CHIRALITY, SYMMETRY PLANES AND ENANTIOMERS
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1 CHIRALITY, SYMMETRY PLANES AND ENANTIOMERS Craig Wheelock October 17th, (copies of slides can be downloaded from my homepage)
2 WHICH OBJECTS ARE SYMMETRIC? (mirror image is identical) asym sym (outside) sym asym asym asym sym sym
3 PLANES OF SYMMETRY
4 A SYMMETRIC OBJECT HAS A PLANE OF SYMMETRY - ALSO CALLED A MIRROR PLANE plane of symmetry mirror plane
5 IF AN OBJECT HAS A PLANE OF SYMMETRY, ITS MIRROR IMAGE WILL BE IDENTICAL IDENTICAL MIRROR IMAGES WILL SUPERIMPOSE (MATCH EXACTLY WHEN PLACED ON TOP OF EACH OTHER)
6 CHIRALITY
7 An object without symmetry is CHIRAL no symmetry The mirror image of a chiral object is different and will not superimpose on the original object. OBJECTS WHICH ARE CHIRAL HAVE A SENSE OF HANDEDNESS AND EXIST IN TWO FORMS
8 An object with symmetry is ACHIRAL (not chiral) The mirror image of an achiral object is identical and will superimpose on the original object. plane of symmetry
9 Cl ENANTIOMERS non-superimposable mirror images Cl rotate H F F H this molecule is chiral Cl H F note that the fluorine and bromine have been interchanged in the enantiomer
10 non-superimposable
11 OPTICAL ACTIVITY PLANE-POLARIZED LIGHT
12 PLANE-POLARIZED POLARIZED LIGHT BEAM single ray or photon wavelength λ All sine waves (rays) in the beam aligned in same plane. SIDE VIEW A beam is a collection of these rays. frequency ( ν ) ν = c λ c = speed of light unpolarized beam. polarized beam NOT PLANE-POLARIZED Sine waves are not aligned in the same plane. END VIEW
13 Optically Active Refers to molecules that interact with plane-polarized light Jean Baptiste Biot French Physicist He discovered that some natural substances (glucose, nicotine, sucrose) rotate the plane of plane-polarized light and that others did not.
14 Optical Activity angle of rotation, α α incident polarized light sample cell (usually quartz) transmitted light (rotated) a solution of the substance to be examined is placed inside the cell
15 TYPES OF OPTICAL ACTIVITY new older Dextrorotatory (+)- d- Rotates the plane of plane-polarized light to the right. new older Levorotatory (-)- l- Rotates the plane of plane-polarized light to the left.
16 t [α] D = Specific Rotation [α] D α cl α = observed rotation c = concentration ( g/ml ) l = length of cell ( dm ) D = yellow light from sodium lamp t = temperature ( Celsius ) This equation corrects for differences in cell length and concentration. Specific rotation calculated in this way is a physical property of an optically active substance. You always get the same t value of [α] D
17 SPECIFIC ROTATIONS OF BIOACTIVE COMPOUNDS COMPOUND [α] D cholesterol cocaine -16 morphine -132 codeine -136 heroin -107 epinephrine -5.0 progesterone +172 testosterone +109 sucrose β-d-glucose α-d-glucose +112 oxacillin +201
18 PASTEUR S DISCOVERY Louis Pasteur 1848 Sorbonne, Paris HOOC CH CH COOH + Na OOC CH CH COO 2- NH 4 + OH OH OH OH tartaric acid ( found in wine must ) sodium ammonium tartrate Pasteur crystallized this substance on a cold day
19 Crystals of Sodium Ammonium Tartrate hemihedral faces Pasteur found two different crystals. mirror images Biot s results : (+) (-) Louis Pasteur separated these and gave them to Biot to measure.
20 Enantiomers non-superimposable mirror images (also called optical isomers) W W X C Z Y Y Z C X Pasteur decided that the molecules that made the crystals, just as the crystals themselves, must be mirror images. Each crystal must contain a single type of enantiomer.
