Properties of Enantiomers

In what properties do enantiomers differ from one another Enantiomers differ only with respect to chirality. In all other respects, they are identical. For this reason, they differ from one another only in properties that are also chiral. Let us illustrate this idea first with familiar objects. 

A left-handed baseball player (chiral) can use the same ball or bat (achiral) as can a right-handed player. But, of course, a left-handed player (chiral) can use only a left-handed baseball glove (chiral). A bolt with a right-handed thread (chiral) can use the same washer (achiral) as a bolt with a left-handed thread, but it can only fit into a nut (chiral) with a right-handed thread. To generalize, the chirality of an object is most significant when the object interacts with another chiral object. 

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Pairs of enantiomers have the same physical and chemical properties they have that same heats of formation, density, melting point, and boiling point. They also have the same chemical properties, and undergo the same reactions in an achiral environment. However, enantiomers can be distinguished in a chiral environment. This difference is important in many processes in hvrng cells. Only one of a pair of enantiomers fits into a specific site in a biological molecule such as an enzyme catalyst because the site on the enzyme that binds the enantiomer is chiral. The binding of this enantiomer is stereospecific. 

An example of a stereospecrfic process is the conversion of the drug levodopa to dopamine, a neuro transmitter in the brain. Levodopa (or L-dopa), the precursor of dopamine, is administered to treat Parkinson s disease. Levodopa has one chiral carbon atom. Therefore, it exists as either of two enantiomers. Only the enantiomer with the configuration shown below is transformed into dopamine. 

The reaction occurs because a stereospecific decarboxylase catalyzes the loss of a carboxyl group by formation of carbon choxide (decarboxylation). This enzyme has a chiral binding site for levodopa, but it does not bind the enantiomer of levodopa. 

As you work with Fischer projections, you may notice that some routine stmctural changes lead to predictable outcomes—outcomes that may reduce the number of manipulations you need to do to solve stereochemistry problems. Instead of listing these shortcuts. Problem 7.11 invites you to discover some of them for yourself. 

Using the Fischer projection of (/ )-2-butanol shown, explain how each of the following affects the configuration of the chirality center. 

Switching the positions of two groups in a Fischer projection reverses the configuration of the chiraiity center. 

We mentioned in Section 7.6 that the d,l system of stereochemical notation, while outdated for most purposes, is still widely used for carbohydrates and amino acids. Likewise, Fischer projections hnd their major application in these same two families of compounds. 

The usual physical properties such as density, melting point, and boiling point are identical for both enantiomers of a chiral compound. 

Enantiomers can have striking differences, however, in properties that depend on the arrangement of atoms in space. Take, for example, the enantiomeric forms of car-vone. (/ )-(—)-Carvone is the principal component of spearmint oil. Its enantiomer, (5)-( + )-carvone, is the principal component of caraway seed oil. The two enantiomers do not smell the same each has its own characteristic odor. 

The difference in odor between (R)- and (5)-carvone results from their different behavior toward receptor sites in the nose. It is believed that volatile molecules occupy only those odor receptors that have the proper shape to accommodate them. Because the receptor sites are themselves chiral, one enantiomer may fit one kind of receptor while 

Click CoachedTutorial Problems to practice using the Cahn-Ingold-Prelog Sequence Rules. 

When do enantiomers have different properties Again, it is helpful to draw analogies with everyday objects that are chiral. When do your hands have different properties They are different when you put on a glove they are different when you write they are different when you shake hands. What do these objects or activities have in common A glove, writing, and shaking hands are all chiral Hands are different when they interact with one enantiomer of a chiral object or activity. Likewise, enantiomeric molecules are different when they are in a chiral environment. Most commonly, the chiral environment is the presence of one enantiomer of another chiral compound. Otherwise, their properties are identical. For example, the naturally occurring ketones (R)- and (5)- 

The most common method used to detect the presence of chiral molecules in a sample employs the interaction of plane-polarized light with the sample. 

Regular light waves consist of electromagnetic fields that oscillate in all directions perpendicular to the direction of travel of the wave. If we could see these fields while viewing the light beam coming directly at us, the oscillations would occur along the arrows. 

