Name: TA Name Lab Section: Day Time OPTICAL ISOMERISM 1. Construct a model that has a central carbon atom with 4 different colored spheres attached to it, representing four different atoms or groups. Draw a solid/dashed-wedge structure of this model here and answer the following questions. a. Does the model have a plane of symmetry? Yes No The central carbon is said to be a stereocenter, stereogenic center, or chiral carbon. Change one of the colored spheres so that two of the four spheres are the same and examine the structure at different angles. b. Does the molecule now posses symmetry? Yes No What kind (plane, axis, etc.)? c. In view of your answers to a and b, what condition is necessary for a carbon to be stereogenic center? d. Draw a structural formula for each of the following and indicate all chiral carbons with an asterisk: 3-chloro-1-pentene 4-heptanol 3-amino-1-butanol 2-bromo-3-chlorobutane 1
2. Reconstruct the original stereogenic carbon with the four different colored balls again. Set the model on the table so that the blue ball (C in the Fig 1) points upwards. a. Looking down on the model from above, record the colors of the balls, proceeding in a clockwise direction. Construct another model that is the mirror image of the first and place it on the table with the blue ball up. b. In what direction must you proceed to observe the same order of substituents as recorded for 2a? counter-clockwise clockwise c. Try to superimpose the 2 models. Are they the same? Yes No If not, how are they different? d. What is the stereochemical term that relates these two models? e. What two important properties must the two molecules (models) each have to be related by the term that is the answer to 2d? f. On your mirror image model, switch any two groups. Is this model still the mirror image of the original model? Yes No g. Try to superimpose the two models. Are they superimposable? h. What is the stereochemical term that relates these two models? Yes No Reconstruct your models so that they are the same as the models in 2a-e. Change the red ball on each model to a blue one (Now models contain two blue balls) i. Are the models still mirror images? Yes No j. Do the models have a plane of symmetry? Yes No If so, describe its location. k. Are the models superimposable? Yes No l. Do the models represent identical or different molecules? 2
Explain. m. Each of the following molecules has one stereocenter, and is therefore chiral. Draw stereorepresentations (dash-wedge formulas) for both enantiomers of each. 3-chloro-1-pentene 3-amino-1-butanol 3. Diastereomers and Meso Forms. When a molecule has two or more stereogenic centers, stereoisomers that are not mirror images can exist; these are called diastereoisomers. Within this general class, there are special types of stereoisomers that are always optically inactive and are called meso forms. Construct a model with four different colored balls about a carbon center. Construct another identical to the first and verify this by the superimposition test. Now remove the same colored balls, blue (C from Fig 1) for example, from each model and join the two carbons at the vacant valence sites as shown in Fig 2. a. How many stereogenic carbons does this model have? Note that each carbon has the same four groups attached to it. b. Conformationally adjust (by rotation about the central C-C bond) the model in a search for symmetry. Does the molecule have a plane of symmetry? Yes No Construct the mirror image of this molecule. c. Are the mirror images identical? Yes No d. What term is used to describe the relationship between the two molecules in 3c? e. Examine each of these models to determine whether they are chiral or achiral. Interchange two balls (A and B for example as shown in Fig 3) on one carbon of one model. 3
f. Are the two models identical now? Yes No g. Are they mirror images? Yes No h. Are they stereoisomers? Yes No i. What stereochemical term describes the two molecules that are related in this way? j. Examine various conformations of the model where the two balls were interchanged. Can you find a conformation with a plane of symmetry? Yes No k. Explain the symmetry that you see if you answered yes. l. Predict whether this molecule's mirror image will be identical with or nonsuperimposable on the original. Explain the basis of your prediction. Test the validity of your prediction by constructing the mirror image and conducting the superimposition test. Record your observations. m. Is this molecule (Fig 3) chiral? Yes No n. Is this compound optically active? Yes No o. What is the stereochemical term used to describe this type of stereo isomer? 2,3-Diaminobutanedioic acid, can come in three different forms, corresponding to the models assembled in 2a-o: a pair of optically active enantiomers and a third optically inactive form called the meso form. The latter is a diastereomer of the optically active enantiomers. p. Draw solid/dashed-wedge structures for the three forms of the acid drawn above. Label which structures are enantiomers or diastereomers to each other, and which one is the meso form. 4
