Chair conformation

Let’s know about Chair conformation. In organic chemistry , cyclohexane are any of the many three-dimensional shapes adopted by molecules of cyclohexane . Since many compounds have structurally similar six-membered rings , the structure and mobility of cyclohexane are important prototypes of a wide range of compounds.

Chair conformation
Chair conformation

The interior angles of a regular , flat hexagon are 120°, preferred, while the angle between consecutive bonds in a carbon chain is about 109.5°, the tetrahedral angle ( of arc cosine -1/3). Therefore, the cyclohexane ring assumes non-planar (distorted) conformations , with all angles closer to 109.5° and therefore a lower strain energy than the flat hexagonal shape .

If we keep the carbon atoms 1, 2, and 3 fixed, with the correct bond length and tetrahedral angle between the two bonds, and then add carbon atoms 4, 5, and 6 with the correct bond length and tetrahedral angle Continuing, we can substitute three dihedral angles for the sequences (2,3,4), (3,4,5), and (4,5,6) . The next bond, from atom 6, also gives a dihedral angle, so we have four degrees of freedom . But that last bond has to end at atom 1’s position, which imposes three terms in three-dimensional space. This means that all bond lengths are equal and all angles between bonds are equal, assuming there is one degree of freedom of conformation. It turns out that there are two sets of solutions to this geometric problem. two chair in one setThere are configurations (one in which the chain of atoms 1, 2, 3, and 4 has a positive dihedral angle, and one in which it is negative). The second set is a continuum, a topological circle where the angle strain is zero, consisting of twist boats and boat conformations. All conformations in this continuum have a two-fold axis of symmetry running through the ring, while chair conformations do not (they have a 3D symmetry, with a three-fold axis running through the ring) . . The Pitzer stresses related to the dihedral angles on this circleBecause energy changes. Twist boat has less energy than boat. To move from a chair structure to a twist-boat structure or other chair structure, the binding angles have to change, leading to a high-energy half-chair structure. So the relative constants are: chair > twist boat > boat > half chair. All relative conformational energies are shown below. [3] [4] Molecules can move freely between these structures at room temperature, but only the chair and twist-boat can be separated in pure form, as the other local energies are not at the minimum.

Boat and turn-boat conformity, as stated, lies along the continuum of zero angle stress. If there are substituents that allow the separation of different carbon atoms, this continuum is analogous to six boats and six twist-boats between them, like a circle with three “right-handers” and three “left-handed”. Is. (Which should be called right-handed is unimportant.) But if the carbon atoms are indistinguishable, as in cyclohexane, then moving along the continuum moves the molecule from boat form to a “right-handed” turn-boat, and then Back to the same boat as (with permutations of carbon atoms), then to a “left-handed” turn-boat, and then back to the achiral boat.

Principal Confirmers

The various conformations are called “conformers”, which are a mixture of the terms “conformation” and “isomer”.

Chair structure

Chair conformation: The structure of the chair is the most stable confirmer. At 25 °C, 99.99% of all molecules of cyclohexane solution adopt this conformation.

The symmetry is D3D . All carbon centers are the same. Six hydrogen centers are located in the axial position, approximately parallel to the C3 axis, and six hydrogen atoms are located near the equator. These H atoms are called axial and equatorial, respectively.

Each carbon has one “up” and one “bottom” hydrogen. The C-H bonds in successive carbons are staggered such that there is little torsional strain . The chair geometry is often preserved when hydrogen atoms are replaced by halogens or other simpler groups .