For a particular compound the observed rotation depends on the concentration of the compound, the path length of the sample tube, and the wavelength of the light that is used. Often the yellow light produced by a sodium lamp, called the sodium D line (wavelength = 589 run), is used. The specific rotation, a constant characteristic of each 

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Because of the extreme sensitivity of monolayers to contamination and the resulting likelihood of erroneous results, every precaution must be taken to ensure that all materials and instruments are clean. A great bonus for the stereochemical studies reported here is that comparison of the properties of enantiomers provides a rigorous check for internal consistency and purity that is not available to studies of achiral systems. Details have been given elsewhere and will not be included here (Thompson, 1981). 

Since the laws of symmetry require that all properties of enantiomers (except their interactions with other chiral systems) be exactly the same, these studies have profited by the application of an absolute test for the presence of impurities, a perennial problem in monolayer research. In every case, all measurements were repeated with both enantiomers. Unless the results agreed within experimental error, the compounds were purified repeatedly until they did agree. 

Two compounds are diastereomers when they contain more than one chiral center. If the number of dissymmetric centers is given by N, then the number of possible diastereomers is given by 2N. Of these 2 v diastereomers, each will be characterized by its mirror image, so that the number of enantiomers is given by 2NI2. Whereas the physical properties of enantiomers in an achiral environment are necessarily identical, the physical properties (including solubility) of diastereomers are normally different. The differences arise since there is no structural requirement that the crystal lattices of different diastereomers be the same. For instance, the solubility of an (SS )-diastereomer could differ substantially from that of the (/ S)-diastereomer. However, it should be remembered that the solubility of the (SS)-diastereomer must be exactly identical to that of the (I 7 )-diastereomer, since these compounds are enantiomers of each other. At the same time, the solubilities of the (SI )-diastereomer and the (I S)-diastereomer must also be identical. 

In psychopharmacology, interest in the properties of enantiomers has been aided by the need to improve the therapeutic efficacy and decrease the side effects and toxicity of drugs. For example, if the therapeutic activity resides entirely in one enantiomer (called a eutomer) then giving a racemic mixture which contains the active and the inactive enantiomer is clearly wasteful. Thus using the single enantiomer (isomer or eutomer) should enable the dose of the drug to be lowered, reduce the interpatient variability in the response and, hopefully, reduce the side effects and toxicity of the drug (see Table 3.4). 

The physical properties of enantiomers and race-mates, except for optical rotation and melting points, are... 

Although most physical properties of enantiomers are identical, pharmacological properties may be different. There are examples of compounds where ... 

Properties of enantiomers Enantiomers share same physical properties, e.g. melting points, boiling points and solubilities. They also have same chemical properties. However, they differ in their activities with plane polarized light, which gives rise to optical isomerism, and also in their pharmacological actions. 

Because the physical properties of enantiomers are identical, they seldom can be separated by simple physical methods, such as fractional crystallization or distillation. It is only under the influence of another chiral substance that enantiomers behave differently, and almost all methods of resolution of enantiomers are based upon this fact. We include here a discussion of the primary methods of resolution. 

Circularly polarized (laser) light is widely used not only to study the absorption properties of enantiomers, but also to generate optically active compounds via enantioselective photochemical process. 

Except for the property of rotating plane-polarized light in opposite directions, the physical properties of enantiomers of the same compound are identical. In addition, their chemical properties are identical, except when they are acted upon by another chiral molecule. One such kind of molecule consists of enzymes, large molecules of proteins that catalyze biochemical reactions. Therefore, many biochemical reactions involve chiral molecules. 

The physical and chemical properties of enantiomers are identical the physical and chemical properties of diastereoisomers differ. Diastereoisomer is sometimes shortened to diastereomer. ... 

The physical properties of meso compounds, diastereomers and racemic mixtures differ from each other and from the properties of enantiomers. 

W. H. Pirkle, The nonequivalence of physical properties of enantiomers in Optically active solvents. Differences in nuclear magnetic resonance spectra. I, /. Am. Chem. Soc. 88 (1966), 1837. 

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