21 Pasteur s hypothesis eventually led to the discovery that tetravalent carbon atoms are tetrahedral. Van t Hoff and LeBel (1874) C tetrahedral carbon Only tetrahedral geometry can lead to mirror image molecules: C C Square planar, square pyrimidal or trigonal pyramid will not work: C C C
22 ENANTIOMERS HAVE EQUAL AND OPPOSITE ROTATIONS W Enantiomers W X C Y Y C X Z Z (+)-nn o dextrorotatory (-)-nn o levorotatory ALL OTHER PHYSICAL PROPERTIES ARE THE SAME
23 TARTARIC ACID from fermentation of wine OH OH Enantiomers OH OH HOOC H H COOH H HOOC COOH H (+)-tartaric acid (-)-tartaric acid HOOC H OH OH meso COOH H meso -tartaric acid ALSO FOUND (as a minor component) [α] D = 0 more about this compound later
24 TARTARIC ACID (-) - tartaric acid (+) - tartaric acid [α] D = o [α] D = o mp o solubility of 1 g 0.75 ml H 2 O 1.7 ml methanol 250 ml ether mp o solubility of 1 g 0.75 ml H 2 O 1.7 ml methanol 250 ml ether insoluble CHCl 3 d = g/ml insoluble CHCl 3 d = g/ml [α] D = 0 o mp 140 o d = g/ml meso - tartaric acid solubility of 1 g 0.94 ml H 2 O insoluble CHCl 3
25 RACEMIC MIXTURE an equimolar (50/50) mixture of enantiomers [α] D = 0 o the effect of each molecule is cancelled out by its enantiomer
26 STEREOISOMERS ENANTIOMERS are a type of STEREOISOMER Stereoisomers are the same constitutional isomer, but differ in the way they are arranged in 3-D space at one or more of their atoms.
27 STEREOCENTERS One of the ways a molecule can be chiral is to have a stereocenter A stereocenter is an atom, or a group of atoms, that can potentially cause a molecule to be chiral stereocenters - can give rise to chirality
28 STEREOGENIC CARBONS ( called chiral carbons in older literature ) Cl This is one type of. stereocenter. others are possible H F A stereogenic carbon is tetrahedral and has four different groups attached. H F Cl
29 F Cl ACHIRAL The plane of the paper is a plane of symmetry F F Cl Cl Cl Cl TWO IDENTICAL GROUPS RENDERS A TETRAHEDRAL CARBON ACHIRAL
30 ONE PAIR OF ATOMS ATTACHED TO A TETRAHEDRAL CARBON IS IN A PLANE PERPENDICULAR TO THE OTHER PAIR plane 1 plane 2 F Cl Look at your model set!!!!
31 TWO VIEWS OF THE PLANE OF SYMMETRY F plane of symmetry F Cl Cl Cl F Cl Cl side view edge view
32 CONFIGURATION ABSOLUTE CONFIGURATION ( R / S )
33 CONFIGURATION The three dimensional arrangement of the groups attached to an atom Stereoisomers differ in the configuration at one or more of their atoms.
34 CONFIGURATION - relates to the three dimensional sense of attachment for groups attached to a chiral atom or group of atoms (i.e., attached to a stereocenter). clockwise 1 4 C C 1 counter clockwise view with substituent of lowest priority in back 3 R (rectus) S 3 (sinister)
35 SPECIFICATION OF CONFIGURATION Enantiomers are assigned a CONFIGURATION using the same priority rules we developed for E / Z stereoisomers. 1. Higher atomic number has higher priority. 2. If priority cannot be decided based on the first atom attached move to the next atom, following the path having the highest prioity atom. 3. Expand multiple bonds by replicating the atoms attached to each end of the bond. CAHN-INGOLD-PRELOG SEQUENCE RULES
36 1 R 4 2 (R)-2-Butanol 3
37 omochlorofluoroiodomethane atomic number H F Cl I 1 I I 1 4 F Cl 3 C 2 4 F 2 C 3 Cl R Enantiomers S
38 NUMBER OF STEREOISOMERS POSSIBLE
39 How Many Stereoisomers Are Possible? maximum number of stereoisomers sometimes fewer than this number will exist = 2 n, where n = number of stereocenters (sterogenic carbons)
40 CH 3 CH 2 OH * OH C CH 2 CH CH CH 3 * CH 3 CH 3 R R R S S R S S 2 2 = 4 stereoisomers CH 3 * * * OH R R R R R S R S R S R R R S S S R S S S R S S S CH 3 CH CH3 2 3 = 8 stereoisomers
41 FINDING STEREOCENTERS (STEREOGENIC CARBONS) O O O * CH 3 * CH 3 CH 3 plane ( indicates no stereoisomers ) stereoisomers (max) : 2 0 = = = 2
42 FINDING STEREOCENTERS (STEREOGENIC CARBONS) O O O * * * * CH 3 CH 3 CH 3 CH 3 CH3 * * CH 3 plane ( but only if cis ) stereoisomers (max) : 2 2 = = = 4
43 FINDING STEREOCENTERS (CHIRAL CARBONS) CH 3 CH 3 * HO * CH 3 * * * * * * CH 3 CH 3 H n = = 256
44 MOLECULES WITH TWO STEREOCENTERS DIASTEREOMERS / MESO
45 2-omo-3-chlorobutane * * CH 3 CHCHCH Cl = 4 stereoisomers possible SR RS SS RR When comparing butanes, the comparisons are done best using the eclipsed conformation. The relationships and planes of symmetry are not easily seen in other conformations.
