How many stereoisomeric tartaric acids exist

As a general rule, a structure having n chiral centers will have 2 n possible combinations of these centers. Depending on the overall symmetry of the molecular structure, some of these combinations may be identical, but in the absence of such identity, we would expect to find 2 n stereoisomers. Some of these stereoisomers will have enantiomeric relationships, but enantiomers come in pairs, and non-enantiomeric stereoisomers will therefore be common.

We refer to such stereoisomers as diastereomers. In the example above, either of the ephedrine enantiomers has a diastereomeric relationship with either of the pseudoephedrine enantiomers. For an interesting example illustrating the distinction between a chiral center and an asymmetric carbon Click Here.

The configurations of ephedrine and pseudoephedrine enantiomers may be examined as interactive models by. Stereogenic Nitrogen Atoms. A close examination of the ephedrine and pseudoephedrine isomers suggests that another stereogenic center, the nitrogen, is present.

As noted earlier, single-bonded nitrogen is pyramidal in shape, with the non-bonding electron pair pointing to the unoccupied corner of a tetrahedral region. Since the nitrogen in these compounds is bonded to three different groups, its configuration is chiral.

The non-identical mirror-image configurations are illustrated in the following diagram the remainder of the molecule is represented by R, and the electron pair is colored yellow. If these configurations were stable, there would be four additional stereoisomers of ephedrine and pseudoephedrine. However, pyramidal nitrogen is normally not configurationally stable. It rapidly inverts its configuration equilibrium arrows by passing through a planar, sp 2 -hybridized transition state, leading to a mixture of interconverting R and S configurations.

If the nitrogen atom were the only chiral center in the molecule, a racemic mixture of R and S configurations would exist at equilibrium. If other chiral centers are present, as in the ephedrin isomers, a mixture of diastereomers will result. In any event, nitrogen groups such as this, if present in a compound, do not contribute to isolable stereoisomers.

The inversion of pyramidal nitrogen in ammonia may be examined by. Fischer Projection Formulas. The problem of drawing three-dimensional configurations on a two-dimensional surface, such as a piece of paper, has been a long-standing concern of chemists.

The wedge and hatched line notations we have been using are effective, but can be troublesome when applied to compounds having many chiral centers.

As part of his Nobel Prize-winning research on carbohydrates, the great German chemist Emil Fischer , devised a simple notation that is still widely used. In a Fischer projection drawing, the four bonds to a chiral carbon make a cross with the carbon atom at the intersection of the horizontal and vertical lines. The two horizontal bonds are directed toward the viewer forward of the stereogenic carbon.

The two vertical bonds are directed behind the central carbon away from the viewer. Since this is not the usual way in which we have viewed such structures, the following diagram shows how a stereogenic carbon positioned in the common two-bonds-in-a-plane orientation x—C—y define the reference plane is rotated into the Fischer projection orientation the far right formula.

When writing Fischer projection formulas it is important to remember these conventions. A model of the preceding diagram may be examined by. Using the Fischer projection notation, the stereoisomers of 2-methylaminophenylpropanol are drawn in the following manner. Note that it is customary to set the longest carbon chain as the vertical bond assembly.

The usefulness of this notation to Fischer, in his carbohydrate studies, is evident in the following diagram. There are eight stereoisomers of 2,3,4,5-tetrahydroxypentanal, a group of compounds referred to as the aldopentoses. Since there are three chiral centers in this constitution, we should expect a maximum of 2 3 stereoisomers. These eight stereoisomers consist of four sets of enantiomers. If the configuration at C-4 is kept constant R in the examples shown here , the four stereoisomers that result will be diastereomers.

Fischer formulas for these isomers, which Fischer designated as the "D"-family, are shown in the diagram. Each of these compounds has an enantiomer, which is a member of the "L"-family so, as expected, there are eight stereoisomers in all.

Determining whether a chiral carbon is R or S may seem difficult when using Fischer projections, but it is actually quite simple. If the lowest priority group often a hydrogen is on a vertical bond, the configuration is given directly from the relative positions of the three higher-ranked substituents.

If the lowest priority group is on a horizontal bond, the positions of the remaining groups give the wrong answer you are in looking at the configuration from the wrong side , so you simply reverse it.

The aldopentose structures drawn above are all diastereomers. A more selective term, epimer , is used to designate diastereomers that differ in configuration at only one chiral center. Thus, ribose and arabinose are epimers at C-2, and arabinose and lyxose are epimers at C However, arabinose and xylose are not epimers, since their configurations differ at both C-2 and C Meso Compounds.

The chiral centers in the preceding examples have all been different, one from another. In the case of 2,3-dihydroxybutanedioic acid, known as tartaric acid, the two chiral centers have the same four substituents and are equivalent.

