3.6 Conformations of Ethane
2 min read•may 7, 2024
Ethane's arise from rotation around its central C-C bond. This rotation creates different spatial arrangements of atoms, with staggered and eclipsed conformations being the main types. Understanding these conformations is crucial for grasping molecular behavior.
Staggered conformations are more stable due to reduced electron repulsion, while eclipsed ones have higher energy. The energy difference between these forms, about 12 kJ/mol, is called the . This concept is key to understanding molecular stability and reactivity.
Conformations of Ethane
Conformations in ethane
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- Different spatial arrangements of atoms resulting from rotation around single bonds ()
- Not distinct chemical structures but different orientations of the same molecule
- In ethane conformations arise due to rotation around the central allowing for free rotation
- Results in an infinite number of possible conformations
- Two main types are staggered and eclipsed conformations (discussed below)
Staggered vs eclipsed ethane conformations
- Staggered conformations have hydrogen atoms on adjacent carbon atoms offset by 60°
- More stable than eclipsed conformations
- Minimized steric strain due to reduced repulsion between electron clouds of C-H bonds
- Energy minima on the
- Eclipsed conformations have hydrogen atoms on adjacent carbon atoms aligned with each other (0° offset)
- Less stable than staggered conformations
- Higher steric strain due to increased repulsion between electron clouds of C-H bonds
- Energy maxima on the potential energy diagram
- Staggered conformations are more stable by approximately 12 kJ/mol compared to eclipsed conformations
- Energy difference between staggered and eclipsed conformations is called the rotational barrier
- Stability differences are influenced by and between atoms
Ethane rotation energy diagram
- Plots the potential energy of the molecule as a function of the (rotation angle around the C-C bond)
- Sinusoidal shape with three energy minima and three energy maxima per 360° rotation
- Energy minima correspond to staggered conformations
- Occur at dihedral angles of 60°, 180°, and 300°
- Represent the most stable conformations of ethane
- Energy maxima correspond to eclipsed conformations
- Occur at dihedral angles of 0°, 120°, and 240°
- Represent the least stable conformations of ethane
- Difference in potential energy between the minima (staggered) and maxima (eclipsed) is the rotational barrier
- Approximately 12 kJ/mol for ethane
Structural considerations
- Ethane exhibits around each carbon atom
- plays a role in the equivalence of staggered and eclipsed conformations
- The tetrahedral arrangement contributes to the overall stability of the molecule
Key Terms to Review (24)
Bond Rotation: Bond rotation refers to the ability of atoms in a molecule to rotate around a single covalent bond, allowing the molecule to adopt different spatial arrangements or conformations. This term is particularly relevant in the context of understanding the structure and behavior of alkanes, such as ethane, as well as the concept of sp3 hybridization.
C-C Single Bond: A C-C single bond is a covalent chemical bond formed between two carbon atoms, where each carbon contributes one electron to the shared electron pair. This type of bond is the most common and fundamental structural unit in organic chemistry, providing the backbone for a wide range of organic molecules and compounds.
Conformational isomers: Conformational isomers are different spatial arrangements of atoms in a molecule that result from rotation around single bonds. These isomers can interconvert through bond rotations without breaking any covalent bonds.
Conformations: Conformations refer to the three-dimensional arrangements that molecules can adopt due to the rotation around single bonds. This concept is essential in understanding the behavior and properties of organic compounds, including alkanes, cycloalkanes, and their substituted derivatives.
Conformers: In the context of organic chemistry, conformers are different spatial arrangements of atoms in a molecule that result from rotation around a single bond. These configurations can interconvert through simple rotations without breaking any bonds.
Dihedral angle: A dihedral angle is the angle between two intersecting planes, which in organic chemistry, often refers to the angle between planes through two sets of three atoms, usually involving a bond between two carbon atoms. In the context of ethane's conformations, it describes the rotational angle around the carbon-carbon bond.
Dihedral Angle: The dihedral angle is the angle between two intersecting planes, specifically the angle between the planes formed by the bonds in a molecule. It is a crucial concept in understanding the three-dimensional structure and conformations of organic compounds.
Eclipsed conformation: In organic chemistry, an eclipsed conformation occurs when atoms or groups attached to adjacent carbons are aligned with each other, maximizing their overlap as viewed along the bond axis between the two carbons. This alignment results in higher torsional strain due to the repulsion between electron clouds of these atoms or groups.