If we think of a carbon atom with four half-bonds sticking out towards the vertices of a tetrahedron , we can imagine that they are standing on a surface with one half-bond pointing straight up. does. Viewed from above, the other three appear to move outwards towards the vertices of an equilateral triangle , so the bondsAn angle of 120° will appear between Now consider six atoms that are standing on the surface so that their non-vertical half-bonds meet and form a perfect hexagon. If we reflect three atoms below the surface, we have something similar to a chair-conforming cyclohexane. In this model, the six vertical semi-bonds are exactly perpendicular, and the ends of the six non-vertical semi-bonds that protrude from the ring are exactly at the equator (that is, at the surface). Since C-H bonds are actually longer than half C-C bonds, the “equatorial” hydrogen atom of chair cyclohexane will actually be below the equator when attached to a carbon that is above the equator, and vice versa. This is true for other substances as well. dihedral angleExactly for a chain of four carbon atoms going around the ring in this model options between +60° and -60° (called Gauche ). (Chair conformation)

The chair structure cannot deform without changing the binding angle or length. We can think of it as two chains, mirror images of each other, with atoms 1-2-3-4 and 1-6-5-4 with opposite dihedral angles. The distance from atom 1 to atom 4 depends on the absolute value of the dihedral angle. If these two dihedral angles change (still opposite each other), it is not possible to maintain the correct bond angles on both carbon 1 and carbon 4.

Boat and turn-boat conformance

Boat conformation has more energy than chair conformation. The interaction between two flagpole hydrogens, in particular, generates steric strain . Torsional strain also exists between the C2–C3 and C5–C6 bonds (carbon number 1 is one of the two on the mirror plane), which are assumed —that is, these two bonds are parallel to each other in a mirror plane. Due to this stress, the boat configuration is unstable (ie there is no minimum local energy).

The molecular symmetry is C 2 V.

Chair conformation: Boat conformation spontaneously distorts into twist-boat conformation. Here the symmetry is D 2 , a purely rotational point group with three two-fold axes. This conformation can be obtained from the boat structure by adding a slight twist to the molecule so as to remove the assumption of two pairs of methylene groups. The turn-boat structure is chiral, existing in right-handed and left-handed versions.

The concentration of the twist-boat structure at room temperature is less than 0.1%, but at 1073 Kelvin it can reach up to 30%. Rapid cooling of a sample of cyclohexane from 1073 K to 40 K will result in a large concentration of twist-bottom conformation, which then gradually converges to the chair structure upon heating.


chair chair

Boat structure (4) via cyclohexane chair flip (ring inversion) reaction. Structures of important conformations are shown: chair (1), half chair (2), twist-boat (3) and boat (4). When the ring flip occurs completely from chair to chair, the hydrogens that were previously axial (blue H in the upper-left structure) become equatorial and equatorial (red H in the upper-left structure) axial. [3] It is not necessary to go as a boat.
The interchange of chair conformers is called ring flipping or chair-flipping . Carbon–hydrogen bonds that are axial in one configuration become equatorial in the other, and vice versa. The composition of the two chairs equilibrates rapidly at room temperature . The proton NMR spectrum of cyclohexane is a singlet at room temperature.

In a chair form, the dihedral angle of the carbon atoms chain 1-2-3-4 is positive while that of the 1-6-5-4 chain is negative, but in other chair forms, the situation is opposite. So both these chains have to undergo dihedral angle reversal. When one of these two four-atom chains is flattened at a dihedral angle of zero, at maximum energy along the conversion path we haveThere is structure. When the dihedral angle of this chain becomes equal (in sign as well as in magnitude) to that of the other four-atom chain, the molecule reaches a continuum of conformation, including the twist boat and the boat, where the bond angle and length are all their normals. values ​​and therefore the energy is relatively low. After that, the other four-carbon chain has to switch the sign of its dihedral angle to obtain the target chair form, so the molecule has to pass through the half-chair again as the dihedral angle of this chain passes through zero. Sequentially switching the signs of the two chains in this way reduces the maximum-energy state along the way (in the half-chair position)—meaning the dihedral angles of both four-atom chains have a simultaneous switch sign. will go through the symmetry due to angle stress on carbon 1 and 4.Due to high energy.