As a result, two of the four possible stereoisomers of this compound are identical due to a plane of symmetry, so there are only three stereoisomeric tartaric acids. Two of these stereoisomers are enantiomers and the third is an achiral diastereomer, called a meso compound. Meso compounds are achiral optically inactive diastereomers of chiral stereoisomers.

Investigations of isomeric tartaric acid salts, carried out by Louis Pasteur in the mid 19th century, were instrumental in elucidating some of the subtleties of stereochemistry.

Some physical properties of the isomers of tartaric acid are given in the following table. Fischer projection formulas provide a helpful view of the configurational relationships within the structures of these isomers. In the following illustration a mirror line is drawn between formulas that have a mirror-image relationship. A model of meso-tartaric acid may be examined by An additional example, consisting of two meso compounds, may be examined by Other Configuration Notations.

Fischer projection formulas are particularly useful for comparing configurational isomers within a family of related chiral compounds, such as the carbohydrates. However, the eclipsed conformations implied by these representations are unrealistic. When describing acyclic compounds incorporating two or more chiral centers, many chemists prefer to write zig-zag line formulas for the primary carbon chain.

Here, the zig-zag carbon chain lies in a plane and the absolute or relative configurations at the chiral centers are then designated by wedge or hatched bonds to substituent groups. This is illustrated for D- - -ribose and the diastereoisomeric D-tetroses erythrose and threose in the following diagram.

These compounds are all chiral and only one enantiomer is drawn the D-family member. Many times, however, we must refer to and name diastereoisomers that are racemic or achiral. For example, addition of chlorine to cisbutene yields a stereoisomer of 2,3-dichlorobutane different from the one obtained by chlorine addition to transbutene.

In cases having two adjacent chiral centers, such as this, the prefixes erythro and threo may be used to designate the relative configuration of the centers. These prefixes, taken from the names of the tetroses erythrose and threose above , may be applied to racemic compounds, as well as pure enantiomers and meso compounds, as shown in the following diagram.

In the commonly used zig-zag drawings substituents may lie on the same side of the carbon chain, a syn orientation, or on opposite sides, an anti orientation. For adjacent vicinal substituents this is opposite to their location in a Fischer formula. Thus, the substituents in the erythro isomer have an anti orientation, but are syn in the threo isomer. The syn-anti nomenclature may be applied to acyclic compounds having more than two chiral centers, as illustrated by the example in the colored box.

The stereogenic center nearest carbon 1 serves as a reference. At sites having two substituents, such as carbon 5, the terms refer to the relative orientation of the highest order substituent, as determined by the C. As noted earlier, chiral compounds synthesized from achiral starting materials and reagents are generally racemic i.

Separation of racemates into their component enantiomers is a process called resolution. Since enantiomers have identical physical properties, such as solubility and melting point, resolution is extremely difficult. Diastereomers, on the other hand, have different physical properties, and this fact is used to achieve resolution of racemates. Reaction of a racemate with an enantiomerically pure chiral reagent gives a mixture of diastereomers, which can be separated.

Reversing the first reaction then leads to the separated enantiomers plus the recovered reagent. Many kinds of chemical and physical reactions, including salt formation, may be used to achieve the diastereomeric intermediates needed for separation. The following diagram illustrates this general principle by showing how a nut having a right-handed thread R could serve as a "reagent" to discriminate and separate a mixture of right- and left-handed bolts of identical size and weight.

Only the two right-handed partners can interact to give a fully-threaded intermediate, so separation is fairly simple. The resolving moiety, i. Chemical reactions of enantiomers are normally not so dramatically different, but a practical distinction is nevertheless possible. To learn more about chemical procedures for achieving resolution Click Here. Conformational Enantiomorphism.

The Fischer projection formula of meso-tartaric acid has a plane of symmetry bisecting the C2—C3 bond, as shown on the left in the diagram below, so this structure is clearly achiral. The eclipsed orientation of bonds that is assumed in the Fischer drawing is, however, an unstable conformation, and we should examine the staggered conformers that undoubtedly make up most of the sample molecules. The four structures that are shown to the right of the Fischer projection consist of the achiral Fischer conformation A and three staggered conformers, all displayed in both sawhorse and Newman projections.

The third conformer C has a center of symmetry and is achiral. Since a significant proportion of the meso-tartaric acid molecules in a sample will have chiral conformations, the achiral properties of the sample e. Equilibria among the various conformations are rapidly established, and the proportion of each conformer present at equilibrium depends on its relative potential energy the most stable conformers predominate.

