|Chiral Stereoisomers||The Difference Between Enantiomers on the Macroscopic Scale|
|The Difference Between Enantiomers on the Molecular Scale|
The cis/trans or E/Z isomers formed by alkenes aren"t the onlyexample of stereoisomers. To understand the second example of stereoisomers, it might beuseful to start by considering a pair of hands. For all practical purposes, they containthe same "substituents" fourfingers and one thumb on each hand. If you clap them together, you will find even moresimilarities between the two hands. The thumbs are attached at about the same point on thehand; significantly below the point where the fingers start. The second fingers on bothhands are usually the longest, then the third fingers, then the first fingers, and finallythe "little" fingers.
In spite of their many similarities, there is a fundamental difference between a pairof hands that can be observed by trying to place your right hand into a left-hand glove.Your hands have two important properties: (1) each hand is the mirror image ofthe other, and (2) these mirror images are not superimposable. The mirror imageof the left hand looks like the right hand, and vice versa, as shown in the figure below.
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Objects that possess a similar handedness are said to be chiral(literally, "handed"). Those that do not are said to be achiral.Gloves are chiral. (It is difficult, if not impossible, to place a right-hand glove onyour left hand or a left-hand glove on your right hand.) Mittens, however, are oftenachiral. (Either mitten can fit on either hand.) Feet and shoes are both chiral, but socksare not.
In 1874 Jacobus van"t Hoff and Joseph Le Bel recognized that a compound that contains asingle tetrahedral carbon atom with four different substituents could exist in two formsthat were mirror images of each other. Consider the CHFClBr molecule, for example, whichcontains four different substituents on a tetrahedral carbon atom. The figure below showsone possible arrangement of these substituents and the mirror image of this structure. Byconvention, solid lines are used to represent bonds that lie in the plane of the paper.Wedges are used for bonds that come out of the plane of the paper toward the viewer;dashed lines describe bonds that go behind the paper.
If we rotate the molecule on the right by 180 around the CH bond we get the structure shown on the rightin the figure below.
These structures are different because they cannot be superimposed on eachother, as shown in the figure below.
CHFClBr is therefore a chiral molecule that exists in the form of a pair ofstereoisomers that are mirror images of each other. As a rule, any tetrahedral atom thatcarries four different substituents is a stereocenter, or a stereogenic atom. However, theonly criterion for chirality is the nonsuperimposable nature of the object. A testfor achirality is the presence of a mirror plane within the molecule. If a molecule has a plane within it that will cut it into two symmetrical halves,then it is achiral. Therefore, lack of such a plane indicates amolecule is chiral. Compounds that contain a single stereo-centerare always chiral. Some compounds that contain two or more stereocenters are achiralbecause of the symmetry of the relationship between the stereocenters.
The prefix "en-" often means "to make, or cause to be," as in"endanger." It is also used to strengthen a term, to make it even more forceful,as in "enliven." Thus, it isn"t surprising that a pair of stereoisomers that aremirror images of each are called enantiomers. They are literallycompounds that contain parts that are forced to be across from each other. Stereoisomersthat aren"t mirror images of each other are called diastereomers. Theprefix "dia-" is often used to indicate "opposite directions," or"across," as in diagonal.
The cis/trans isomers of 2-butene, for example, are stereoisomers, but they are notmirror images of each other. As a result, they are diastereomers.
|Practice Problem 10: |
Which of the following compounds would form enantiomers because the molecule is chiral?
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The Difference Between Enantiomers onthe Macroscopic Scale
If you could analyze the light that travels toward you from a lamp, you would find theelectric and magnetic components of this radiation oscillating in all of the planesparallel to the path of the light. However, if you analyzed light that has passed througha polarizer, such as a Nicol prism or the lens of polarized sunglasses, you would findthat these oscillations were now confined to a single plane.
