What Are Enantiomers

Enantiomers are stereoisomers that are nonsuperposable mirror images. The significance of enantiomerism is that, except for inorganic and a few simple organic compounds, the vast majority of molecules in the biological world show this type of isomerism, including carbohydrates (Chapter 17), lipids (Chapter 19), amino acids and proteins (Chapter 18), and nucleic acids (DNA and RNA, Chapter 20). Further, approximately one-half of the medications used in human medicine also show this type of isomerism. 

As an example of a molecule that exhibits enantiomerism, let us consider 2-butanol. As we go through the discussion of this molecule, we focus on carbon 2, the carbon bearing the —OH group. What makes this carbon of interest is that it has four different groups bonded to it. The most common cause of enantiomerism among organic molecules is a carbon bonded to four different groups. 

The structural formula we have just drawn does not show the shape of 2-butanol or the orientation of its atoms in space. To do this, we must consider the molecule as a three-dimensional object. On the left are a ball-and-stick model of 2-butanol and a perspective drawing of what we will call the original molecule. In this drawing, the —OH and —CH3 groups on carbon-2 are in the plane of the paper the —H is behind the plane and the — CH2CH3 group is in front of the plane. 

The key point here is that either an object is superposable on its mirror image or it isn t Now let us look at 2-butanol and its mirror image and ask, Are they or are they not superposable The following drawings illustrate one way to see that the mirror image of 2-butanol is not superposable on the original molecule  

Stereoisomers that are nonsuperposable mirror images the term refers to a relationship between pairs of objects. 

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A What is optical isomerism What are enantiomers A Which of the following compounds would exhibit optical isomerism ... 

How many stereoisomers of 1,2-cyclopentanediol are there What are their stereochemical relationships Are they enantiomers ... 

What are the facts of life One of the most striking is that all known living systems involve the same types of polymers, i.e., three varieties of homochiral biopolymers. That is, each variety is composed of unique molecular building blocks having the same three-dimensional handedness. Thus, with rare exceptions, the proteins found in cells are composed exclusively of the 1-enantiomers of 19 optically active amino acids (Fig. 11.1). Similarly, only D-ribose and 2-deoxy-D-ribose sugars are found in the nucleic acid polymers that make up the RNAs and DNAs, which are essential for protein synthesis in the cell and for the transmission of genetic information from one generation to the next. 

Problem 5.25 (a) What is the necessary and sufficient condition for the existence of enantiomers (6) What is the necessary and sufficient condition for measurement of optical activity (c) Are all substances with chiral atoms optically active and resolvable (d) Are enantiomers possible in molecules that do not have chiral carbon atoms (e) Can a prochiral carbon ever be primary or tertiary (/) Can conformational enantiomers ever be resolved ... 

Exercise 5-13 Write structures for all the configurations possible for 2,4-dibromo-pentane. Which stereoisomers are enantiomers Which are diastereomers What combination of isomers would give a racemic mixture Which isomer is achiral ... 

To our knowledge, topologically chiral molecules have not yet been resolved into enantiomers. However, we may anticipate that their energy barrier to racemization will be extremely high, compared to Euclidean chiral molecules. Therefore they are expected to be useful in enantioselective interactions or reactions. For example, it has been shown that tetrahedral copper(I) bis-2,9-diphenyl-l,10-phenanthroline complexes (which form the catenate subunits) are good reductants in the excited state [97] therefore the chiral Cu(I) catenates could be used for enantioselective electron-transfer reactions. Alternatively, the resolution of topologically chiral molecules would allow to answer fundamental questions, such as what are the chiroptical properties of molecular trefoil knots ... 

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. 

Problem 36.9 Besides threonine, there are four amino acids in Table 36.1 that can exist in more than two stereoisomeric forms, (a) What are they (b) How many isomers are possible in each case Indicate enantiomers, diastereomers, any meso compounds. 

The selectivity of a chemical reaction is a very important criterion. Besides the chemo- and regioselectivity, the stereoselectivity, i.e. the favored or excluded formation of one or several stereoisomers in the course of a chemical reaction, plays an important role. If there is a formation of (S)- and (K)-enantiomers from a prochiral compound, an enantioselective reaction takes place. What are the reasons for the growing interest in enantioselective reactions and preparation of homochiral compounds Firstly, it is certainly the wish of the chemist to imitate the ability of nature by stereospecific synthesis in the laboratory. Secondly, there are some practical and economic reasons many natural products and a great number of synthetic drugs have a chiral structure and the enantiomers can differ markedly in their biological activity. Sometimes only one of the enantiomers exhibits the wanted optimal activity, while the other is less active or totally inactive, or even toxic. 

What are the stereochemical relations (identical, enantiomers, diastereoisomers) of the following four molecules (I-L) Assign absolute configurations at each stereogenic centre. 

Lipkowitz [63] used molecular dynamics simulations to answer the following questions a) What are the intermolecular forces responsible for analyte binding to the CSP b) Where on or in the host does the analyte bind c) What are the differential interactions giving rise to chiral discrimination d) What differences do R and S-enantiomers experience in the CD cavity e) Are existing chiral recognition mechanisms valid His computational work was based on experimental separations... 

Approximately one in every four drugs currently on the market can be considered to be an isomeric mixture, yet for many of these compounds, the biological activity may reside in only one isomer (or at least predominate in one isomer). The majority of these isomeric mixtures are what are referred to as racemic mixtures (or racemates ). These are compounds, usually synthetic, that contain equal amounts of both possible enantiomers, or optical isomers. 

Compared with other drug products, what are the costs of and technical barriers to obtaining safety and efficacy data for a drug product whose active ingredient is a single enantiomer of a previously approved racemate ... 

Fig. 3.1 The energies of enantiomers are different because of a symmetry violation. The energy difference Ap Eo = ApyE and the reaction enthalpy Apv/fo = /lApv o for the reaction R = S can be described with the spectroscopic schematic diagram shown here. This is estimated to be 10 J moL for CHFClBr [12]. How important is this energy difference for chemistry What are the consequences for biology (see also [13])...

Chiral objects have nonsuperimposable mirror images. What does that mean Superimpos-able means one object can be placed over another, or less technically, that they re identical. So enantiomers are not identical, but tho are mirror images, lyce a look at your hands— they are enantiomers. If you put one hand up to a mirror, it looks like your other hand (so they are mirror images). But if you try to put one hand on top of your other (no, not palm to palm, that s cheating), you see they re not identical (therefore nonsuperimposable). 

The dihydroxy acid shown was prepared as a single enantiomer and underwent spontaneous cyclization to give a 8-lactone, What are the R-S configurations of the chirality centers in this lactone (No stereochemistry is implied in the structural drawing.)... 

Demonstrate the validity of what we have represented in Fig. 5.5 by constructing models. Demonstrate for yourself that III and IV are related as an object and its mirror image and that they are not superposable (i.e., that III and IV are chiral molecules and are enantiomers), (a) Take IV and exchange the positions of any two groups. What is the new relationship between the molecules (b) Now take either model and exchange the positions of any two groups. What is the relationship between the molecules now ... 

The compounds represented by structures 1 and 2 are enantiomers. The compounds represented by structures 3 and 4 are also enantiomers. But what is the isomeric relation between the compounds represented by 1 and 3 ... 

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