Introduction to Conformational Enantiomorphism
The meso-tartaric acid's Fischer projection formula has a plane of symmetry bisecting the C2-C3 bond, as displayed on the left in the picture below, thus this structure is clearly achiral. The eclipsed orientation of bonds i.e. assumed in the Fischer drawing is, though, an unstable conformation and we should inspect the staggered conformers that without any doubt make up most of the sample molecules. The four structures which are displayed to the right of the Fischer projection contain the achiral Fischer conformation (A) and three staggered conformers; all shown in both Newman and sawhorse projections. The second and fourth conformations (B & D) are dissymmetric and are actually enantiomeric structures. The third conformer (C) has a center of symmetry and is achiral.
Since an important proportion of the meso-tartaric acid molecules in a sample will have chiral conformations, the sample's achiral properties (for example optical inactivity) should not be attributed to the symmetry of the Fischer formula. Equilibria among the several conformations are fastly established and the proportion of each conformer exist at equilibrium depends on its relative potential energy (the most stable conformers predominate). Because enantiomers have equivalent potential energies, they will be exists in equal concentration, so cancelling their macroscopic optical activity and other chiral behaviour. Simply put, any chiral species that are exists are racemic.
It is interesting to describing that chiral conformations are exists in several conformationally mobile compounds, although in the absence of any chiral centers. The butane's gauche conformers, for an instance, are chiral and are present in similar concentration in any sample of this hydrocarbon. The following example displayed the enantiomeric relationship of these conformers, which are an illustration of a chiral axis rather than a chiral center.
Other class of compounds that show conformational enantiomorphism are the substituted biphenyls. As displayed in the diagram, itself biphenyl is not planar, one benzene ring being little twisted or canted in relation to the other like a consequence of steric crowding. The resulting chiral conformation, having a dihedral angle approximately of 45º, equilibrates fastly with its enantiomer by rotation about the connecting single bond. Note: a conformation having a 90º dihedral angle is achiral, like a consequence of a plane of symmetry.
If each one of the phenyl rings of a biphenyl has two different meta or ortho substituents, even the twisted 90º dihedral angle conformer becomes chiral. To interconvert this type of conformers with their mirror image structures, a rotation from the higher energy coplanar form must be made. The easiness with which these interconversions occur will depend on the size of the ortho substituents, because these groups must slide past each other. The 2,2'-dicarboxylic acid on the left side below cannot be resolved at room temperature, because thermal (kinetic) energy is enough to provide the necessary activation energy for racemization. The 2 additionally substituted diacids to its right have a higher activation energy for racemization, and can be determined if care is taken to avoid heating them. According to a rule, an activation energy obstacle of 16 to 19 kcal/mole is required to avoid spontaneous room temperature racemization of substituted biphenyls. Because fluorine is smaller than a nitro group, the center compound racemizes more fastly on heating than does the nitro compound to its right. Conformational isomers which are isolable because of high energy barriers are called atropisomers.
The 2,2'-disulfonic acid (compound A) may be resolved with care, confirming the greater size of SO3H compared with CO2H. Compounds B and C present additional insight into the racemization of biphenyls. Even though these biphenyls have similar ortho substituents, the Meta nitro substituent adjacent to methoxyl group in C exerts a buttressing affect that increases the effective size of that ortho substituent. At last, in a twisted conformation, the left hand compound is held by the bridging carbon chain. Racemization necessitates passing from a planar configuration and get increased eclipsing strain and angle in this structure contribute to large activation energy. Subsequently, this compound is simply resolved into enantiomeric stereoisomers. The right hand side compound is heavily ortho-substituted and most definitely resists assuming a planar configuration. Though, the right benzene ring has two similar ortho substituents, therefore the stable 90º dihedral angle conformer has a plane of symmetry. All chiral twisted conformers are exists as racemates, therefore this compound cannot be resolved.
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