4.5 Conformations of Cyclohexane
3 min read•may 7, 2024
's are key to understanding its behavior. The chair form is most stable, minimizing strain through ideal bond angles and staggered hydrogens. This stability impacts cyclohexane's reactivity and properties.
Conformational dynamics play a crucial role in cyclohexane's flexibility. Ring flips and allow interconversion between forms, affecting how substituents are positioned. This knowledge is essential for predicting reactivity in organic reactions.
Cyclohexane Conformations
Chair conformation of cyclohexane
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- Draw two parallel lines representing the top and bottom faces of the ring slightly offset from each other
- Top line represents the face closest to the viewer
- Bottom line represents the face farthest away
- Add two vertical lines on the left and right sides connecting the top and bottom faces representing the bonds perpendicular to the ring faces
- Place four carbon atoms at the ends of the vertical lines, two on the top face and two on the bottom face
- Add one hydrogen atom to each of these four carbon atoms
- Place the remaining two carbon atoms in the middle of the parallel lines, one on the top face and one on the bottom face
- Add two hydrogen atoms to each of these two carbon atoms, one pointing up and one pointing down
Stability of chair conformation
- Minimizes by having bond angles close to the ideal tetrahedral angle of 109.5°
- Other conformations (boat and twist-boat) have more distorted bond angles leading to higher
- Minimizes as all adjacent C-H bonds are staggered minimizing eclipsing interactions
- Other conformations have eclipsed or partially eclipsed C-H bonds resulting in higher
- Minimizes by positioning the axial and equatorial hydrogens in a way that reduces steric repulsion
- Other conformations have more unfavorable steric interactions between hydrogens or substituents
- Exhibits lower compared to other conformations
Chair vs twist-boat conformations
- is approximately 5.5 kcal/mol more stable than the due to the higher strain present in the
- has bond angles close to the ideal tetrahedral angle (109.5°) while the twist- has more distorted bond angles resulting in higher angle strain
- In the chair conformation, all adjacent C-H bonds are staggered minimizing eclipsing interactions, but the twist-boat conformation has partially eclipsed C-H bonds leading to higher torsional strain
- Chair conformation minimizes steric repulsion between hydrogens or substituents, while the twist-boat conformation has more unfavorable steric interactions particularly between the two "flagpole" hydrogens on the same side of the ring
Conformational dynamics of cyclohexane
- Cyclohexane exists in a between different conformations
- The chair conformation can undergo a , converting axial substituents to equatorial positions and vice versa
- Pseudorotation allows for interconversion between different twist-boat conformations
- An can be used to visualize the relative energies of different cyclohexane conformations
- can influence the stability and preferred conformation of substituted cyclohexanes
Key Terms to Review (33)
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.
Angle strain: Angle strain occurs when the bond angles in a molecule, especially in cycloalkanes, deviate from their ideal values, leading to increased instability and reactivity. It is a type of strain energy that affects the stability of cyclic compounds by forcing the bond angles to be either larger or smaller than what is energetically favorable.
Angle Strain: Angle strain refers to the distortion or deviation from the ideal bond angles in a molecule, which results in increased potential energy and decreased stability. This concept is particularly relevant in the context of cyclic organic compounds, where the inherent ring structure can impose geometric constraints that lead to angle strain.
Axial Position: The axial position refers to the orientation of a substituent or atom in a cyclohexane ring, where it is positioned parallel to the central axis of the ring. This term is particularly relevant in the context of understanding the conformations of cyclohexane and how they impact reactivity in organic chemistry reactions.
Axial position (cyclohexane): In cyclohexane, an axial position refers to the orientation of substituents parallel to the axis of the ring, projecting outward from or inward towards the center of the molecule. These positions alternate up and down around the ring in a chair conformation.
Boat Conformation: The boat conformation is a three-dimensional arrangement of atoms in cyclic organic compounds, particularly cyclohexane, where the ring adopts a distorted, non-planar shape resembling the hull of a boat. This conformation is one of the key conformations observed in cyclic molecules and is crucial in understanding their stability and reactivity.
Boat cyclohexane: Boat cyclohexane is a less stable conformation of cyclohexane, characterized by the molecule adopting a shape that resembles a boat. This conformation arises due to the twisting of the carbon ring to reduce torsional strain but introduces steric strain between hydrogen atoms on the flanking carbons.
Chair conformation: The chair conformation is a three-dimensional shape that cyclohexane (a six-carbon ring) can adopt, characterized by its stability due to minimized steric hindrance and torsional strain. It resembles a chair, with alternating carbon atoms serving as the "seat" and "legs" or "backrest" of the chair.
