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ch.6 lec outline - Rice CHMQBl Lecture Outline Chapter 6...

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Unformatted text preview: Rice, CHMQBl, Lecture Outline. Chapter 6. Chapter 6: Isomers and Stereochernistry Please bring your model kits to class for use with this chapter © Everything in this chapter applies to biochemistry and nature @ (1) Isomers. Constitutional vs. stereoisomers (1.1) Constitutional isomer: Differ in the way the atoms are connected. (1.2) Stereoisomers: The atoms are connected in the same way, they are just arranged differently in space. (2) Chirality. (2.1) What is a chiral molecule? A chiral object/ molecule has a non~superimposable mirror image. (Therefore a left and right-handed form exists). Examples: (2.2) What is an achiral molecule? An achirai object/ molecule has a superimposable mirror image. {Therefore only one version of it exists). Examples: (3) What is an asymmetric center(s)? An asymmetric center causes chiraiity in a molecule. It is an atom bonded to four different groups. {Note the difference between groups and atoms herei). Asymetric (or chiral) centers are usually marked with an asterix). Examples: Page 1/10 Rice, CHM-231, Lecture Outline. Chapter 6. (3.1) What is an enantiomer? Lets consider 2wbrornobutane. It has one asymmetric center and so two stereoisomers exist. Enantiomer is the name given to molecules that are nonsuperimposable mirror images of each other. The two stereoisomers of 2—bromobutane are enantiomers. (3.2) Invciass activity: Using your model kit build both enantiomers of 2~bromobatane Use four different colors to represent the different atoms (use white for H, green for Br, blue for ~CH3 and red for WCHECHZ). After you have buiit the compounds try and superimpose them so that the same colors match up! You shouldn’t be able to do it because these are chiral! Now replace the blue atom with another White atom. Now try to superimpose them. You should be able to do it because they are achirall (3.3) Perspective formulas and drawing enantiomers. (1) They involve two ‘normal’ bonds (in the piane of the paper); a solid wedge (protruding forward out of the paper) and a hashed wedge (protruding backward into the paper). (2) The two ‘norrnai’ bonds must always be drawn next to each other. (3) The atoms are added in any order around the central atom (chiral cente )). (4) The mirror image of the first structure can be drawn to get the second enantiomer. (Look at your models again to convince yourself that the bonds are doing What the above statement says). Rage 2/10 Rice, CHM—231, Lecture Outline. Chapter 6. (4) R/S System for naming enantiomers. With only one asymmetric center the enantiomers are named as having either an Rmconfiguration or an S~ configuration, (4.1) Rules to determine R/S. {1) Rank the groups (or atoms) that are bonded to the asymmetric center in order of priority. (3 is the highest priority and 4 is the lowest). (Tins is the same system for figuring out E and Z with alkenes: the Cahnslngold- Prelog rules or priority system). (2) If the group (or atom) with the lowest priority (4) is bonded with the bashed wedge then draw an arrow from priority 1 to priority 2. Clockwise arrow means R—configuran'on; anticlockwise arrow means S-configuration. (3) If the group (atom) with the lowest priority (4) is not bonded by the hashed wedge then interchange two groups to put the lowest priority group on the hashed wedge. Then follow rule 2 to assign R and 5. Note: Remember, in this case you have figured out the configuration of the switched molecule; the original molecule will be the opposite configuration. (4) When drawing your arrows from group 1 to group 2; you can draw past the group with the lowest priority (4), but never past the group with the next lowest priority (3). Page 3/10 Rice, CHM—231, Lecture Outline. Chapter 6. (4.2) Practice problems Q. Answer problems 11 and 12 from the book. Problem 11: Indicate whether each of the following structures has the R or 5 configuration. a) H b) 0... Br / "’CHa H‘ OH COOH Problem 12: Name the following: a) B? b) l /’0”"UCHECH3 // CH3 0 H (5) Properties of Enantiomers Enamtiomcrs share many physical properties that are similar: boiling point, melting point and solubility. The optical activity of enantiomers is different. (5.1) Optical activity Enantiorners rotate the plane of polarized light and are thus optically active. (Achirai molecules don’t). (If you don’t understand ‘polarized light’ then look in the book on pages 151—152). (5.2) How do we distinguish between compounds that rotate polarized light? (1) Dextrorotatory: The plane of polarized light is rotated ciockwise {add the prefix (4-) to the name). (2) Levorotatory: The piane of polarized light is rotated anticlockwise (add the prefix (~) to the name). Do not confuse R and S with (+) and (—). R and S is the arrangement of groups around the asymmetric center. (fl and (m) can only be determined by measuring the direction that plane polarized light is rotated using a polarirneter. In 232 you wili learn an additional system called D and L. Q? Page 4/10 Rice, CHMQBI, Lecture Outline. Chapter 6. (5.3) Specific rotation See pages 153—155 for more information. Specific rotation is measured with a polarimeter and measures the direction and amount that plane polarized light is rotated. Two enantiomers will rotate the light a certain amount in equal, but opposite directions. A formula is used to calculate this rotation. (5.4) Racemic mixtures This is an equal mixture of both enantiorners. These compounds are optically inactive. Why? (6) Two or more Asymmetric Centers A compound can have a maximum of 2“ steteoisomers, where n