Since enantiomers have equal potential energies, they will be present in equal concentration, thus canceling their macroscopic optical activity and other chiral behavior. Simply put, any chiral species that are present are racemic. It is interesting to note that chiral conformations are present in most conformationally mobile compounds, even in the absence of any chiral centers. The gauche conformers of butane, for example, are chiral and are present in equal concentration in any sample of this hydrocarbon.

The following illustration shows the enantiomeric relationship of these conformers, which are an example of a chiral axis rather than a chiral center. Another class of compounds that display conformational enantiomorphism are the substituted biphenyls. As shown in the following diagram, biphenyl itself is not planar, one benzene ring being slightly twisted or canted in relation to the other as a consequence of steric crowding.

This crowding will be demonstrated by clicking on the diagram. In order to interconvert such conformers with their mirror image structures, a rotation through the higher energy coplanar form must be made.

The ease with which this interconversion occurs will depend on the size of the ortho substituents, since these groups must slide past each other. The 2,2'-dicarboxylic acid on the left below cannot be resolved at room temperature, since thermal kinetic energy is sufficient to provide the necessary activation energy for racemization.

The two additionally substituted diacids to its right have a higher activation energy for racemization, and can be resolved if care is taken to avoid heating them.

Since fluorine is smaller than a nitro group, the center compound racemizes more rapidly on heating than does the nitro compound to its right. Conformational isomers that are isolable due to high energy barriers are called atropisomers. By clicking on the diagram , three additional examples of resolvable biphenyls will be displayed. Compounds B and C provide additional insight into the racemization of biphenyls.

Although these biphenyls have identical ortho substituents, the meta nitro substituent adjacent to the methoxyl group in C exerts a buttressing influence that increases the effective size of that ortho substituent.

Finally, by clicking on the diagram a second time two additional examples of substituted biphenyls will be shown. The left hand compound is held in a twisted conformation by the bridging carbon chain. Racemization requires passing through a planar configuration, and the increased angle and eclipsing strain in this structure contribute to a large activation energy. Consequently, this compound is easily resolved into enantiomeric stereoisomers.

The enantiomer of erythrose is its mirror image, and is named L-erythrose once again, you should use models to convince yourself that these mirror images of erythrose are not superimposable. In a pair of enantiomers, all of the chiral centers are of the opposite configuration. What happens if we draw a stereoisomer of erythrose in which the configuration is S at C 2 and R at C 3?

This stereoisomer, which is a sugar called D-threose, is not a mirror image of erythrose. D-threose is a diastereomer of both D-erythrose and L-erythrose. The definition of diastereomers is simple: if two molecules are stereoisomers same molecular formula, same connectivity, different arrangement of atoms in space but are not enantiomers, then they are diastereomers by default.

In practical terms, this means that at least one - but not all - of the chiral centers are opposite in a pair of diastereomers. By definition, two molecules that are diastereomers are not mirror images of each other.

L-threose, the enantiomer of D-threose, has the R configuration at C 2 and the S configuration at C 3. L-threose is a diastereomer of both erythrose enantiomers. In general, a structure with n stereocenters will have 2 n different stereoisomers. We are not considering, for the time being, the stereochemistry of double bonds — that will come later.

For example, let's consider the glucose molecule in its open-chain form recall that many sugar molecules can exist in either an open-chain or a cyclic form. There are two enantiomers of glucose, called D-glucose and L-glucose. The D-enantiomer is the common sugar that our bodies use for energy.

In L-glucose, all of the stereocenters are inverted relative to D -glucose. That leaves 14 diastereomers of D-glucose: these are molecules in which at least one, but not all, of the stereocenters are inverted relative to D-glucose. One of these 14 diastereomers, a sugar called D -galactose, is shown above: in D-galactose, one of four stereocenters is inverted relative to D-glucose. Diastereomers which differ in only one stereocenter out of two or more are called epimers.

D-glucose and D-galactose can therefore be refered to as epimers as well as diastereomers. We know that enantiomers have identical physical properties and equal but opposite degrees of specific rotation. In addition, the specific rotations of diastereomers are unrelated — they could be the same sign or opposite signs, and similar in magnitude or very dissimilar.

Determine the stereochemistry of the following molecule:. Objectives After completing this section, you should be able to calculate the maximum number of stereoisomers possible for a compound containing a specified number of chiral carbon atoms. Key Terms Make certain that you can define, and use in context, the key term below. Introduction It is easy to mistake between diasteromers and enantiomers. Figure 5. Diastereomers vs. Enantiomers Tartaric acid, C 4 H 6 O 6 , is an organic compound that can be found in grape, bananas, and in wine.

Notice that both chiral centers in L-erythrose both have the S configuration. Note In a pair of enantiomers, all of the chiral centers are of the opposite configuration.