Chapter 5
Stereochemistry
05-01

Labeled
Title
 Mirror Test for Chirality
Caption
 Figure 5-1 Use of a mirror to test for chirality. An object is chiral if its mirror image is different from the original object.
Notes
 The mirror images of chiral objects cannot be superimposed so that all atoms coincide with the equivalent atom of the other molecule.
Keywords
 chiral, mirror images, superimposable
05-04

Labeled
Title
 Enantiomers of 2-Bromobutane
Caption
Notes
 2-Bromobutane has one asymmetric carbon, also called chiral carbon. On close inspection the mirror images of 2-bromobutane are not superimposable.
Keywords
 asymmetric carbon, chiral carbon, superimposable
05-06

Labeled
Title
 Enantiomers
Caption
 Figure 5-6 Enantiomers of a chiral carbon atom. These two mirror images are nonsuperimposable.
Notes
 Nonsuperimposable mirror images of a chiral molecule are known as enantiomers.
Keywords
 enantiomers
05-07

Labeled
Title
 Non-chiral Molecules
Caption
 Figure 5-7 A carbon atom bonded to just three different types of groups is not chiral.
Notes
 A chiral carbon has four different substituents. Carbons with only three different groups on them are not chiral because their mirror images can be superimposed.
Keywords
 chiral carbon
05-08

Labeled
Title
Caption
 Figure 5-8 cis-1,2-Dichlorocyclopentane has a mirror plane of symmetry. Any compound with an internal mirror plane of symmetry cannot be chiral.
Notes
 A plane of symmetry divides the molecule into two internal mirror images, all atoms reflect each other across the internal mirror plane. Planes of symmetry can go through atoms dividing them in two. Cis cycles such as 1,2-dichlorocyclopentane have an internal mirror plane of symmetry so they are not chiral.
Keywords
 plane of symmetry, internal mirror image
05-08-01UN

Labeled
Title
 Plane of Symmetry
Caption
 Notice below that the chiral trans isomer of 1,2-dichlorocyclopentane does not have a mirror plane of symmetry. The chlorine atoms do not reflect into each other across our hypothetical mirror plane. One of them is directed up, the other down.
Notes
 Disubstituted trans cyclic compounds such as 1,2 dichloropentane are chiral because they do not have an internal mirror plane of symmetry.
Keywords
 plane of symmetry
05-09

Labeled
Title
Caption
 Figure 5-9 A carbon atom with two identical substituents (only three different substituents) usually has an internal mirror plane of symmetry. The structure is not chiral.
Notes
 Only compounds with chiral carbons have the possibility of being chiral.
Keywords
 chiral, internal mirror plane, plane of symmetry
05-09-05UN

Labeled
Title
Caption
 These mirror images are different, and this difference is reflected in their biochemistry. Only the enantiomer on the left can be metabolized by the usual enzyme; the one on the right is not recognized as a useful amino acid. Both are named alanine, however, or 2-aminopropanoic acid in the IUPAC system. We need a simple way to distinguish between enantiomers and to give each of them a unique name.
Notes
 Even though enantiomers have most of the same physical properties, nature can easily differentiate between enantiomers. Nature is selective.
Keywords
 enantiomers, selectivity
05-09-12UN

Labeled
Title
 Assigning (R) and (S) Absolute Configuration
Caption
 2. Using a three-dimensional drawing or a model, put the fourth priority group in back and view the molecule along the bond from the chiral carbon to the fourth priority group. Draw an arrow from the first priority group, through the second, to the third. If the arrow points clockwise, the chiral carbon atom is called (R) (Latin, rectus, ÒuprightÕÕ). If the arrow points counterclockwise, the chiral carbon atom is called (S) (Latin, sinister, ÒleftÕÕ).
Notes
 Since nature can differentiate between enantiomers we need to be able to distinguish between them as well. We assign each enantiomer an absolute configuration based on the three dimensional structure of the asymmetric carbon.
Keywords
 Cahn-Ingold-Prelog convention, enantiomers
05-10

