Conformational Isomerism

Isomerism of Class 11

Conformational Isomerism

A bond between two carbon atoms is formed by the overlap of sp3 hybrid orbitals of carbon atom along their internuclear axis as a result of which the electron distribution within the molecular orbital thus formed is cylindrically symmetrical along the internuclear axis. Due to this symmetry, rotation about carbon-carbon single bond is almost free, as a result number of momentary arrangements of atoms called conformations or rotational isomers or rotamers can result.

(a)  Conformations of ethane

In ethane, the two carbon atoms are connected by a s-bond. If one of the methyl group in ethane molecule is kept fixed and the other is rotated about the C-C bond, infinite number of possible conformations of ethane result. The two extreme ones are termed as staggered & eclipsed and all the conformations lying between them are termed as gauche or skew conformations. The conformations can be represented by Sawhorse and Newman projections.

(i)   Sawhorse projection

In this projection, the molecule is viewed along the axis of the model from above and right. The central C-C bond is drawn as a straight line, slightly tilted to right for the sake of clarity. The line is drawn some what longer. The front carbon is shown as the lower left hand carbon whereas the rear carbon is shown as the upper right hand carbon. Each carbon has three lines corresponding to three atoms/groups (H atoms in the case of ethane).

(ii)  Newman projection

These projection formulae are obtained by viewing the molecule along the bond joining the two carbon atoms. The atom near the eye is represented by a point and the three atoms or groups attached to it by three equally spaced radii. The carbon atom farther from the eye is designated by a circle and the three atoms or groups attached to it by three equally spaced radical extensions.

(b) Conformations of n-butane

In order to examine the conformations of n-butane, it is considered as a derivative of ethane in which one hydrogen atom of each carbon is replaced by a methyl group. Thus, butane is considered as dimethyl ethane as shown below,

Thus, each of the two central carbon atoms (C2 and C3) in n-butane is linked to one methyl group and two hydrogen atoms. If now one of these carbon atoms is fixed and the other is rotated around the central bond through an angle of 360°, an infinite number of conformations are possible. Out of them, six important ones are as follows,

Relative stabilities of conformations

Out of the six conformations listed above, anti-conformation (I) is the most stable since in this conformations the two non-bonded methyl groups (dihedral angle 180°) and the four non-bonded hydrogen atoms are as far apart as possible. The next in order of higher energy come the two gauche conformations (III and V) in which the two non-bonded methyl groups are only 60° apart and hence causing crowding or streric strain. As a result of this streric strain, the two gauche conformations (III and V) are slightly less stable than the anti-conformation (I). However, the two gauche conformations are themselves of equal energy. Experimentally, it has been found that the gauche conformations are about 3.35 kJ mol-1 less stable than the anti conformation.

Next in order of higher energy fall the two partially eclipsed conformations (II and VI).
In these conformations, there are two methyl-hydrogen eclipsing interactions and one hydrogen-hydrogen eclipsing interaction. Since each methyl-hydrogen eclipsing interaction introduces energy of 5.35 kJ mol-1, therefore, partially eclipsed conformations of n-butane are less stable than anti and gauche conformations. However, the two partially eclipsed conformations are themselves of equal energy. Experimentally, it has found that partially eclipsed conformation (II and VI) is less stable than gauche conformation (III or V) by about 10.85 kJ mol-1 and than anti conformation (I) by about 14.2 kJ mol-1.

The fully eclipsed conformation (IV) is however, the least stable. This is due to the reason that in this conformation, there is one methyl-methyl eclipsing interaction and two weak hydrogen-hydrogen eclipsing interactions. Experimentally, it has been found that fully eclipsed conformation is about 18.4 - 25.5 kJ mol-1 less stable than the most stable anti conformation. Thus, the relative energies of the four distinct conformations of n-butane follow the order,

Anti > Gauche > Partially eclipsed > Fully eclipsed.

(b)       Conformations of Cyclohexane

The cyclic compounds most commonly found in nature contain six-membered rings because such rings can exist in a conformation that is almost completely free of strain. This conformation is called the chair conformation. In the chair conformer of cyclohexane, all the bond angles are 111°, which is very close to the ideal tetrahedral bond angle of 109.5°, and all the adjacent bonds are staggered.

Each carbon has an axial bond and an equatorial bond. The axial bonds are vertical and alternate above and below the ring. The axial bond on one of the uppermost carbons is up, the next is down, the next is up, and so on. The equatorial bonds point outward from the ring. Because the bond angles are greater than 90°, the equatorial bonds are on a slant. If the axial bond points up, the equatorial bond on the same carbon is on a downward slant. If the axial bond points down, the equatorial bond on the same carbon is on an upward slant.

Cyclohexane can also exist in a boat conformation, Like the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable as the chair conformer because some of the bonds in the boat conformer are eclipsed, giving it torsional strain. The boat conformer is further destabilized by the close proximity of the flagpole hydrogens (the hydrogens at the “bow” and “stern” of the boat), which causes steric strain.

Cyclohexane rapidly interconverts between two stable chair conformations because of the ease of rotation about its carbon–carbon bonds. This interconversion is known as ring flip. When the two chair conformers interconvert, bonds that are equatorial in one chair conformer become axial in the other chair conformer and vice versa.

The conformations that cyclohexane can assume when interconverting from one chair conformer to the other. To convert from the boat conformer to one of the chair conformers, one of the topmost carbons of the boat conformer must be pulled down so that it becomes the bottommost carbon. When the carbon is pulled down just a little, the twist-boat (or skew-boat) conformer is obtained. The twist-boat conformer is more stable than the boat conformer because there is less eclipsing and, consequently, less torsional strain and the flagpole hydrogens have moved away from each other, thus relieving some of the steric strain. When the carbon is pulled down to the point where it is in the same plane as the sides of the boat, the very unstable half-chair conformer is obtained. Pulling the carbon down farther produces the chair conformer.

The graph shows the energy of a cyclohexane molecule as it interconverts from one chair conformer to the other; the energy barrier for interconversion is 12.1 kcal mol (50.6 kJ mol). From this value, it can be calculated that cyclohexane undergoes 105 ring flips per second at room temperature. In other words, the two chair conformers are in rapid equilibrium.

Because the chair conformers are the most stable conformers, at any instant more molecules of cyclohexane are in chair conformations than in any other conformation. It has been calculated that, for every thousand molecules of cyclohexane in a chair conformation, no more than two molecules are in the next most stable conformation—the twist-boat.

Unlike cyclohexane, which has two equivalent chair conformers, the two chair conformers of a monosubstituted cyclohexane such as methylcyclohexane are not equivalent. The methyl substituent is in an equatorial position in one conformer and in an axial position in the other, because substituents that are equatorial in one chair conformer are axial in the other The chair conformer with the methyl substituent in an equatorial position is the more stable conformer because a substituent has more room and, therefore, fewer steric interactions when it is in an equatorial position.

 

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