General Chemistry II
General Chemistry II CHM 204
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Date Created: 11/01/15
Chapter 17 Stereoisomerism We have already encountered isomers in the organic chemistry chapters For example in Chapter 12 Unsaturated Hydrocarbons we say that alkenes can exhibit cistrans isomerism We will brie y review constitutional isomerism then stereoisomerism as it applies to alkenes Most of the chapter is devoted to a different type of stereoisomerism that has an immense impact on biochemical systems l7l Types of Isomerism Constitutional isomerism occurs when the atoms in a molecule can be rearranged such that different atoms are bound to one another ie the atom connectivity is the same For example the following are some of the constitutional isomers of C6H12 O W A B C D In contrast in stereoisomers the same atoms are bound to one another but their orientation in space differs We have already seen one type of stereoisomer the cistrans orientation around a double bond Chapter 12 Now compounds C and D have cis isomers as well N W E F What relationship are these molecules to the first 4 and to each other Molecule E is a constitutional isomer of molecules A B D and F but not C Molecule F is a constitutional isomer of molecules A B C and E but not D Molecules C and E are stereoisomers as are molecules D and F Thus C and E are constitutional isomers of A B D and F because the double bond is located in a different place C and E are stereoisomers because the identical groups attached to the double bond are arranged in a different way Stereoisomers can also occur in molecules possessing only single bonds These isomers are the subject ofthe next section 172 Molecular Chirality The remaining isomers we will discuss are a type of stereoisomer that incorporate a structural feature that is surprising at rst Typically only one molecule can be built imagine using a modeling kit where a molecule would be built like using Tinker Toys if the atom connectivity is predetermined Molecules for which stereoisomers are possible have more than one structure that can be drawn however Here the spatial arrangement of those atoms makes a difference In a molecule such as methane CH4 the placement of the hydrogen atoms around the carbon doesn t matter because they are identical In fact as long as any two or more of the groups bound to carbon are the same the way they are connected doesn t matter The situation changes if all four groups are different Your book chooses to look at an amino acid because it is relevant to this course but I m going to use an example that might be a little easier to visualize The book uses abbreviations to simplify its example and the graphics are quite nice Ifyou can see how this works using the book s graphics you may be able to skip the explanation given here Consider bromochlorofluoromethane CHBrClF F Cl C Br H The position to which each atom is attached matters here Let s look at why In the next set of gures we will rotate the molecule 1800 about the CF bond That is the CF will not change Remember the H C amp F atoms begin in the plane of the paper The Br leans out toward you and the Cl is pressed in behind the paper In the middle picture C amp F are still in the plane of the paper but H has moved behind the paper and Cl amp Br point out of the paper towards you In the nal picture H C amp F are back in the plane in the paper but now Cl points out towards you and Br back away from you the reverse of the beginning picture F F F gt C1 CltBr gt Cl CH H CIIBr Cl W Now lets take the rst View of the molecule and take its mirror image Ellquot T C Cw CI quotquotCl Br H H Br The nal thing we need to do is see if we can superimpose a molecule of the initial molecule in black on its mirror image in light blue Clearly the molecule as written to the left of the mirror in the preVious gure won t work but what about the molecule that was rotated It would look like this F F BCI As you can see the best we can do is get 2 of the 4 atoms bound to carbon to line up with each other The other two will always be in mirror positions These structures are called enantiomers and are related as nonsuperimposable mirror images All stereoisomers that are not enantiomers are diastereomers Figure 172 p 502 and the blue text in the margin of that paper may be helpful here So why does this matter as opposed to being simply a chemical oddity There are only two differences between these isomers they rotate the plane of polarized light by exactly equal amounts but in opposite directions and they react with other enantiomers differently In all other respects they are indistinguishable Because the mirror images of methanol CH3OH are identical they would react with it identically Enantiomers have the same color boil at the same temperature and have the same density Nevertheless it is important to remember that they are different molecules Interestingly they may or may not have the same odor Can you guess why or why not We will now examine each of these properties and their significance What is plane polarized light and what do we mean by rotating it Imagine a light wave like the one shown below A Now when you turn on a light bulb or when a star shines there are large numbers of these light waves coming out Either they are all in the same plane e g this paper or they aren t It turns out they aren t Let s assume for now the plane is simply the plane of this sheet of paper and the first light wave we look at is traveling down the dotted line that is in the center of the light wave shown above If we add a light wave it will be out of this plane perhaps perpendicular The next would probably add at a different angle maybe 30 Ifyou could stand behind the source and see all of the different light waves coming out what you d see is light coming out at every angle eg For six light waves we might see Plane polarized light is light that has passed through a special lter A blue light lter allows only blue light to pass through it All other wavelengths are absorbed A polarized light lter consists of a set of slits all lined up with one another so the light that passes through all lies in the same plane If the light is out of the plane it gets absorbed Try this analogy Imagine light waves 12 inch high and as thin as a human hair Now imagine a comb as the polarizing lter Ifthe light wave lines up with the slits in the comb it gets through If it comes in at an angle it hits the slats and can t get through A polarizing lter works like this You can prove this to yourself Go to a store that sells polarized sunglasses Take two pairs and put one on Look at the ceiling lights Now take the other pair and line them up so they re just like the rst Look through both pairs at the lights Slowly rotate the second pair At first you won t notice much difference but suddenly it will begin getting a lot darker very quickly By time you have rotated the glasses 900 no light whatsoever will get through Ifyou keep going to 1800 you ll get back to where you began because the slits will line back