3.7 Conformations of Other Alkanes
3 min read•may 7, 2024
Alkanes, seemingly simple molecules, have fascinating spatial arrangements. Rotations around single bonds create different conformations, each with unique energy levels. Understanding these arrangements is key to predicting molecular behavior and stability.
Staggered and eclipsed conformations are the stars of the show. Staggered wins the stability contest by keeping substituents far apart. Anti and gauche conformations in butane further illustrate how spatial arrangements affect energy levels and molecular populations.
Conformations of Alkanes
Alkane conformations: staggered vs eclipsed
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- Conformations represent different spatial arrangements of atoms that can interconvert by rotation about single bonds (ethane, butane)
- arranges substituents as far apart as possible minimizing steric strain and resulting in lower energy and greater stability compared to ()
- arranges substituents as close together as possible maximizing steric strain and leading to higher energy and reduced stability relative to ()
- measures the angle between two planes each containing one of the C-C bonds with staggered conformations having a of 60° and eclipsed conformations having a dihedral angle of 0°
- quantifies the energy required to rotate from one conformation to another determined by the energy difference between staggered and eclipsed conformations ()
- This barrier is influenced by torsional strain, which arises from electron-electron repulsion during
Anti vs gauche conformations in butane
- represents the staggered conformation where methyl groups are positioned 180° apart resulting in the lowest energy conformation of butane by minimizing steric strain and torsional strain (extended zigzag)
- depicts a staggered conformation where methyl groups are oriented 60° apart leading to higher energy than the due to increased steric strain (sickle shape)
- Energy difference between anti and gauche conformations amounts to approximately 0.9 kcal/mol with the being less stable than the anti conformation by this margin
- Population distribution of anti and gauche conformations is determined by the with a ratio of approximately 3:1 anti to gauche conformations at room temperature (entropy)
Strain energy calculations for alkanes
- quantifies the energy difference between a given conformation and the lowest energy conformation (ground state)
- Interaction values represent energy contributions from specific interactions between substituents:
- occurs between substituents separated by a dihedral angle of 60° with a typical interaction value of 0.9 kcal/mol (butane)
- arises between axial substituents separated by three bonds commonly observed in cyclohexane and its derivatives with a typical interaction value of 3.0 kcal/mol (cis-1,3-dimethylcyclohexane)
- Calculating strain energy involves summing all interaction values present in a given conformation such as two gauche interactions resulting in a strain energy of 2 × 0.9 kcal/mol = 1.8 kcal/mol
- Comparing strain energies allows predicting the preferred conformation of a given alkane with the conformation exhibiting the lowest strain energy being the most stable ()
Conformational analysis and equilibrium
- involves studying the different possible spatial arrangements of a molecule and their relative energies
- Bond rotation allows molecules to interconvert between different conformations
- , caused by the spatial proximity of atoms or groups, influences the stability of different conformations
- exists between different conformations, with the population of each determined by their relative energies
Key Terms to Review (28)
1,3-Diaxial interaction: In organic chemistry, specifically in the context of cyclohexanes and their stereochemistry, 1,3-diaxial interactions are repulsive forces between axial substituents on a cyclohexane ring that are positioned on carbon atoms three positions apart (e.g., carbons 1 and 4). These interactions are a type of steric hindrance that affects the stability and conformational preferences of cyclohexanes.
1,3-Diaxial Interaction: A 1,3-diaxial interaction is a type of steric strain that occurs in cyclohexane conformations when two axial substituents are positioned directly across from each other on the ring. This interaction creates significant repulsive forces that destabilize the molecular structure.
Anti conformation: Anti conformation is a specific arrangement of atoms in an alkane where the two largest groups are positioned 180 degrees apart from each other on adjacent carbon atoms, minimizing steric hindrance. This conformation is considered the most energetically favorable for linear alkanes due to reduced electron repulsion.
Anti Conformation: The anti conformation is a specific spatial arrangement of atoms in a molecule, particularly in alkanes, where the substituents or functional groups are positioned as far apart from each other as possible. This orientation minimizes steric interactions and is generally the most stable conformation for these types of compounds.
Boltzmann Distribution: The Boltzmann distribution is a statistical distribution that describes the relative probability of particles occupying different energy states in a system in thermal equilibrium. It is a fundamental concept in statistical mechanics and plays a crucial role in understanding the conformations of various alkanes.
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.
Conformational analysis: Conformational analysis is the study of the different shapes (conformations) that molecules can adopt due to rotation around single bonds. It particularly focuses on how these shapes affect the molecule's chemical properties and reactivity in organic chemistry.
Conformational Analysis: Conformational analysis is the study of the three-dimensional arrangements or conformations that a molecule can adopt. It involves examining the relative stability and interconversion of different conformations, which is crucial for understanding the behavior and reactivity of organic compounds.
Conformational Equilibrium: Conformational equilibrium refers to the dynamic balance between different spatial arrangements or conformations of a molecule. It describes the interconversion and coexistence of multiple conformations that a molecule can adopt under given conditions.
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.
Gauche conformation: Gauche conformation is a specific spatial arrangement of atoms in organic molecules where two substituents on adjacent carbon atoms are closer than the most energetically favorable position, typically within a 60-degree dihedral angle. This conformation is less stable than the staggered conformation due to increased steric hindrance.
Gauche Conformation: The gauche conformation is a specific spatial arrangement of atoms in organic molecules, particularly alkanes, where adjacent carbon-carbon bonds are oriented at approximately 60 degrees relative to each other. This conformation arises from the minimization of steric interactions between substituents on the carbon atoms.
Gauche Interaction: A gauche interaction is a type of steric interaction that occurs when two bulky substituents on adjacent carbon atoms in an alkane are oriented in a gauche (60° dihedral angle) conformation. This interaction leads to increased potential energy and destabilization of the molecule compared to the anti (180° dihedral angle) conformation.
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.
Rotational Energy Profile: The rotational energy profile, also known as the torsional energy profile, is a graphical representation of the potential energy of a molecule as a function of its dihedral angle or torsion angle. This profile provides insights into the conformational stability and flexibility of organic molecules, particularly alkanes, and is crucial in understanding their three-dimensional structure and behavior.
Sawhorse Representation: The sawhorse representation is a visual tool used to depict the three-dimensional structure of organic molecules, particularly alkanes. It provides a simplified way to represent the spatial arrangement of atoms and bonds in a molecule, highlighting the steric interactions and conformational preferences.
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.
Strain Energy: Strain energy is the potential energy stored in a molecule or structure due to the distortion or bending of chemical bonds. It arises when the geometry of a molecule deviates from its most stable, relaxed configuration, creating internal stress and tension within the structure.
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.