Add the A-Values for each Axial Substituent to determine chair conformation stability. The lower the value, the more stable the system. For example, a methyl group has an A-Value of 19.9 kcal/mol, while an ethyl group has an A-Value of 3.8 kcal/mol.
Axial substituents can affect stability in two ways: by increasing the size of the ring structure, or through inductive effects. Inductive effects are present when there is a change in the electronic environment of the ring system. In general, larger axial substituents create more stable systems because they increase the size of the ring structure and make it less likely that rotations around single bonds will occur. Electronic effects can also influence stability if the substituent has a significant impact on the electron density of the molecule. For example, nitrogen atoms have high polarizability due to their p electrons, so they tend to attract electrons from adjacent atoms (i.e., electron-withdrawing groups). This reduces the energy of the system and makes it more stable.
Stable molecules are generally not reactive. However, some reactive compounds may be stabilized by resonance.
Because there is no steric obstruction or steric repulsion between the hydrogen bonds in the chair conformation, it is more stable. We can see how the H's are positioned by sketching cyclohexane in a chair conformation. These are hydrogen atoms in axial form. These hydrogens have an equitorial structure. They are located on the same side of the ring system.
The chair conformation is most often seen in small non-polar molecules like cyclohexane. However, larger polycyclic aromatic hydrocarbons (PAHs) may also take on this conformation. In these cases, the presence of chiral centers may cause the molecule to adopt one of two different chair conformations - meso- or dextro-rotorua. The former is more common than the latter.
Cyclohexane can be converted into adiphene, which is similar to hexamethylene tetramine (see diagram below). Adiphene can be obtained by heating cyclohexane under pressure. The product will precipitate out if the temperature is lowered slowly enough. Otherwise, it can be collected by filtration or decantation.
Adiphene has been used as a heat stabilizer for asphalt and bitumen. It prevents them from melting at temperatures lower than what they would otherwise do.
The most stable chemical conformation of a six-membered single-bonded carbon ring, such as cyclohexane, is the chair conformation. In this conformation, the plane of the ring is perpendicular to its axis, and the carbons are in four equivalent positions: equidistant from a common point along their axial line.
In general, all chair conformations are equivalent; that is, they can be interchanged without changing the geometry of the molecule. This is not true for non-chair conformations, which are not equivalent. For example, the boat conformation cannot be interchanged with the chair conformation. A molecule in the boat conformation has two pairs of opposite sides, whereas a molecule in the chair conformation has only one pair of opposite sides.
A carbon-carbon double bond is said to have chair conformation when it is in a plane perpendicular to its axis; that is, when it is in an "all-chair" configuration. A carbon-carbon double bond will always take on some degree of rotational freedom around its axis, allowing it to explore other configurations, such as half-chair or twisted-boat. The more restricted the double bond, the less room it has to move.
Cyclopropane adopts a chair conformation.
Between chair conformer and boat conformer, chair conformer is more stable because boat conformation has larger steric and torsional tensions. The boat conformer shape has a high energy while the chair conformer form has a low energy. When two molecules adopt similar chair conformations, they are likely to bond together.
The chair form is also more stable than the boat because there are less angles for atoms to occupy. Chairs have fewer possible configurations than boats which makes chairs more stable.
Atoms in chairs can rotate more freely than those in boats due to lack of constraints from other atoms or groups of atoms. This allows chairs to fit into smaller cavities than boats and be more stable overall.
Chairs are more stable than boats because they have less potential energy: they have lower total energies than boats. A chair does not need to be as tightly packed as a boat to be as stable.
The chair form is more stable than the boat form because it has less angle strain. Angles in boats need to be almost straight lines while chairs allow some curving motion which reduces the amount of angle strain.
Bonds in chairs tend to be shorter than those in boats because there are less constraints on where atoms can be located.
To identify the most stable conformation, we select the shape with the fewest big axial groups; the least stable form has the greatest axial groups. Also, consider how many pairs of atoms are within a distance of 4.0 angstroms of each other in at least one of the structures. If many pairs of atoms are close together in more than one structure, then that structure is unstable and cannot be the most stable form.
The correct answer is C: The alpha helix has the most stable conformation because it has the fewest big-axial groups (3 vs. 7 for beta sheet). It also has the most pairs of atoms within 4.0 angstroms of each other in its crystal structure (16 pairs). Thus, the alpha helix is the most stable conformation.
Beta sheets have more big-axial groups than alpha helices (7 vs. 3), so they are less stable. Beta strands have more pairs of atoms within 4.0 angstroms of each other in their crystal structure than alpha helices (14 pairs vs. 16), so they are also less stable. In general, objects with more big-axial groups or elements that come closer together in space are less stable.
The "chair conformation" refers to the most stable cyclohexane ring shape. As a result, the torsional strain in the chair shape is minimal. Because the overall strain in the chair conformation is modest, the chair conformation is relatively stable. It can be easily converted into other stable conformations by rotations about single bonds.
In contrast, the strained gauche conformation has twice as many rotatable bonds as the chair conformation and thus exhibits greater stability. The energy difference between these two structures can be as large as 150 kJ/mol or more. However, even this large energy difference cannot prevent isomerization to the chair form during chemical reactions or thermal fluctuations.
The term "chair conformation" was first introduced by Linus Pauling and Robert Corey in their book "Organic Chemistry" published in 1969. They wrote that in cyclohexane the chair conformation is the most stable form.
Cyclohexane has two stable forms - chair and twist. The ratio of chair to twist is approximately 1:1. Chairs are more common than twists because they are easier to create when starting from linear molecules like those found in polymers. Also, chairs are more stable than twists due to the smaller number of intermolecular contacts that need to be broken to convert one conformation to another.
The chair form, depicted on the right, is the most stable cyclohexane shape. Because the C-C-C bonds are so near to 109.5o, there is essentially no angle strain. It is also a totally staggered shape, which eliminates torsional strain. These factors help make the chair form highly stable.
The chair conformation is particularly stable because it allows the maximum number of non-bonded pairs of electrons. The more pairs that can be formed, the more stable the molecule will be.
In contrast, the boat conformation (depicted on the left) has an angle strain of 17.5 degrees between the two aromatic rings. This makes the boat less stable than the chair form.
Molecules in the chair conformation can rotate freely around their central covalent bond. They cannot move further away from each other or closer together. This leaves all the space along the ring available for bonding with other molecules or atoms inside the cell.
Molecules in the boat conformation can rotate around only one of its covalent bonds. The free rotation site is usually at the "bottom" of the boat structure, where there is room for movement. However, if two molecules in the boat conformation happen to line up correctly, they could form a stable double bond without rotating either molecule out of the chair conformation.