up At 900 all ofthe light getting through the rst lter is perpendicular to the second and gets completely absorbed Your book shows pictures on these topics on p 509510 Now back to chemistry We all know light interacts with matter The most obvious types of examples are that you can t see through a brick wall and that a car steering wheel gets warm on a sunny day but there are more subtle and relevant examples Take a smooth clear and preferably colorless dinner glass and ll it with water Hold it up to a light bulb at an angle and look at the light through the water Now slowly lower the glass You can see how the water bends the light waves as the surface of the water passes between your eyes and the light In a similar fashion some molecules twist light waves Thus they may strike the molecule in one plane say 0 and come out at a different angle say 27 This value is called the optical rotation With ordinary light this makes no difference because since all angles come in and all are rotated by the same amount we can t tell a difference If the light is polarized though we can measure the rotation because the light strikes the molecule at only one angle Now for a few de nitions Optically active molecules are those that rotate the plane of polarized light Molecules that exhibit optical activity are said to be chiral Molecules that don t are m The word chiral comes from the Greek word for hand Your hands are for all practical purposes mirror images of one another but can t be superimposed on each other Although other optically active structures are possible the only time you will encounter chirality is in molecules possessing tetrahedral carbons with 4 different groups attached Remember that anv carbon with four different groups attached is chiral This property of optically active molecules was discovered in 1815 by JeanBaptiste Biot but remained an unexplained curiosity until a very famous experiment by Louis Pasteur in 1848 He had just received his PhD and wanted to learn about crystallography To do this he decided to learn by repeating an experiment that someone before him did In the experiment he grew crystals of sodium ammonium tartrate a byproduct of the winemaking process Unlike the first experimenter he noticed under a magnifying glass that crystals could be divided into two groups each crystal in the first group the nonsuperimposable mirror image of a crystal in the second Their optical rotations were equal in size but opposite in sign In 1874 the Dutch chemist Jacobus Van t Hoff and the French chemist J osephAchille LeBel independently closed the loop by proposing tetrahedral carbon atoms to account for this and other chemical phenomena Now we will turn our attention to the chemical properties of chiral molecules As was mentioned earlier enantiomers of chiral molecules react with achiral molecules identically When we discuss this we are assuming the reaction occurs at the chiral carbon There are two possibilities The first is that the product can t be chiral For example imagine an alcohol with a chiral carbon say 2butanol Ifit is dehydrated to an alkene the alkene doesn t have a chiral center so the fact that the acid catalyst isn t chiral doesn t make any difference H OH N Hgt H20 So why do chiral molecules react with other chiral molecules differently Enzymes are perhaps the easiest chiral molecules to explain Typically an enzyme is a huge molecule with a relatively rigid structure It has numerous chiral centers within it Usually the substrate molecule must either enter a pocket or bind to the surface for a reaction to occur If the shape of the molecule doesn t line up with the shape of the pocket or surface it can t bind If it can t bind it won t react Your book has a nice illustration that shows this on p 504 To use an analogy your hands are to a first approximation chiral Let them be enantiomers Now a glove is like an enzyme It will interact correctly with only one of the two hands Now something in the preVious paragraph warrants further comment Molecules can have multiple chiral centers and there are some consequences of this fact The first is that each additional chiral center potentially doubles the number of stereoisomers We will see shortly why we don t always double the number To Visualize this imagine a carbon atom to which 4 different groups are bound Call the groups A B C and D The picture below shows the two different enantiomers with A B and C in the plane of the paper the carbon atom in the center behind the paper and D is directly behind the carbon atom To move from A to B to C in the first enantiomer one moves to the right clockwise Lets call this enantiomer R The same movement on the second enantiomer requires moving to the left counterclockwise This is enantiomer L As you might imagine one could draw such A A Thus for a molecule with two chiral centers one would have two pairs of enantiomers R amp pictures for any enantiomer L and R amp L They could be paired 4 different ways R R R L L R L L For three chiral centers there would be three pairs of enantiomers R amp L R39 amp L39 and R amp L This results in 8 different triplets R R R R R L R L R R L L L R R L R L L L R L L L Mathematically we can say that if there are n chiral centers in a molecule there will be 2n stereoisomers For the most part this is straightforward but in one scenario something different happens When the two chiral centers are identical pairs of the enantiomers turn out to be identical This is best seen pictorially A B This molecule which has two chiral centers is the mirror image of itself This type of molecule is said to have a meso structure Meso isomers are not optically active don t rotate the plane of polarized light They are composed of different enantiomers however Ifyou look carefully you may be able to see that the left meso form is L R top bottom while the right meso form is R L39 The two other forms of the previous molecule are optically active and are not superimposable These molecules are enantiomers Thus this type of molecule has 3 stereoisomers not the expected 4 A IIIU39J Recall diastereomers are stereoisomers that are not related as mirror images Thus L R39 and R L are diastereomers of R R and L L The chemical and physical properties of diastereomers are different from one another although they are usually quite similar The two exceptions occur when some of the diastereomers are optically active In that case the optical rotations may vary widely e g the meso form will always have an optical rotation of zero Chemically they will react differently with chiral centers 173 Optical Activity Most of this has already been covered but there are a few points worth addressing now Recall that enantiomers rotate the plane of polarized light by equal amounts but in opposite directions This means that a solution containing equal amounts of two enantiomers of the same molecule will not rotate the plane of polarized light Such a solution is called a racemic mixture It is optically inactive March 22 2002
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