When a molecule has two different types of stereogenic centers, it can exist in 2 n stereoisomeric forms, where n is the number of stereogenic carbons. Remove any one ball from any onecarbon center and replace it with a color not already being used. Your model should have two stereogenic centers, with two of the three colored balls on one center being the same as on the other. Construct the four stereoisomers now possible. q. Record the diagrams for these 4 possible stereoisomers; label the pairs of enantiomers and diastereomers. r. Explain why 4 stereoisomers cannot be achieved with 2,3-diaminobutanedioic acid. 4. Absolute Configuration and R/S Sequence Rules. In 3q you constructed models with two stereocenters that represented 4 stereoisomers. 3-Hydroxy-2-methylbutanal exemplifies such a compound. Make models for the isomers of this compound. Note that the C of the aldehyde is C1. 5
a. Draw solid/dashed-wedge structures for the 4 possible stereoisomers and label the pairs of enantiomers and diastereomers. b. Specify the sequence priority for the substituents on carbon-2 and carbon-3. c. Label each structure above with the proper R/S notation in the drawings above, for example (2R,3S). d. Draw Newman projections for all 4 possible stereoisomers. e. Specify the sequence of priority for the substituents on C2 and C3 for 2,3- diaminobutanedioic acid (see 3p). Write your answer for the priority sequence for the molecule drawn in 3p here. 5. Conformational Enantiomers and Cyclic Compounds a. Construct a model of methanol (CH 3 OH). Is methanol chiral? Why or why not? b. Orient your model so that the plane of symmetry (indicated by the dotted rectangle) contains the H-C-O-H bonds that are in the plane of the paper, as shown below: 6
Now rotate the C-O bond, so that the plane no longer contains the O-H bond. All of the bonds in methanol freely rotate at room temperature, so you are just showing a different conformation of methanol. Does this conformation of methanol have a plane of symmetry? Yes No c. Even though one conformation of methanol has a plane of symmetry, and one doesnʼt, methanol is still considered to be achiral. It is only necessary to find one conformation with a plane of symmetry for a molecule to be achiral. d. Now consider cyclohexanol, shown below. Looking at the flat, hexagonal representation of cyclohexanol, does cyclohexanol contain a plane of symmetry? Yes No e. Of course, cyclohexanol never exists in a flat hexagonal form. It is just drawn this way for convenience. It exists in one of two chair conformations that are rapidly interconverting at room temperature, the conformation with the hydroxyl in the equatorial position being favored. f. Make a model of cyclohexanol with the OH in the equatorial position. Now do a chair flip, so that the OH is in the axial position. Can you find a plane of symmetry for each configuration? Is cyclohexanol chiral? f. Now consider a disubstituted cyclohexane derivative, cis-1,2-dibromocyclohexane: Looking at the flat, hexagonal representation of cis-1,2-dibromocyclohexane, does this molecule contain a plane of symmetry? Yes No 7
g. Once again, we know that cyclohexane rings never exist in a flat hexagonal form. They exist as rapidly interconverting chair conformations. Only at absolute zero can you isolate a single chair conformation of any cyclohexane derivative. If you convert cis-1,2-dibromocyclohexane to a chair conformation, however, both chair conformations are chiral: Both chair conformations are non-superimposable mirror images of eachother and there is no plane of symmetry, BUT, they are interconvertible by a ring flip! Make a model of each of the chair conformations of cis-1,2-dibromocyclohexane, and show that they are nonsuperimpoasble mirror images of each other. Is cis-1,2-dibromocyclohexane isomer optically active? The planar structure has a plane of symmetry, but the chair conformation doesnʼt! Yes No Although each conformer is chiral, the rapid interconversion between the two at room temperature means they cannot be separated. The compound is not chiral. Identifying chirality in cyclohexane derivatives can seem daunting. Fortunately we can greatly simplify by making using the following rule: If you can find a plane of symmetry in the flat hexagonal drawing of a substituted cyclohexane ring, the molecule is achiral. If you cannot find a plane of symmetry, it is chiral. h. Using this simplification, determine whether the following substituted cyclo-hexane rings are chiral or achiral. Draw plane of symmetry for any achiral molecules 8