Question

Draw perspective formulas and Newman projections for the eclipsed and staggered forms of propane.

Short answer

The perspective formulas and Newman projections for eclipsed and staggered conformations of propane are as follows: Eclipsed conformation (perspective formula): ``` H H | | H-C-C-H H H ``` Staggered conformation (perspective formula): ``` H | H-C-H | H | H C-H | H ``` Eclipsed conformation (Newman projection): ``` H | H-C-H | H | H | H-C-H | H ``` Staggered conformation (Newman projection): ``` H | H-C-H | H H | H-C-H | H ``` Staggered conformations are more stable due to lower torsional strain than eclipsed conformations.

Step-by-step solution

1. Identify the structure of propane

Propane has the molecular formula C3H8, and it is an alkane. The structure of propane consists of three carbon atoms in a straight chain, each attached to hydrogen atoms. The carbon atoms make single bonds with each other, and the hydrogen atoms are attached to each carbon atom. The structural formula of propane is: CH3-CH2-CH3

2. Understand eclipsed and staggered conformations

In eclipsed conformations, the hydrogen atoms on adjacent carbon atoms are directly in line with each other. This results in higher torsional strain and less stability. In staggered conformations, the hydrogen atoms on adjacent carbon atoms are as far apart as possible and thus have the lowest torsional strain, which makes them more stable.

3. Draw perspective formulas for the eclipsed and staggered conformations of propane

To draw perspective formulas of propane, we will use the skeletal structure and represent the 3-dimensional arrangement of atoms in the molecule. Eclipsed conformation: ``` H H | | H-C-C-H H H ``` Staggered conformation: ``` H | H-C-H | H | H C-H | H ```

4. Draw Newman projections for the eclipsed and staggered conformations of propane

Newman projections are a way to represent molecules in which we view the molecule along the bond between two carbon atoms. In the case of propane, we will look along the C1-C2 or C2-C3 bond. Eclipsed conformation: ``` H | H-C-H | H | H | H-C-H | H ``` Staggered conformation: ``` H | H-C-H | H H | H-C-H | H ``` In summary, we have drawn perspective formulas and Newman projections for the eclipsed and staggered conformations of propane. Remember that staggered conformations are more stable than eclipsed conformations due to their lower torsional strain.

Key concepts

Newman Projections

Understanding the spatial arrangement of atoms within a molecule is crucial for grasping its chemical behavior. One valuable tool for visualizing such arrangements is the Newman projection. This method represents the 3-dimensional orientation of bonds in a molecule from a specific vantage point, often looking straight down the axis connecting two carbon atoms. It simplifies the understanding of rotational conformations of alkanes like propane by 'flattening' the view into a 2-dimensional representation.

To create a Newman projection for propane, we imagine standing directly in front of one carbon atom and looking towards the other along the bond. The front carbon's bonds are shown as a 'Y' with its hydrogen atoms or groups at each point. The rear carbon's bonds are depicted as a circle with its groups drawn relative to the front carbon's bonds. This perspective enables us to assess the different conformations—the eclipsed and the staggered— as the molecule rotates around its bonds.

Eclipsed Conformation

When we mention the eclipsed conformation in relation to propane, we are referring to one of the least stable arrangements of the molecule's atoms. In this form, the hydrogen atoms attached to the front and rear carbon atoms are directly in line with each other, as if one set is 'eclipsing' the other when viewed from a Newman projection. This alignment produces repulsion between the electron clouds of the hydrogen atoms due to their close proximity.

The visual representation shows a front carbon with three hydrogen atoms spread at 120-degree angles and a rear carbon directly behind it with a corresponding alignment. The direct overlap of these atoms leads to increased torsional strain, and consequently, the molecule is in a higher energy state, making it less stable compared to other conformations.

Staggered Conformation

The staggered conformation stands in stark contrast to the eclipsed conformation because it is the conformation where propane achieves its maximum stability. This stability arises due to the hydrogen atoms of adjacent carbons being as far away from each other as possible, which minimizes repulsions. This arrangement decreases the torsional strain significantly.

When propane adopts a staggered conformation, as depicted in a Newman projection, each hydrogen atom on the front carbon is offset from the hydrogen atoms on the back carbon. The resulting molecular structure displays a zigzag pattern, with the hydrogen atoms interlocking like pieces of a puzzle. This arrangement favours a decrease in energy, making it the molecule's preferred state under normal conditions.

Torsional Strain

Torsional strain is an important concept when discussing the conformations of alkanes like propane. It refers to the strain experienced by a molecule due to the repulsion between electrons in bonds that are eclipsing or closely aligned with each other in space. Think of torsional strain as the molecular equivalent of tension within a twisted elastic band; it represents potential energy that increases as the atoms get closer to an eclipsed configuration.

It's important to realize that torsional strain directly influences the stability of different conformations. The higher the strain, the less stable the conformation. That's why the eclipsed conformation of propane, with its atoms aligned and creating maximum repulsion, has higher torsional strain and is less stable than the staggered conformation where atoms are spread apart, resulting in lower strain and enhanced stability.

Molecular Structure of Propane

Propane's molecular formula is C3H8, and it belongs to the alkane series with a linear chain of three carbon atoms. Each carbon atom in propane forms four single covalent bonds either with hydrogen atoms or other carbon atoms. The central carbon is bonded to two other carbons, while the carbons at the ends of the chain (terminal carbons) each bond to three hydrogen atoms.

The molecular structure can easily be imagined as a flexible string of atoms where rotations around bonds give rise to different conformations. Propane's structure allows for rotations around the C1-C2 and C2-C3 bonds, enabling the exploration of various spatial arrangements that lead to either staggered or eclipsed conformations. Appreciating the structural flexibility of propane is key to understanding its behavior under different physical conditions and the resulting implications for chemical reactions and energy changes.