Eclipsed Conformation: The eclipsed conformation is a specific arrangement of atoms in a molecule where the bonds of adjacent carbon atoms are aligned directly with each other, resulting in a high-energy, less stable configuration.
Eclipsing strain: Eclipsing strain is the increase in energy that occurs when atoms in adjacent groups of a molecule are in closest proximity, causing repulsion between their electron clouds. This phenomenon is most commonly discussed in the context of ethane conformations as it rotates around its carbon-carbon bond.
Molecular Symmetry: Molecular symmetry refers to the arrangement and orientation of atoms within a molecule that allows for the identification of symmetry elements such as planes, axes, and centers of symmetry. This concept is crucial in understanding the conformations of molecules, their handedness, and the characteristics of nuclear magnetic resonance (NMR) spectroscopy.
Newman projection: A Newman projection is a method used in organic chemistry to visualize the spatial arrangement of bonds and atoms in a molecule from a specific viewpoint, which is looking down the bond axis connecting two carbon atoms. This visual representation helps in understanding the different conformations (spatial arrangements) that molecules can adopt due to rotation around single bonds.
Newman Projection: The Newman projection is a way of representing the three-dimensional structure of organic molecules, particularly alkanes, on a two-dimensional plane. It provides a simplified view of the spatial arrangement of atoms and their relative positions, allowing for the analysis of conformational preferences and steric interactions.
Potential Energy Diagram: A potential energy diagram is a graphical representation that illustrates the changes in potential energy of a system as a function of a specific reaction coordinate or structural parameter. It provides a visual depiction of the energy barriers and energy minima associated with the different conformations or states of a molecule or a reaction pathway.
Rotational Barrier: The rotational barrier is the energy required to rotate around a carbon-carbon single bond in an alkane molecule. This term is particularly relevant in the context of understanding the conformations and flexibility of alkane structures.
Sawhorse representation: A sawhorse representation is a method used in organic chemistry to depict the three-dimensional spatial arrangement of bonds in molecules, specifically focusing on staggered and eclipsed conformations of alkanes like ethane. It shows the molecule from a slightly angled side view, allowing for easier visualization of the spatial relationships between atoms or groups.
Sawhorse representations: Sawhorse representations are a way to depict the three-dimensional spatial arrangement of atoms in molecules, particularly focusing on the bond angles and orientations between atoms. They provide a visual representation that helps in understanding the conformational structure of molecules like ethane by illustrating the spatial relationships between hydrogen atoms attached to adjacent carbon atoms.
Staggered conformation: In organic chemistry, staggered conformation is a specific spatial arrangement of atoms in ethane and similar molecules where the hydrogen atoms attached to adjacent carbon atoms are as far apart as possible, minimizing repulsion between electron clouds. This arrangement results in a more stable, lower energy state for the molecule.
Staggered Conformation: The staggered conformation is a three-dimensional arrangement of atoms in a molecule where the substituents are positioned as far apart from each other as possible to minimize steric repulsion. This conformation is particularly important in the study of alkanes and their conformations.
Steric Hindrance: Steric hindrance, also known as steric strain or steric effect, refers to the repulsive forces that arise between atoms or groups of atoms in a molecule due to their physical size and spatial arrangement. This phenomenon can significantly impact the stability, reactivity, and conformations of organic compounds.
Tetrahedral Geometry: Tetrahedral geometry refers to the three-dimensional spatial arrangement of atoms or groups of atoms in a molecule, where the central atom is bonded to four other atoms or groups in a symmetrical tetrahedral configuration. This geometric structure is a fundamental concept in understanding the structure and properties of various organic and inorganic compounds.
Torsional strain: Torsional strain arises from the resistance to twisting of the molecular bonds in a molecule, observed when atoms on adjacent atoms are rotated about their bond axis. It is most commonly discussed in the context of ethane conformations, where varying degrees of this strain affect the molecule's stability.
Torsional Strain: Torsional strain refers to the distortion or twisting of a molecule's structure due to the unfavorable interactions between atoms or functional groups. This strain arises when the rotation around a bond is restricted, leading to a deviation from the most stable conformation.
Van der Waals forces: van der Waals forces are weak intermolecular attractive forces that arise between neutral molecules. These forces, while individually weak, can collectively contribute to the physical and chemical properties of substances.