equals the number of asymmetric centers. A stereoisomer does not have to be an enantiomer. A stereoisomer that is not an enantiomer is called a diastereomer. (6.1) What is the difference between an enantiomer and a diastereomer? Use the amino aci threonine as an example: How many asymmetric centers does it have? How many stereoisomers are there? goo; + goo" goo; + goo“ 'H\%,Na3 H3Ncéj,a Hc%,NH3 Hawaiina HWS‘OH HOVE‘H HO’E‘H HVEVOH CH3 CH3 CH3 CH3 1 2 ' 3 4 Copydght® 2936 Pearson Education. Inc (i) Which are enantiomers? (2) Which isomers are not identical and are not mirror images? (3) Which are pairs of diastereorners? (4) What is different and what is the same in the pairs of diastereomers? Page 5/10 Rice, CHM—231, Lecture Outline. Chapter 6. (6.2) Physical properties T he physical properties of enantiomers are the same and so are chemical properties. (React at same rate with achiral reagents, however they react differently with chiral reagents). The physical properties and chemical properties of diastereomers are different. (React at different rates with achiral reagents). (6.3) Mesa compounds {1) A meso compound is achirai. {2) A meso compound has two or more asymmetric centers and a plane of symmetry. (3) If a compound has two asymmetric centers with the same four groups bonded to each of the asymmetric centers, one of its stereoisomers wiil be a meso compound. CHgCEI-KIIHCfig Example: fir Br 2.3~dihrome§autane 9H3 c143 9H3 HATEABI‘ HKCFABr BfkéAH urge; BrrE‘wH HVE‘VBI CH3 CH3 CH3 1 2 3 Copyrighl @2008 Pearson Pruhnce Halli inn (7) Chirafity and reactions; chirah'ty and biochemistry Chirality is found throughout nature and in the body. That is why it is such an important concept to get your head around © Chirality is also important in chemicai reactions! (7.1) Reactions Think about alkenes. If an asymmetric center is formed in the product then we need to figure out What enantiomer has formed. B I When a reactant that does not have an asymmetric centers forms a product that has one, then it always produces the racemic mixture. CH3CHQ {Si—Z—htomobutane xw.t___a\g \\\H ‘1‘ CH3 Br (muzbromobutane CWDZDOGP-mflmm Page 6/10 Rice, CHM-231, Lecture Outline. Chapter 6. (7.2) Receptors (proteins that bind a molecule) R enantiomer S enantiomer 1 ._.‘. é i gt err" ' lee-tit bindmg sate of the receptor b all ”site-of the Cmyrlm n was Fem mum-t Im Has applicabiiity in drug design w need the right enantiomer to bind to the receptor. (7.3) Chiral drugs Homework: Read up on Thalidomide and chiral drugs on pg. 161. (7.4) Biochemistry Enzymes are the body’s reaction catalystsl They lower the energy of activation of reactions in the body. This is a good thing since physiological conditions are really miidl A lot of enzymes are stereospecific and so work only with one particuiar enantiomer. They are also so specific that when a product is formed with an asymmetric center oniy one enantiomer is formed! Why? You xvii} encounter the Citric Acid Cycle in Biochemistry and other subjects? One step of the CAC involves the conversion of fumarate into maiate. This next reaction does not work in the body: Why? Page 7/10 Rice, CHM—231, Lecture Outiinc. Chapter 6. (7.5) I’ractice probiems: (1) Which of the {oliowing moiecuies are chiral? (1.21) 3—btomopentanc (1.1)) 1,3—dibromopcntane (1.1:) 3—mcthyiw1~hcxcnc (1 .d) (5&1 ,4—dimcthylcydohcxanc (2) Alanine, an amino acid found in proteins is chira}. Draw the two cnantiorncis of alanine using perspective formulas. Assign the R and 5 configuration to each one. CH3CHc02H NH2 (3) Using the Cahnglngold—Ptclog priority rules rank the following substiuents in order of priority. (3.9.) —H, -OH, mcnzcns, —CH2CHZCH3 (3.13) 40214, ~COZCH3, wcnzon, on (4) Assign R, S configuzations to the foliowing moieculcs: (4.3) Er (4.5} H "'“H o H JV” H38)\ 0 2 HO cozH H CH3 Page 8/10 Rice, CHM~231, Lecture Outline. Chapter 6. (8) Summary of Chapter 6: Complete the following summary information: (1) Stereochemistry is the field of chemistry that deais with: (2) Isomers are defined as: (3) Isomers fall into two classes, They are: (4) A constitutional isomer differs (5) Stereoisomers differ (6) What are the two types of stereoisomers? (7) Define ‘chiral moiecule’: (8) Define ‘achirai molecule’: (9) What feature most often causes chirality? (10) What is an asymmetric center? (11) Define the term enantiomer: (12) What is a diastereomer? (13) Enantiomers have the physical and chemical properties and react the with achiral reagents (They react differently with chiral reagents). Diastereomers have physical and chemical properties and react with achiral reagents. (14) A mixture containing equal amounts of two enantiomers is called a: (15) What do the letters R and 5 designate? (36) Enantiomers are compounds with both configuration. Identicai molecules are either both R or both 5. (17) Chiral compounds are optically active and this means that they rotate . Achiral compounds are opticaliy inactive. Page 9/10 Rice, CHM—231, Lecture Outline. Chapter 6. {18) If one enantiomer rotates the plane of polarized light clockwise it is designate , its mirror image will rotate it the same amount in the other direction and is designated (1 9) Each optically active compound has a characteristic (20) Racernic mixtures are optically inactive. Why? (2%) Define the term ‘meso compound“: (22) Are meso compounds achiral? (23) A compound with the same four groups bonded to two asymmetric centers will have the following: (24) When does a reaction form the racemic mixture as the products? (25) Enzyme catalyzed reactions form only one stereoisomer and typically catalyze the reaction of only one stereoisomer. Page ill/10 ...
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