In 1813 Jean Baptiste Biot noticed that plane-polarized light was rotated either to theright or the left when it passed through single crystals of quartz or aqueous solutions oftartaric acid or sugar. Because they interact with light, substances that can rotateplane-polarized light are said to be optically active. Those that rotatethe plane clockwise (to the right) are said to be dextrorotatory (fromthe Latin dexter, "right"). Those that rotate the planecounterclockwise (to the left) are called levorotatory (from the Latin laevus,"left"). In 1848 Louis Pasteur noted that sodium ammonium tartrate forms twodifferent kinds of crystals that are mirror images of each other, much as the right handis a mirror image of the left hand. By separating one type of crystal from the other witha pair of tweezers he was able to prepare two samples of this compound. One wasdextrorotatory when dissolved in aqueous solution, the other was levorotatory. Since theoptical activity remained after the compound had been dissolved in water, it could not bethe result of macroscopic properties of the crystals. Pasteur therefore concluded thatthere must be some asymmetry in the structure of this compound that allowed it to exist intwo forms.
Once techniques were developed to determine the three-dimensional structure of amolecule, the source of the optical activity of a substance was recognized: Compoundsthat are optically active contain molecules that are chiral. Chirality is aproperty of a molecule that results from its structure. Optical activity is a macroscopicproperty of a collection of these molecules that arises from the way they interact withlight. Compounds, such as CHFClBr, that contain a single stereocenter are the simplest tounderstand. One enantiomer of these chiral compounds is dextrorotatory; the other islevorotatory. To decide whether a compound should be optically active, we look forevidence that the molecules are chiral.
The instrument with which optically active compounds are studied is a polarimeter,shown in the figure below.
Imagine a horizontal line that passes through the zero of a coordinate system. Byconvention, negative numbers are placed on the left and positive numbers on the right ofzero. Thus, it isn"t surprising that levorotatory compounds are indicated with a negativesign (-).and dextrorotatory compounds are with a positive sign (+).
The magnitude of the angle through which an enantiomer rotates plane-polarized lightdepends on four quantities: (1) the wavelength of the light, (2) the length of the cellthrough which the light passes, (3) the concentration of the optically active compound inthe solution through which the light passes, and (4) the specific rotationof the compound, which reflects the relative ability of the compound to rotateplane-polarized light. The specific rotation of the dextrorotatory isomer of glucose iswritten as follows:
When the spectrum of sunlight was first analyzed by Joseph von Fraunhofer in 1814, heobserved a limited number of dark bands in this spectrum, which he labeled A-H. We nowknow that the D band in this spectrum is the result of the absorption by sodium atoms oflight that has a wavelength of 589.6 nm. The "D" in the symbol for specificrotation indicates that it is light of this wavelength that was studied. The"20" indicates that the experiment was done at 20C. The "+" signindicates that the compound is dextrorotatory; it rotates light clockwise. Finally, themagnitude of this measurement indicates that when a solution of this compound with aconcentration of 1.00 g/mL was studied in a 10-cm cell, it rotated the light by 3.12.
The magnitude of the rotations observed for a pair of enantiomers is alwaysthe same.
The only difference between these compounds is the direction in which they rotateplane-polarized light. The specific rotation of the levorotatory isomer of this compoundwould therefore be -3.12.
The Difference Between Enantiomers on theMolecular Scale
A strategy, which is based on the Latin terms for left (sinister) and right (rectus),has been developed for distinguishing between a pair of enantiomers. Arrange the four substituents in order of decreasing atomic number of the atoms attached to the stereocenter. (The substituent with the highest atomic number gets the highest priority.) The substituents in 2-bromobutane, for example, would be listed in the order: Br > CH3 = CH2CH3 > H.
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In this example, the path curves to the left, so this enantiomer is the (S)-2-bromobutanestereoisomer.
It is important to recognize that the (R)/(S) system is based on thestructure of an individual molecule and the (+)/(-) system is based on the macroscopicbehavior of a large collection of molecules. The most complete description of anenantiomer combines aspects of both systems. The enantiomer analyzed in this section isbest described as (S)-(-)-2-bromobutane. It is the (S) enantiomerbecause of its structure and the (-) enantiomer because samples of the enantiomer withthis structure are levorotatory; they rotate plane-polarized light clockwise. Notethat the sign of the optical rotation is not correlated to the absolute configuration.
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