Why do stereoisomers exist

Practice Problems. Conformational Isomerism. Structural formulas show the manner in which the atoms of a molecule are bonded together its constitution , but do not generally describe the three-dimensional shape of a molecule, unless special bonding notations e. The importance of such three-dimensional descriptive formulas became clear in discussing configurational stereoisomerism, where the relative orientation of atoms in space is fixed by a molecule's bonding constitution e.

Here too it was noted that nomenclature prefixes must be used when naming specific stereoisomers. In this section we shall extend our three-dimensional view of molecular structure to include compounds that normally assume an array of equilibrating three-dimensional spatial orientations, which together characterize the same isolable compound.

We call these different spatial orientations of the atoms of a molecule that result from rotations or twisting about single bonds conformations. We know this is not strictly true, since the carbon atoms all have a tetrahedral configuration. The actual shape of the extended chain is therefore zig-zag in nature. However, there is facile rotation about the carbon-carbon bonds, and the six-carbon chain easily coils up to assume a rather different shape.

Many conformations of hexane are possible and two are illustrated below. Extended Chain Coiled Chain. For an animation of conformational motion in hexane. Ethane Conformers. The simple alkane ethane provides a good introduction to conformational analysis.

Here there is only one carbon-carbon bond, and the rotational structures rotamers that it may assume fall between two extremes, staggered and eclipsed. In the following description of these conformers, several structural notations are used. The first views the ethane molecule from the side, with the carbon-carbon bond being horizontal to the viewer. The hydrogens are then located in the surrounding space by wedge in front of the plane and hatched behind the plane bonds.

If this structure is rotated so that carbon 1 is canted down and brought closer to the viewer, the "sawhorse" projection is presented. Finally, if the viewer looks down the carbon-carbon bond with carbon 1 in front of 2, the Newman projection is seen. To see an eclipsed conformer of ethane orient itself as a Newman projection, and then interconvert with the staggered conformer and intermediate conformers. The most severe repulsions in the eclipsed conformation are depicted by the red arrows.

There are six other less strong repulsions that are not shown. In the staggered conformation there are six equal bond repulsions, four of which are shown by the blue arrows, and these are all substantially less severe than the three strongest eclipsed repulsions.

Consequently, the potential energy associated with the various conformations of ethane varies with the dihedral angle of the bonds, as shown below. For a discussion of this feature. The above animation illustrates the relationship between ethane's potential energy and its dihedral angle. Butane Conformers. The hydrocarbon butane has a larger and more complex set of conformations associated with its constitution than does ethane. Of particular interest and importance are the conformations produced by rotation about the central carbon-carbon bond.

As in the case of ethane, the staggered conformers are more stable than the eclipsed conformers by 2. Since the staggered conformers represent the chief components of a butane sample they have been given the identifying prefix designations anti for A and gauche for C. Four Conformers of Butane The following diagram illustrates the change in potential energy that occurs with rotation about the C2—C3 bond. The model on the right is shown in conformation D , and by clicking on any of the colored data points on the potential energy curve, it will change to the conformer corresponding to that point.

Each carbon atom is then numbered in order through the end of the chain. When numbering stereoisomers that have more than three carbon atoms we look at the position of the OH group on the penultimate or next to last carbon atom because this determines whether it is an L or D stereoisomer. In this example we will look at the numbering of D-Glucose. First we must find the reactive end of the molecule and assign its carbon the number one.

We then number the remaining carbons in order through the end of the chain. In theory, in glucose, the position of the OH group on each of the asymmetric carbon atoms, numbers two, three, four, and five could be flipped, producing a distinct stereoisomer each time, for a total of 16 or 2 4 stereoisomers. However, not all of these actually exist in nature. For fructose, there are only three asymmetric carbons, so only 8 or 2 3 stereoisomers can be produced.

Only a few of the monosaccharides exist free in nature. Most of them are usually found as sugar units in polysaccharides or in more complex molecules. Monosaccharides are often called simple sugars, and are sub-divided according to the number of C-atoms.

These compounds are important metabolic intermediates in the oxidation of glucose to produce energy. Pentoses C 5 H 10 O 5 Three important pentoses are:. D-ribose — a component of RNA, ribonucleic acid, vitamins riboflavin , and coenzymes. In its reduced form, deoxyribose, it is a component of DNA. L-arabinose — occurs in conifer heartwood and is a component of hemicelluloses where it occurs with xylose. It is also a component of pectin and can be a major component of gums gum Arabic.

Bacterial action in making silage can yield free arabinose. Arabans are polymers of arabinose. D-xylose — there are small amounts of D-xylose free in fruits, but it occurs mainly in hemicellulose, as xylans and hetero-xylans.

Hemicellulose is a polysaccharide of xylose and arabinose a heteroxylan. The ratio of xylose to arabinose seems to affect digestibility as digestibility is reduced as the proportion of xylose increases. Hemicelluloses constitute a considerable portion of the cell walls of plants so herbivores eat large amounts of them. These sugars are all aldopentoses. This type of stereoisomer is the essential mirror-image, non-superimposable type of stereoisomer introduced in the beginning of the article.

Figure 3 provides a perfect example; note that the gray plane in the middle demotes the mirror plane. Figure 2: Comparison of Chiral and Achiral Molecules. Rotation of its mirror image does not generate the original structure.

To superimpose the mirror images, bonds must be broken and reformed. Note that even if one were to flip over the left molecule over to the right, the atomic spatial arrangement will not be equal. This is equivalent to the left hand - right hand relationship, and is aptly referred to as 'handedness' in molecules. This can be somewhat counter-intuitive, so this article recommends the reader try the 'hand' example.

Place both palm facing up, and hands next to each other. Now flip either side over to the other. One hand should be showing the back of the hand, while the other one is showing the palm.

They are not same and non-superimposable. This is where the concept of chirality comes in as one of the most essential and defining idea of stereoisomerism. Chirality essentially means 'mirror-image, non-superimposable molecules', and to say that a molecule is chiral is to say that its mirror image it must have one is not the same as it self. Whether a molecule is chiral or achiral depends upon a certain set of overlapping conditions.

Figure 4 shows an example of two molecules, chiral and achiral, respectively. Notice the distinct characteristic of the achiral molecule: it possesses two atoms of same element. In theory and reality, if one were to create a plane that runs through the other two atoms, they will be able to create what is known as bisecting plane: The images on either side of the plan is the same as the other Figure 4.

In this case, the molecule is considered 'achiral'. In other words, to distinguish chiral molecule from an achiral molecule, one must search for the existence of the bisecting plane in a molecule. All chiral molecules are deprive of bisecting plane, whether simple or complex.

As a universal rule, no molecule with different surrounding atoms are achiral. Chirality is a simple but essential idea to support the concept of stereoisomerism, being used to explain one type of its kind. The chemical properties of the chiral molecule differs from its mirror image, and in this lies the significance of chilarity in relation to modern organic chemistry.

We turn our attention next to molecules which have more than one stereocenter. We will start with a common four-carbon sugar called D-erythrose. A note on sugar nomenclature: biochemists use a special system to refer to the stereochemistry of sugar molecules, employing names of historical origin in addition to the designators ' D ' and ' L '.

You will learn about this system if you take a biochemistry class. As you can see, D -erythrose is a chiral molecule: C 2 and C 3 are stereocenters, both of which have the R configuration. In addition, you should make a model to convince yourself that it is impossible to find a plane of symmetry through the molecule, regardless of the conformation. Does D-erythrose have an enantiomer? Of course it does — if it is a chiral molecule, it must. 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.

Notice that both chiral centers in L-erythrose both have the S configuration. 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?