Labeled
Title
 Plane Polarized Light
Caption
 Figure 5-10 The waves of plane-polarized light vibrate primarily in a single plane.
Notes
 Unpolarized light is composed of waves that vibrate in every direction. When a polarizing filter is used, only waves that vibrate in the same direction as the axis of the filter can pass through it. Plane-polarized light vibrates only in one direction.
Keywords
 unpolarized light, polarized light, plane-polarized light, filter, wave
05-13

Labeled
Title
 Schemicatic Diagram of a Polarimeter
Caption
 Figure 5-13 Schematic diagram of a polarimeter. The light originates at a source (usually a sodium lamp) and passes through a polarizing filter and the sample cell. The analyzing filter is another polarizing filter equipped with a protractor. It is turned until a maximum amount of light is observed, and the rotation is read from the protractor.
Notes
 The light of a sodium lamp is passed through a filter that selects the yellow emission light (the D line). This polarized light is passed through the sample cell containing a solution of the compound being analyzed. Only chiral compounds have optical activity and can rotate the plane of polarized light. The light leaving the sample cell is then passed through a second polarizing filter that can be rotated until light is observed to determine the magnitude and direction of the optical rotation.
Keywords
 sodium lamp, polarizing filter
05-15

Labeled
Title
 Chiral Recognition
Caption
 Figure 5-15 Chiral recognition of epinephrine by an enzyme. Only the levorotatory enantiomer fits into the active site of the enzyme.
Notes
 Nature can easily differentiate between enantiomers. The active sites of enzymes are generally design to accomodate only one of the enantiomers to form the enzyme-substrate complex. The other enantiomer will not fit the enzyme's active site so it will show no biochemical activity.
Keywords
 active site, enzyme
05-15-02UN

Labeled
Title
 Racemic Mixtures
Caption
 Sometimes a racemic mixture is called a racemate, a (6) pair, or a (d,l) pair. A racemic mixture is symbolized by placing (6) or (d,l) in front of the name of the compound. For example, racemic 2-butanol would be symbolized by Ò(6)-2-butanolÕÕ or Ò(d,l)-2-butanol.ÕÕ
Notes
 When a solution contains equal amounts of both enantiomers it will show no optical rotation. This 1:1 mixture is called a racemic mixture or racemate.
Keywords
 racemic mixture, racemate
05-16

Labeled
Title
Caption
 Figure 5-16 Hydrogenation of 2-butanone forms racemic 2-butanol. Hydrogen adds to either face of the double bond. Addition of H2 to one face gives the (R) product, while addition to the other face gives the (S) product.
Notes
 Many reactions form racemic mixtures of products, especially when the starting material is achiral and the product is chiral.
Keywords
 chiral, racemic mixtures
05-17

Labeled
Title
 Conformational Enantiomerism
Caption
 Figure 5-17 This biphenyl cannot pass through its symmetric conformation because there is too much crowding of the iodine and bromine atoms. The molecule is ÒlockedÕÕ into one of the two chiral, enantiomeric, staggered conformations.
Notes
 Some molecules can be chiral even though they do not have any asymmetric carbons. These compounds are typically so bulky that they cannot interconvert between its two mirror image conformations. The molecules are "locked" into one of the two possible conformations which are non-superimposable mirror images and thus considered enantiomers.
Keywords
 conformational enantiomerism
05-19-02UN

Labeled
Title
 Chirality in Allenes
Caption
 Carbon atom 3 is the sp hybrid allene carbon atom. Carbons 2 and 4 are both sp2 and planar, but their planes are perpendicular to each other. None of the carbon atoms are attached to four different atoms, so there is no chiral carbon atom. Nevertheless, 2,3-pentadiene is chiral, as you should see from your models and from the following drawings of the enantiomers.
Notes
 The inner carbon of an allene is sp hybridized and linear while the outer carbons are sp2 hybridized and trigonal. Allenes are chiral when each of the carbons at the end have two different substituents.
Keywords
 allenes
05-20