Chair conformation: The detailed mechanism of chair-to-chair inter-conversion has been the subject of much study and debate. [6] The half-chair state ( D , in the figure below) is the major transition state in the interconversion between chair and twist-boat conformations. The half-chair has C 2 symmetry. The inter-conversion between two chairs includes the following sequence:

chair → half chair → twist-boat → half chair′ → chair′.

twist-boat – twist-boat

The boat structure ( c , bottom) is a transition state, which allows for the interchange between two different turn-boat conformations. While the boat structure is not necessary for the inter-conversion between the two chair structures of cyclohexane , it is often included in the reaction coordination diagram used to describe this inter-conversion because of its energy relative to the half-chair. So the energy to go from boat to chair with any molecule has enough energy to go from boat to boat. Thus, there are several pathways by which a molecule of cyclohexane in the twist-boat structure can acquire the chair structure again.

substituted derivatives

In cyclohexane, the two chair structures have the same energy. With substituted derivatives the situation becomes more complicated. The two chair conformers in methylcyclohexane are not isoenergetic. The methyl group prefers the equatorial orientation. The preference of a substituent towards the equatorial structure is measured in terms of its A value , which is the Gibbs free energy difference between the two chair conformations. A positive A value indicates a preference for the equatorial position. The magnitude of A values ​​for very small substituents such as deuterium range from about zero to about 5 kcal/mol (21 kJ/mol) for very heavy substituents such as the tert-butyl group.

displaced cyclohexane

For 1,2- and 1,4-dissociated cyclohexane, a cis configuration leads to an axial and an equatorial group. Such species undergo rapid, degenerate chair flipping. For the 1,2- and 1,4-displaced cyclohexane, a trans configuration, the biaxial structure is effectively prevented by its high steric stress. For 1,3-dissociated cyclohexane, the cis form is diquatorial and there is additional steric interaction between the two axial groups in the flipped structure. Trans -1,3-dissubstituted cyclohexane are cis – 1,2- and cis – 1,4- like and can be flipped between the two equivalent axial/equatorial forms. [2]

The cis -1,4-di- tert – butylcyclohexane has an axial tert-butyl group in the chair structure and the conversion to the twist-boat structure places both groups in a more favorable equatorial position. As a result, the twist-bottom structure is more stable than 0.47 kJ/mol (0.11 kcal/mol) at 125 K as measured by NMR spectroscopy. [6]

Heterocyclic analogs

Heterocyclic analogues of cyclohexane are widespread among sugars, piperidines, dioxins, etc. They generally follow the trends observed for cyclohexane, i.e. the chair conformer is the most stable. The axial–equatorial equilibrium (A values) are however highly affected by the replacement of methylene by O or NH. Examples are the compositions of glucosides. [2] 1,2,4,5-tetrathion ((SCH 2 ) 3 ) lacks the unfavorable 1,3-biaxial interactions of cyclohexane. As a result its turn-boat structure is populated; In the corresponding tetramethyl structure, the 3,3,6,6-tetramethyl-1,2,4,5-tetrathion, twist-bottom conformation dominates.

historical background

In 1890, a 28-year-old assistant in Berlin, Hermann Sachs  [de] published instructions for folding a piece of paper to represent the two forms of cyclohexane, which he called symmetric and asymmetric (what we now call chair and boat ). ) Told. He clearly understood that there were two positions for hydrogen atoms in these forms (again, to use modern terminology, axial and equatorial ), that the two bases would probably be interconnected, and even that how some substituents can favor one of the chair forms (True-Mohr principle  [D], Because he expressed all this in mathematical language, few chemists of the time understood his arguments. He made several attempts to publish these ideas, but none succeeded in capturing the imagination of the chemists. His death in 1893 at the age of 31 meant that his thoughts sank into obscurity. It was only in 1918 that Ernst Mohr  [d] , based on the molecular structure of diamond, which had recently been resolved using the then-new technique of X-ray crystallography, [7] [8] was able to successfully argue that Was that the chair of Sachse was the main motive. [9] [10] [11] [12] [13] [14]Derek Barton and Aud Hasel shared the Nobel Prize in 1969 for work on the structure of cyclohexane and various other molecules.