences a molecule's shape and reactivity due to its unique characteristics.
The double bond consists of one sigma (\( \sigma \)) bond and one pi (\( \pi \)) bond.

• The \( \sigma \) bond forms by the head-on overlap of orbitals, while the \( \pi \) bond results from the side-to-side overlap of p orbitals.
• The \( \pi \) bond is less stable than the \( \sigma \) bond, making it a site for chemical reactions.
  This engagement in reactions is why many alkene reactions, such as hydrogenation or halogenation, involve the addition across the double bond.

Additionally, the double bond affects the rigidity and shape of molecules; it restricts rotation leading to the formation of geometric isomers as seen in 2-butene.
Understanding carbon-carbon double bonds is crucial for predicting reactivity and properties of organic compounds.