Part A: Chirality (How can you tell if an object is chiral?)

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1 hemactivity 7 hirality 1 hemactivity 7 Part A: hirality (ow can you tell if an object is chiral?) Model 1: riterion for Identical-ness Two molecules are identical if models of the two molecules can be superimposed without breaking any bonds. and identical if super-imposable 1. Make two identical models of the following molecule and confirm that they can be superimposed on one another. Use different colored balls for each of the four substituents. If your set has green, orange and purple, use the following color code. green ball = chlorine atom orange ball = bromine atom I purple ball = iodine atom Br 2. witch any two balls on one of your models (leave the other model unchanged). Is this new model identical to your original model? No. 3. Which of the following words describe the relationship between these two models? ircle more than one choice, if appropriate. B and E A. Identical B. tereoisomers (same atom connectivity, but not identical). onformers (can be made identical via single bond rotation) D. onstitutional isomers (same formula, different atom connectivity) E. Mirror images (look like reflections of one another in the mirror)

2 hemactivity 7 hirality 2 4. onsider the following set of molecules. Note: assume each molecule is in its simplest conformation, even if it is not the lowest energy conformation. In this case, assume each cyclohexane ring is in a planar conformation (rather than a chair). A B trans trans cis a) Label each of A- with the word cis or trans, as appropriate. b) Are any two of the set an identical pair? If so which ones. No. c) Which are more similar A&B or B& [circle one]? A&B d) What do A&B have in common that B& do not? A&B are both trans. Definitions: chiral = not identical to its mirror image. (a property of an object) (hiral is Latin for handed. ) enantiomers = a pair of objects that are mirror images but not identical. (a relationship between two objects) (Enantiomers is Latin for opposites. ) diastereomers= a pair of objects that are stereoisomers but not enantiomers (a relationship between two objects) racemic mixture = a 1:1 mixture of a pair of enantiomers. (a special sample of objects) 5. Give an example of an everyday object that is chiral and one that is not chiral. hiral: glove, screw, car, fan, your hand Not hiral: soccer ball, your face (disregarding moles, scars and other imperfections), pencil, any 2D object such as a letter or written word Note: students often say that writing is chiral because, if you hold a page up to a mirror it is impossible to read. But if the writing itself could be lifted off the page and flipped over like a pancake, it would be correct again. 6. Give an example of a pair of everyday objects that are enantiomers. a pair of gloves, or your hands, and L bicycle pedals, righty and lefty scissors, shoes 7. ircle each structure in TQ 4 that is chiral (it may help to draw the mirror image of each). A and B are chiral. a) What is the relationship between A&B (be as specific as possible). A& B are enantiomers (they are also stereoisomers but this is not as specific.) b) What is the relationship between B& (be as specific as possible). B& are diastereomers (they are also stereoisomers but this is not as specific.) c) What is the relationship between A& (be as specific as possible). A& are diastereomers (they are also stereoisomers but this is not as specific.)

3 hemactivity 7 hirality 3 8. T or F: All chiral objects have exactly one enantiomer. If false, give an example from TQ 4. True! Your left hand has exactly one mirror image. An object can not have two different mirror images!! 9. T or F: All diastereomers are chiral. If false, give an example from TQ 4. False. tructure in TQ 4 is a diastereomer of A and B, but it is identical to its mirror image, so it is not chiral. Model 2: Internal Plane of ymmetry (Mirror Plane) It is always true that an object (or molecule) with an internal plane of symmetry is not chiral (not chiral = achiral = identical to its mirror image). internal plane of symmetry bject A bject B 10. bject has a very obvious plane of symmetry (marked with a dotted line), and a not so obvious one. Where is this second plane of symmetry? (Assume the balls are uniform spheres.) The second plane of symmetry is in the plane of the paper.

4 hemactivity 7 hirality 4 bject 11.

5 hemactivity 7 hirality bjects D and E are NT chiral. Indicate the internal mirror plane in each. bject D bject E In bject D there is a mirror plane that cuts the white and gray spheres in half and crosses halfway between the two black spheres. In bject E there is a mirror plane in the plane of the paper. Part B: Absolute onfiguration (ow can you tell if a chiral center is right handed or left handed?) Model 3: A Trick for Determining If a Molecule Is hiral chiral D not chiral chiral not chiral chiral center = carbon (or other atom) with four different groups attached to it. By convention, chiral centers are marked with an. A molecule with one chiral center is always chiral. If a molecule has more than one chiral center it is usually chiral. 13. onstruct an explanation for why each carbon indicated with a 1 is considered to have two identical groups, while the carbons with an are chiral, with four different groups. (int: It has to do with weather the ring is symmetrical or not.) Not hiral hiral Br 3 Br considered two identical groups 3 3 considered two different groups For each of the chiral centers, a mental journey around the ring encounters the substituents in a different order depending on which way you go. For the two s labeled 1, a journey around the ring in either direction is indistinguishable. This means the two parts of the ring attached to this carbon are considered identical groups. 3 3

