Chemical Properties of Enantiomers

When two enantiomers react with an achiral reagent, they react at the same rate, but when they react with a chiral, non-racemic reagent, they react at different rates. 

For an everyday analogy, consider what happens when you are handed an achiral object like a pen and a chiral object like a right-handed glove. Your left and right hands are enantiomers, but they can both hold the achiral pen in the same way. With the glove, however, only your right hand can fit inside it, not your left. 

We will examine specific reactions of chiral molecules with both chiral and achiral reagents later in this text. Here, we examine two more general applications. 

A living organism is a sea of chiral molecules. Many drugs are chiral, and often they must interact with a chiral receptor or a chiral enzyme to be effective. One enantiomer of a drug may effectively treat a disease whereas its mirror image may be ineffective. Alternatively, one enantiomer may trigger one biochemical response and its mirror image may elicit a totally different response. 

For example, the drugs ibuprofen and fluoxetine each contain one stereogenic center, and thus exist as a pair of enantiomers, only one of which exhibits biological activity. (S)-IbupFofen is the active component of the anti-inflammatory agents Motrin and Advil, and (fi)-fluoxetine is the active component in the antidepressant Prozac. 

Although (R)-ibuprofen shows no anti-inflammatory activity itself, it is slowly converted to the S enantiomer in vivo. 

---

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

Purpose To explore the physical and chemical properties of enantiomers. 

Molecules with nonsuperimposable mirror images are called chiral and a pair of such mirror-image molecules are called enantiomers. Most of the chemical properties of enantiomers and all their physical properties are identical. Their chemical properties differ only in reactions that involve another chiral species, such as a chiral molecule or a receptor site that is shaped to fit only one enantiomer. Most biochemical processes consist of a series of chemically specific reactions that use chiral receptor sites to facilitate reaction by allowing only the specific reactants to fit (and thus react). 

As shown by the example of the herbicide mecoprop in Figure 14.13, chiral molecules are three-dimensional molecules with structures such that a molecule cannot be directly imposed on its mirror image. Chiral molecules have different groups arranged around an atom, usually of carbon, that constitutes a chiral center. Two chiral molecules of the same compound are called enantiomers, commonly designated as R and S. The physical and chemical properties of enantiomers are... 

Some of the physical and chemical properties of enantiomers are indistinguishable from one another. For example, both of the optical isomers of 3-methylhexane have identical freezing points, melting points, and densities. However, the properties of enantiomers differ from one another in two important ways (1) in the direction in which they rotate polarized light and (2) in their chemical behavior in a chiral environment. 

The deoxyinositols (quercitols, cyclohexanepentols) are useful model compounds which display many of the physical and chemical properties of true deoxy sugars. Although (-b)-proto-quercitol, the best known isomer, was isolated from nature 118 years ago, no synthesis has been reported up until now. The synthesis here described is actually that of the (-)-enantiomer, starting with (-)-inositol however, identical procedures applied to the readily available ( + ) or dl-inositol would give (- -) or DL-proto-quercitol, respectively. The natural occurence of, )-proto-quercitol has... 

Another aspect of the chemical properties of mixmres of enantiomers has been reported by Wynberg and Feringa in 1976. These authors have smdied some dia-stereoselective reactions on chiral molecules (such as the LiAlH4 reduction of camphor) in the absence of chiral auxiliaries. They found that the product distribution was significantly different if the substrate was enantiopure or racemic. Similarly, it is known that reduction of enantiopure or racemic camphor by K/liquid NH3 gives rise to different isobomeol/bomeol ratios, a detailed mechanistic analysis has been done by Rautenstrauch. °... 

Problem 5.19 Compare physical and chemical properties of (a) enantiomers, (h) an enantiomer and its racemic form, and (c) diastereomers. 

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. 

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 properties of enantiomers are identical in an achiral environment. However, chemical reactions that add another asymmetric center create a diastereomeric pair, each of which has physical properties that are not completely the same. Therefore, although an enantiomeric pair cannot be separated by ordinary chromatographic means or fractional recrystallization, the diastereomeric pair can often be separated easily by these means, as is indicated in the chapter by Joseph Gal. After separation, the pure enantiomers can then be regenerated by chemical means. This is today the most fundamental way of resolving a racemate. 

Diastereomers are optical isomers that are not related as an object and its mirror image. Unlike enantiomers, the physical and chemical properties of diastereomers can differ and it is not unusual for them to have different melting and boiling points, refractive indices, solubilities, etc. Their optical rotations can differ in both sign and magnitude. 

Identical chemical and physical properties of enantiomers represent a potential source for enantiomer-enantiomer interactions at both pharmacokinetic and pharmacodynamic levels. Whether by competition for plasma- or tissue-binding sites or for drug-metabolizing enzymes, enantiomers may exhibit changes in pharmacokinetics when administered as a racemate compared to individual stereoisomers. The enantiomers of disopyramide exhibit similar clearance and volumes of distribution when given separately. " However, when administered as the racemate, the 5... 

Enantiomers of a chiral molecule have identical melting and boiling points, densities, and other physical and chemical properties. However, enantiomers show different behaviour towards plane-polarized light. When a beam of plane polarized light passes through an enantiomer, the plane of polarization rotates. For this reason chiral molecules are known as optical isomers and are said to be optically active. 

The synthesis of chiral molecules in laboratories results in the formation of a racemic mixture, an equal percentage mixture of both enantiomers. The extraction of enantiomers from racemic mixtures is almost impossible because of the identical physical and chemical properties of the enantiomers. Racemic mixtures are optically inactive, as they contain equal amounts of the D and L isomers. Some big organic molecules may contain more than one asymmetric carbon atom in their structure. An increasing number of asymmetric carbon atoms (n) increases the number of enantiomers by a factor of 2n. 

A molecule that cannot be superimposed on its mirror image is said to be chiral. When a carbon atom is bonded to four different atoms or groups of atoms, it is called a chiral carbon. Two stereoisomers that are nonsuperimposable mirror images of one another are a pair of enantiomers. As mentioned in Section 17.3, the chemical and physical properties of enantiomers are identical, with the exception that they rotate plane-polarized light to the same degree but in opposite directions. This is exactly the phenomenon that Pasteur observed with the mirror-image crystals of tartaric acid salts. 

The principle of this technique relies on the following the enantiomers possess the same free energy and all physico-chemical properties except chiro-optical ones. However, the free energy and, consequently, physico-chemical properties of the diastereomers are different. Thus, to be resolved using this technique, a mixture of enantiomers must be transformed in a mixture of diastereomeric compounds by stereoselective interaction with an optically pure reagent. 

---