Chair Conformation: The chair conformation is a stable three-dimensional arrangement adopted by cyclohexane and other six-membered ring compounds. This specific configuration minimizes steric strain and allows for the most stable positioning of substituents on the ring.
Cis-decalin: cis-Decalin is a bicyclic organic compound consisting of two fused cyclohexane rings in a cis orientation. It is an important structural motif in many natural products and pharmaceuticals, and its conformational properties are crucial to understanding the behavior of polycyclic molecules.
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.
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.
Cyclohexane: Cyclohexane is a saturated, cyclic hydrocarbon compound with the chemical formula C6H12. It is a key component in understanding various aspects of organic chemistry, including the naming and stability of cycloalkanes, conformational analysis, and its role in the structure and properties of aromatic compounds and steroids.
Energy diagram: An energy diagram is a graphical representation that shows the energy changes during a chemical reaction, illustrating the transition from reactants to products and highlighting intermediate states and activation energy. It helps in understanding the energetics and kinetics of a reaction, including whether it is exothermic or endothermic.
Energy Diagram: An energy diagram, also known as a reaction energy profile or potential energy diagram, is a graphical representation that depicts the changes in energy throughout the course of a chemical reaction. It provides a visual aid to understand the energetic factors that influence the progress and feasibility of a reaction.
Equatorial Position: The equatorial position refers to the orientation of a substituent or functional group on a cyclohexane ring. It describes a position on the ring that is perpendicular to the plane of the ring, resulting in a more stable and less sterically hindered arrangement.
Equatorial position (cyclohexane): In cyclohexane, the equatorial position refers to the placement of substituents parallel or nearly parallel to the ring's equator, offering a more stable and less strained conformation due to decreased steric hindrance. This contrasts with axial positions that align perpendicular to the plane of the ring.
Methyl Group: The methyl group (CH3-) is a functional group in organic chemistry composed of a single carbon atom bonded to three hydrogen atoms. It is a commonly occurring substituent in many organic compounds and plays a crucial role in the structure and properties of various molecules.
Pseudorotation: Pseudorotation is a conformational change that occurs in cyclic organic compounds, particularly cyclohexane, where the molecule appears to rotate without any actual bond breaking or formation. This dynamic process allows the molecule to interconvert between different conformations without changing the overall connectivity of the atoms.
Ring Flip: A ring flip, also known as a chair-chair interconversion or a ring inversion, is a conformational change that occurs in cyclic molecules, particularly cyclohexane and its derivatives. This term describes the process by which a cyclic structure, such as a cyclohexane ring, can transition between two distinct chair conformations by the rotation of the carbon-carbon bonds within the ring.
Ring Strain: Ring strain refers to the inherent instability and high-energy state of cyclic organic compounds, particularly those with small ring sizes, due to the distortion of bond angles and bond lengths from their ideal values. This concept is central to understanding the properties and reactivity of cycloalkanes and other cyclic structures.
Steric strain: Steric strain is the repulsion between adjacent atoms or groups in a molecule due to their physical size, causing a decrease in stability. This strain affects the molecule's spatial configuration and can influence its reactivity.
Steric Strain: Steric strain refers to the distortion or destabilization of a molecule caused by the repulsive interactions between bulky groups or atoms that are in close proximity within the molecule's structure. This concept is particularly relevant in the context of understanding the conformations and stability of cyclic compounds, such as cyclohexane, as well as the stability of alkenes.
Substituent Effects: Substituent effects refer to the influence that specific functional groups or atoms have on the chemical and physical properties of a molecule. These effects can significantly impact the reactivity, stability, and behavior of organic compounds in various contexts, including conformational analysis, electrophilic and nucleophilic substitutions, and acidity determination.
Tert-Butyl Group: The tert-butyl group is a bulky, three-dimensional substituent composed of three methyl groups attached to a central carbon atom. It is a common functional group in organic chemistry and has a significant impact on the conformations and reactivity of molecules.
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.
Trans-Decalin: trans-Decalin is a bicyclic organic compound consisting of two fused cyclohexane rings in a trans configuration. It is an important structural motif in the study of conformations of polycyclic molecules and the conformations of cyclohexane rings.
Twist-boat conformation: The twist-boat conformation is a specific geometric arrangement of atoms in cyclohexane where the molecule adopts a shape that reduces torsional strain by twisting. This form is less stable than the chair conformation but more stable than the boat conformation due to minimized steric hindrance and torsional strain.
Twist-Boat Conformation: The twist-boat conformation is a distorted, non-planar configuration of the cyclohexane ring. Unlike the stable chair and boat conformations, the twist-boat represents a higher energy state that is less favored, but still an important intermediate in the interconversion between the more stable conformations.