6 hemactivity 7 hirality onsider the following structures, some of which are chiral. Br N N N 2 3 N ME MPUND a) For some of the chiral centers above (for example, the first one), the is not shown. ow can you tell if an is going into the page or coming out of the page? The "implied" is going the opposite direction of the group shown. For example, in the top-left structure the is going into the page since the group is coming out of the page. b) Mark each chiral center on the structures above with an. c) ircle the three structures at the top of the page that are NT chiral, and for each circled structure, indicate the plane of symmetry (mirror plane). d) ne of the structures you circled is an example of the rare case where a structure contains chiral centers but is not chiral. These molecules are called meso compounds. Label the meso compound above. Information Like gloves each chiral center is either right or left handed. The convention for determining handedness requires we rank four groups attached to a chiral center. ules for anking the Four Groups from highest (3) to lowest (0): The higher the atomic number of an element, the higher its rank. For example: N (atomic # = 7) beats (atomic # = 6), and everything beats (atomic # = 1) In the first round, look at the 1 st shell. This is comprised of the four atoms directly attached to the chiral center of interest. (see 1 st shell, below) If there is a tie in the first round, a second round is necessary in which we look to break the tie by examining atoms attached to the tied groups that are in the 2nd shell. (see example). If there is a tie in the 2 nd round, atoms in the third shell are compared, etc. 3rd shell 2nd shell 1st shell

7 hemactivity 7 hirality For the example on the previous page, a) Based on examination of the first shell, an overall winner (3) and an overall loser (0) can be determined. Mark these atoms with 3 and 0, respectively. b) In the first round there is a tie for ranks 2 and 1. Identify the two atoms that are tied based only on examination of the 1 st shell. c) In this case, examination of the 2 nd shell does NT break the tie and help determine ranks 2 and 1. Explain. d) Based on the 3 rd shell. Mark the two carbons in the 1 st shell as 2 and 1 to complete your ranking of the four atoms attached to the chiral center. Model 4: Nomenclature for hiral enters Each chiral center is designated as either right handed () or left handed (). ight handed chiral centers are called from the Latin word for right: rectus. Left handed chiral centers are called from the Latin word for left: sinister. The or assignment is called the absolute configuration of that chiral center. To determine the absolute configuration of a chiral center such as the one below: I. ank the four groups attached to the chiral center 3, 2, 1 and 0, respectively. II. With your IGT hand, grab the bond to the lowest rank group (use a model at first). Be sure that the 0 group (usually ) is at the pinky end, and the three larger groups are sticking out at the thumb end like a bouquet of flowers. III. If the natural curl of your fingers matches the progression from rank = 1, to rank = 2, to rank = 3 (highest) than the center is right handed or. IV. If the curl of the fingers on your right hand does not match this progression (1-2-3) then the chiral center is left handed (). onfirm the assignment by grabbing this same bond with your left hand and checking that the progression from (highest rank) matches the curl of your fingers. Try your technique on the following examples. ankings are shown using the numbers 0 = lowest, 3 = highest N (#4) going into paper is implied (#4) coming out of paper is implied

8 hemactivity 7 hirality 8 2 F Determine the absolute configuration of each chiral center in the following structures. F Note that the structures in the previous question are the enantiomers of the first row of structures at the top of the page. What happens to the absolute configuration of a molecule if you switch exactly two groups (e.g. and ) attached to a chiral center. If you switch exactly two groups attached to a chiral center, it changes the absolute configuration from to or vice versa. 18. Determine the absolute configuration ( or ) of each chiral center in TQ N Br N 2 3 N N ME MPUND Exercises for Part A: 1. Draw an example of a pair of stereoisomers that are NT enantiomers. What is the name for the relationship between such structures? 2. hade or mark the balls on the object below to generate a chiral object.

9 hemactivity 7 hirality 9 3. onsider the structures below mirror image Br 3 F a) Draw the mirror image of each molecule (the first one is done for you.) b) ircle the structures that are not chiral and use a dotted line to show any internal symmetry planes. Label these structures meso compound. 4. ead the assigned sections in your text and do the assigned problems. Exercises for Part B 5. ometimes a molecule with one or more chiral centers is drawn without giving enough information to determine if the chiral center/s are or. Mark each chiral center below with an. By convention, if there is not enough information given to assign or, we are to assume that the structure represents an even mixture of all stereochemical possibilities. For each structure above, draw all stereoisomers implied by the drawing. ircle those structures that imply a racemic mixture (racemic mixture = 1:1 mixture of exactly two enantiomers).

10 hemactivity 7 hirality Determine the absolute configuration of each chiral center in Exercise 3. Explain why a meso compound must always have a 1:1 ratio of chiral centers to chiral centers. (int: consider what happens to an chiral center when it is reflected in an internal mirror plane.) 7. Label every chiral center on the following molecules with an and an or ; and state whether the molecule as a whole is chiral. N Penicillin G N Alleve (Naproxen) 8. ow many different stereoisomers are there of each molecule in previous question? 9. Most books teach a different and equally good way of figuring out and. Find this section in your textbook by looking in the index under chiral center or absolute configuration. 10. ead the assigned sections in your text and do the assigned problems.

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