CHE 331-800 10.13-10.15

CHE 331-800 10.13-10.15 - 1 r lbo r& H 01 O Tests for...

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Unformatted text preview: 1 r lbo r& H 01 10/4/13 O Tests for Chirality: Planes of Symmetry IbPlane of symmetry ~k An imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other * A molecule with a plane of symmetry cannot be chiral. ‘JE 1 € {7,00 {Lani/(Q6713 cm W saw 35’ ’73 inO/UL O Nomenclature of Enantiomers: The ES System 0 Also called the Cahn-lngold-Prelog system c The four groups attached to the stereogenic carbon are assigned priorities from highest (a) to lowest (d). o Priorities are assigned as follows: * Atoms mama to the stereogenic center are compared. * Atoms with higher atomic number are given higher priority I) If priority cannot be assigned based on directly attached atoms, the next layer of atoms is examined H I (r) (11,11,171) H—(i—H (a) HO\&’H (d) I H-—~("-—H | (b) ((7.11, m n—(E~n H E S (:owfiifgwmfi it WWIOUGLQ nah/1W6 6) 44’ (m m Saw C \OCY— L065 COMM ClOOUO’SQ ‘ (3) .00 10/4/13 0 The molecule is rotated to put the lowest priority group back I} If the groups descend in priority (a,b then c) in clockwise direction, the enantiomer is R. I} If the groups descend in priority in counterclockwise direction, the enantiomer is S c (c) E” 6 ; Arrows are clockwise (R)-2-Butanol CH,H2 (b) 0 Problem: Are A and B identical or enantiomers? Is the configuration R or S? C, (EH3 d HKEACI b aBrnéACH, C l fir cm into B) the same. enantiomers. #Exchange two groups to try to convert B into A (or A ‘k If an even # of exchanges converts B into A, they are * If an odd # of exchanges converts B into A, they are b g1 l F‘_' Hd B E (1 ‘. NUT/MW} Chapter 5 10/4/13 0 Groups with double or triple bonds are assigned priorities as if their atoms were duplicated or triplicated. | C=Y asii‘itwcrc (Y) (C) and (T) (C‘T) —'CEY asiriiwcre —(|?—\|( (Y) (C) . H H . IS treated | I which —CH=CH2 as though —C—C—H has higher it were l 1 priority than (C) (C) If What is the configuration (R or S) of this molecule? 10/4/13 Lo} \\ d O "W 3 9 Properties of Enantiomers: Optical Activity ‘0 L’R’Q Db PW} o Enantiomers have identical physical properties (i.e. _ melting point, boiling point, density) 0 However, enantiomers rotate the plane of plane- polarized light in equal but opposite directions. a Plane polarized light I) Oscillation of the electric field of ordinary light occurs in all possible planes perpendicular to the direction of propagation I} If the light is passed through a polarizer only one plane emerges. Specific Rotation, [a] [01 = 'f“ where [a] = the specific rotation 01 = the observed rotation r; = the concentration of the solution in grams per milliliter of solution (or den- sity in g mL" for neat liquids) I = the length of the tube in decinteters (1 dm = l0 cm) Chapter 5 16 [0C1 -: [tbs—mm VOW?” , ( (oncemafim agommll Oi WW” 8 N0 (Qmofl Ween s/g Omd +/« 10/4/13 0 The specific rotation of the two pure enantiomers of 2- butanol are equal but opposite. 9”} TH} H H HO \ OH I \I C C l l (. I n: (lrnl {in} (:u“ (Rl-Z-Butanol (S)-2-Butanol [a ,2,‘ = -—l3.52° [or 3,5 = +l3.52° 0 There is no direct correlation between the R,S designation of an enantiomer and the direction (+) or (-) in which it rotates plane polarized light. For example: \ few deck 1 (Inf/e .)l » _ . e (S)-(-)-2-iodobutane [c1]D = -15.9° v-7 QWOh DWV tom + (S)-(+)-1-chloro-2-methylbutane [(110 = +1.6° ‘V Q/flC‘lNfiW $3 10m b ’ Chapter 5 O Racemic Forms and Enantiomeric Excess o Often a mixture of enantiomers will be enriched in one enantiomer: enantiomeric excess (ee) moles of one enantiomer - moles of other enantiomer x 100 % Enantiomeric excess . total moles of both enantiomers observed specific rotation ’70 Enantiomeric excess* = X 100 - specxlic rotation of the pure enantiomer 0 Example : Su -.e...: he optical rotation of a sample of (S)-2- ‘ iodobutane *@1 What is the enantiomeric excess? ee = -9.54°/-15.9° (pure enantiomer) x 100 = 60% The composition of the sample is 60% (S) and 40% (R+S) = 60% (S) + 40/2% (8) = 80% (5) Chapter 5 @090 it) (6 other 40%(m/x ob (1+5) WMJEOM “VOW/6 is 80% Wfld C33 9 l 9 file Synthesis of Chiral Molecules 0 Most chemical reactions which produce chiral molecules roduce them in racemic form. _ '° QUHCMW Ni momw UZ 9 CH CH,CCH + H--H —> ‘: —CH CH CHCH. , 3 -” 3 ‘ ) -‘ 2| J emnharmrb O - OH Butanone Hydrogen (:)-2-Butan0l (achiral (achiral [chiral molecules " CCn p molecules) molecules) but 50:50 mixture (R) and (S )] ‘(avfij +6 Chapter 5 [fix “I 3' "“ cola 4< M Ms reacfion 5 cc at (56+ fl 0776 MW flu ear. 0 Molecules with More than One Stereogenic Center 0 The maximum number of stereoisomers available will not exceed 2", where n is equal to the number of tetrahedral stereogenic centers. t. a * l , CH3C|IHC|3HCH2CH3 -—v 22 ’4 Br Br 2,3-Dibromopentane Chapter 5 10 l and a are, enmowtrs 03 and 4 are enantiomas 10/4/13 . 9H} lirgé‘gfl BrgéAH H'gVBr BrVSVH xirnggl. l;x.r€VH C2H5 C3115 CQH5 1 , 2 3 0 There are two pairs of enantiomers (1, 2) and (3,4) 9 Enantiomers are not easily separable so 1 and 2 cannot be separated from each other. e Diastereomers: stereoisomers which are not mirror images of each other I} For instance 1 and 3, or 1 and 4 I) Have different physical properties and cm be separated. Chapter 5 O Fischer Projection Formulas o A two-dimensional representation of chiral molecules I) Vertical lines represent bonds that pro'egt behind the plane of the paper I) Horizontal lines represent bonds that Ero'ect out of the plane of the paper (EH; CH3 T h rec- BrKC A H Br H Fischer dinwusimml 2 projection lormula ‘Br H Br formula EH3 CH3 A A Chapter 5 11 10/4/13 0 Mesa Compounds 0 Sometimes molecules with two or more stereogenic centers will have less than the maximum number of stereoisomers. (91:4 (moo: W) $143 > Mlfi JAG”) 5. *CHBr ‘ I *CHBr 1 CH1 2,3-Dibr0m0butane Chapter 5 23 0 Mesa compounds are achiral despite the presence of stereogenic centers. l>Not optically active QSuperposable on its mirror image 9Has a plane of symmetry Chapter 5 24 12 ...
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