Chemical synthesis of compounds very often produces an mixture of both forms, called a racemic mixture, where only one form is effective, and the other form is not used or may even do damage. It is possible to make L-glucose its mirror-image by chemical synthesis. The human body cannot use L-glucose. It tasts just as sweet, but no calories can be gained from it. It will be secreted by the kidneys, where it may do some damage in the long term, or part of it may cause fermentation in the intestines flatulence.
A more notorious example is thalidomide aka softenon and other names , prescribed in the 50s and early 60s for pregnant women to deal with morning sickness a. The laboratory stuff that was tested contained only one stereo-isomer, but the industrial product also contained the other one, which led to great deformities in newborn babies.
A molecule is considered chiral if there exists another molecule that is of identical composition but which is arranged in a non-superposable mirror image. Also the presence of an asymmetric carbon atom is often the feature that causes chirality in molecules. Two mirror images of a chiral molecule are called enantiomers or optical isomers.
Pairs of enantiomers are often designated as "right-" and "left-handed". Molecular chirality is of interest because of its application to stereochemistry in inorganic chemistry, organic chemistry, physical chemistry, biochemistry, and supramolecular chemistry. Note that only after examining a potential chiral carbon can you determine any achirality it actually has. After you determine what atoms are stereocenters, if there is an even number of stereocenters, check whether or not the molecule has a symmetry axis dividing down the middle of the stereocenters.
If so, you might have a meso isomer , in which case the molecule as a whole is NOT chiral. Can you see that carbons 5, 6, 9, 13, and 14 are stereocenters? Can you identify which configuration they are? Key Questions Why is the study of chiral molecules important in biochemistry? Because organisms may react differently to stereo-isomers.
What are chiral and achiral molecules? How can I identify chiral and achiral molecules? When you figure it out, check out the link above and see what they actually are. If the molecule has more than one chiral center, it is most likely chiral. The exceptions are meso-compounds , which have chiral centers but are not chiral due to the presence of a plane of symmetry. Molecule 3 has a single chiral center carbon 2.
Therefore, this molecule is chiral. Note that in this depiction, we have not specified if the NH2 group is up or down. This means usually means that it is racemic mixture of the two. However, the molecule can still be chiral even if it is a racemate.
Molecule 4 has two chiral centers. Therefore, it is most likely chiral. If we use test 1 or 2, we find that it is in fact chiral. However, if the two methyl groups were cis, then the molecule would still have two chiral centers but not be chiral and would be a meso compound. Ephedrine from Ma Huang: m. Since these two compounds are optically active, each must have an enantiomer. Although these missing stereoisomers were not present in the natural source, they have been prepared synthetically and have the expected identical physical properties and opposite-sign specific rotations with those listed above.
Each may assume an R or S configuration, so there are four stereoisomeric combinations possible. These are shown in the following illustration, together with the assignments that have been made on the basis of chemical interconversions.
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 Clicking Here.
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 clicking on the following diagram. 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 showing the above rotation into a Fischer projection may be examined by Clicking Here.
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 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 Clicking Here. An additional example, consisting of two meso compounds, may be examined by Clicking Here. Other methods of designating configuration have been proposed.
These will be shown by Clicking Here. 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 difficult. Diastereomers, on the other hand, have different physical properties, and this fact may be 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.
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. Try to line up your left hand perfectly with your right hand, so that the palms are both facing in the same directions. Spend about a minute doing this. Do you see that they cannot line up exactly? The same thing applies to some molecules.
Figure 1 : Mirror symmetry. A Chiral molecule has a mirror image that cannot line up with it perfectly- the mirror images are non superimposable. The mirror images are called enantiomers. But why are chiral molecules so interesting? A chiral molecule and its enantiomer have the same chemical and physical properties boiling point, melting point,polarity, density etc It turns out that many of our biological molecules such as our DNA, amino acids and sugars, are chiral molecules.
It is pretty interesting that our hands seem to serve the same purpose but most people are only able to use one of their hands to write.
Similarily this is true with chiral biological molecules and interactions. Just like your left hand will not fit properly in your right glove, one of the enantiomers of a molecule may not work the same way in your body.
This must mean that enantiomers have properties that make them unique to their mirror images. One of these properties is that they cannot have a plan e of symmetry or an internal mirror plane. So, a chiral molecule cannot be divided in two mirror image halves. Another property of chiral molecules is optical activity. Organic compounds, molecules created around a chain of carbon atom more commonly known as carbon backbone , play an essential role in the chemistry of life.
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