Question

Draw the most stable conformation of pentane, using wedges and dashes to represent bonds coming out of the paper and going behind the paper, respectively.

Short answer

The most stable conformation of pentane is a zigzag chain in an anti conformation with alternating bonds shown in wedges and dashes.

Step-by-step solution

Identify the Structure of Pentane

Pentane is a straight-chain alkane with the molecular formula \( C_5H_{12} \). It has five carbon atoms connected in a linear arrangement with single bonds.

Construct the Zigzag Framework

In organic chemistry, the most stable conformation of alkanes is often a zigzag, where the carbon chain is in a staggered formation. Begin by drawing a zigzag line with five carbon atoms, each representing a carbon atom.

Add Hydrogen Atoms

Each carbon atom in pentane forms four bonds. After drawing the carbon backbone, complete the structure by adding hydrogen atoms to ensure each carbon has four bonds. For instance, the terminal carbon atoms will have three hydrogens each.

Utilize Wedges and Dashes

In a 3D space, the bonds protruding out of the plane are drawn as wedges while those going behind the plane are drawn as dashes. For this zigzag conformation, at least one hydrogen on each asymmetric center can be shown using wedges and dashes to reflect its three-dimensional structure.

Check for Stability

To ensure stability, verify that the chain is in an extended form, free of severe steric hindrance or strain. This occurs in what is known as an 'anti' conformation where the largest substituents on adjacent carbons are as far apart as possible.

Draw the Complete Structure

Complete the drawing using the guidelines from previous steps: a zigzag carbon backbone, hydrogens added to satisfy tetra-valency, and wedges and dashes to convey three-dimensional shape. Ensure the drawing is free of any overlap which depicts strain or instability.

Key concepts

Zigzag Formation

In organic chemistry, molecules often adopt shapes that minimize energy and maximize stability. One such optimal shape for carbon chains like pentane is the zigzag formation. This formation occurs because linear carbon chains tend to adjust themselves into a zigzag pattern to reduce the overall torsional strain. Torsional strain is the resistance to twisting about a particular bond. When pentane molecules are drawn, their carbon atoms typically appear as a zigzag line, where each vertex represents a carbon atom. This simple line drawing actually represents a 3D shape. The zigzag conformation allows the hydrogen atoms attached to each carbon to spread out into space optimally. It is important to show this arrangement accurately for clarity, as it plays a key role in the stability of the molecule.

Wedges and Dashes

Visualizing molecular structures in three dimensions can be tricky when using two-dimensional drawings. This is where wedges and dashes come into play. These symbols are crucial for indicating the spatial orientation of bonds in a molecule, and they are especially important when drawing pentane in its most stable form.

• **Wedges** represent bonds coming out of the plane of the paper, towards the viewer.
• **Dashes** signify bonds that extend back behind the plane of the paper.

Using wedges and dashes helps depict how the atoms are arranged around the central carbon atoms, providing a clearer understanding of the molecule's three-dimensional structure. In the case of pentane, apply wedges and dashes to represent the bonds that do not lie in the main plane of the zigzag, giving a true-to-life representation of the molecule.

Staggered Conformation

Staggered conformation is an energetically favorable arrangement often observed in alkanes, including pentane. In this conformation, the substituents on adjacent carbon atoms are positioned as far apart as possible, minimizing repulsive forces. This results in a stable molecular orientation. When we draw pentane, ensuring a staggered conformation means that adjacent hydrogens and carbons will be splayed out in such a way that their respective electrons have more room to 'breathe.' This arrangement reduces torsion and steric strain - factors that can destabilize a molecule. Practically, this is achieved through careful orientation of the wedges and dashes in the molecular structure, which plays a role in reducing any strain introduced by crowded atomic positions.

Tetra-valency

Carbon’s unique propensity for forming stable compounds with diverse structures lies in its tetra-valency. Tetra-valency refers to carbon's ability to form four covalent bonds. In the case of pentane, this remarkable feature is what allows carbon atoms to connect in a continuous chain, while also attaching to hydrogen atoms to complete their valency. Each carbon in the pentane molecule forms four sigma bonds:

• Two bonds connect to neighboring carbons in the chain.
• The remaining two (or three, for terminal carbons) bonds connect with hydrogen atoms.

Understanding this bonding capability is essential when drawing molecules like pentane, as it dictates how we position atoms and is fundamental in ensuring the accuracy of the structural diagram.

Steric Hindrance

In organic chemistry, steric hindrance refers to the crowding that occurs when atoms are forced into close proximity, resulting in destabilizing repulsions. In the context of pentane, minimizing steric hindrance means arranging the carbon and hydrogen atoms so they do not experience undue crowding. One way to mitigate steric hindrance is by adopting conformations like the staggered formation. This ensures that atoms, specifically hydrogen atoms in a molecule like pentane, are not unnecessarily bumping into one another. By strategically placing wedges and dashes, the three-dimensional structure accurately represents this minimized crowding, leading to a more stable and realistic molecular form.