Labeled
Title
Caption
 Figure 5-20 The Fischer projection uses a cross to represent a chiral carbon atom. The horizontal lines project toward the viewer, and the vertical lines project away from the viewer.
Notes
 Fisher projections allow for the determination of the absolute configuration without having to rotate the molecule.
Keywords
 Fisher projections, absolute configuration
05-20-17UN

Labeled
Title
 Cis-trans Isomerism on Rings
Caption
 Cis-trans isomerism is also possible when there is a ring present. Cis- and trans-1,2-dimethylcyclopentane are geometric isomers, and they are also diastereomers. The trans diastereomer has an enantiomer, but the cis diastereomer has an internal mirror plane of symmetry, and it is achiral.
Notes
 Typically trans-1,2 disubstituted rings are chiral and thus have a mirror image. Cis disubstituted rings contain an internal plane of symmetry that makes the mirror images superimposable rendering the cis stereoisomer achiral.
Keywords
 trans, cis, enantiomers
05-20-19UN

Labeled
Title
 Diastereomeric Forms
Caption
 Either member of one pair of enantiomers is a diastereomer of either member of the other pair. We have now seen all the types of isomers we need to study, and we can diagram their relationships and summarize their definitions.
Notes
 Diastereomers are found in molecules with two or more chiral carbons. There are 2 pairs of enantiomers (mirror images). The relationship between the stereoisomers that are not mirror images is diastereomeric, i.e., they are diastereomers of each other.
Keywords
 diastereomer, diastereomeric forms
05-20-20Summ

Labeled
Title
 Types of Stereisomers
Caption
 SUMMARY: The Types of Isomers
Notes
 Isomers are compounds that have the same molecular formula. Conformational isomers differ in the order the atoms are connected to each other. Stereoisomers are connected in the same order but differ in the orientation of the atoms in space. Among the stereoisomers there are enantiomers, which are mirror images, and diastereomers which are not mirror images of each other. Diastereomers are found in compounds with two or more chiral carbons in the molecule.
Keywords
 isomers, constitutional isomers, stereoisomers, enantiomers, diastereomers
05-20-28UN

Labeled
Title
 Meso Compounds
Caption
 2,3-Dibromobutane is an example of a compound having fewer than 2n stereoisomers. It has two chiral carbons, C2 and C3, and the 2n rule predicts a maximum of four stereoisomers. The four permutations of (R) and (S) configurations at C2 and C3 are shown below. Make molecular models of these structures to compare them.
Notes
 When there are two or more chiral centers in the molecule there is a possibility of meso compounds. Meso compounds have an internal mirror plane of symmetry making the mirror images superimposable and rendering the compound achiral. Compounds that have the meso isomer have fewer than 2n stereisomers.
Keywords
 meso compounds
05-21

Labeled
Title
 Enantiomeric Resolution
Caption
 Figure 5-21 Formation of (R)- and (S)-2-butyl tartrate. The reaction of a pure enantiomer of one compound with a racemic mixture of another compound produces a mixture of diastereomers. Separation of the diastereomers, followed by hydrolysis, gives the resolved enantiomers.
Notes
 The process of separating the enantiomers is called resolution. Separating the enantiomers of a racemic mixture requires a pure optically active compound, such as tartrate, to be able to separate them. The reaction between a racemic mixture and pure (R),(R)-(+)-tartaric acid produces two diastereomers which can be separated easily because of their difference in physical properties. Once the diastereomers have been separated, the tartaric acid can be hydrolyzed giving the pure enantiomers.
Keywords
 resolution, enantiomeric resolution
05-22

Labeled
Title
 Chiral Chromatoghaphy of Enantiomers
Caption
 Figure 5-22 Chromatographic resolution of enantiomers. The enantiomers of the racemic compound form diastereomeric complexes with the chiral material on the column packing. One of the enantiomers binds more tightly than the other, and its movement through the column is slower.
Notes
 The different interaction between the diastereomers and the column packing is the basis for chiral chromatography. One diastereomer will interact more with the column packing while the other diastereomer, which interacts less, can pass through the column and elute (come out) from the column first.
Keywords
 chiral chromatography, chiral resolution, chromatography
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