46 PRIORITIES AND DETERMINING R and S 3 1 Cl 2 CH 3 CH 3 H H 4 rotate 4 2 H H 1 Cl CH 3 3 CH 3 S 2 Cl 1 CH 3 CH 3 H H 4 3 rotate 4 H 1 CH CH 3 H Cl R
47 diastereomers 2-omo-3-chlorobutane S Cl R CH 3 CH 3 H H Cl CH 3 H H CH 3 mirror enantiomers 1 enantiomers 2 S Cl R CH 3 CH 3 H H Cl S S R R H CH 3 CH 3 H
48 Models of the Isomers of 2-omo2 omo-3-chlorobutane (methyl groups are reduced to a single red atom) Cl CH 3 H enantiomers-1 diastereomers enantiomers-2
49 MESO ISOMER plane of symmetry Cl Cl * * CH 3 CH 3 H H S CH 3 H Cl mirror Cl R CH 3 H Meso isomer - has a plane of symmetry and the mirror image is identical to the original molecule. - must have 2 stereocenters
50 Models of the Isomers of 2,3-Dichlorobutane (methyl groups are reduced to a single red atom) Cl CH 3 H meso diastereomers enantiomers
51 CIS / TRANS RINGS DIASTEREOMERS
52 1-omo-2-chlorocyclopropane note that the cis / trans isomers are also diastereomers Cl R S Cl R S cis diastereomers enantiomers R R S S Cl Cl trans enantiomers
53 1,2-Dibromocyclopropane mirror image identical cis diastereomers meso R R S S trans enantiomers
54 DISUBSTITUTED CYCLOHEXANES USING PLANAR RINGS FOR STEREOCHEMICAL ANALYSIS
55 1-omo-2-chlorocyclohexane cyclohexanes may be analyzed using planar rings diastereomers Cl Cl S R S R enantiomers cis trans S S R R Cl enantiomers Cl
56 1,2-dichlorocyclohexane mirror image identical Cl Cl Cl Cl cis diastereomers meso Cl Cl trans S S R R Cl enantiomers Cl
57 CONCLUSION 2 n = the maximum number When a molecule has 1. multiple stereocenters and 2. there is a possibility of an arrangement with a plane of symmetry you will not always find all of the 2 n stereoisomers that are possible. Some of the stereoisomers may be meso isomers, and their mirror images will be superimposable (identical) - this will eliminate at least one of the possible stereoisomers, and sometimes more.
58 PHYSICAL PROPERTIES OF ENANTIOMERS, DIASTEREOMERS AND MESO COMPOUNDS
59 REMEMBER : ENANTIOMERS HAVE EQUAL AND OPPOSITE ROTATIONS W Enantiomers W X C Z Y Y Z C X (+)-nn o dextrorotatory (-)-nn o levorotatory ALL OTHER PHYSICAL PROPERTIES ARE THE SAME
60 IN CONTRAST : DIASTEREOMERS MAY HAVE DIFFERENT ROTATIONS AND ALSO DIFFERENT PHYSICAL PROPERTIES
61 RELATIONSHIPS AMONG STEREOISOMERS
62 COMPARING TWO STEREOISOMERS Two Stereoisomers nonsuperimposable mirror images not mirror images Enantiomers Diastereomers
63 DETERMINING CHIRALITY / OPTICAL ACTIVITY no plane of symmetry molecule has one or more stereocenters no mirror image superimposes yes plane of symmetry has an enantiomer is meso is chiral is achiral optically active optically inactive
64 ISOMERS Different compounds with the same molecular formula STEREOISOMERS Isomers with the same order of attachment, but a different configuration (3D arrangement) of groups on one or more of the atoms double bond or ring cis/trans ISOMERS (geometric) each isomer could have stereoisomers with a ring both can apply CONSTITUTIONAL ISOMERS Isomers with a different order of attachment of the atoms in their molecules ENANTIOMERS Stereoisomers whose molecules are nonsuperimposible mirror images of each other DIASTEREOMERS Stereoisomers whose molecules are not mirror images of each other TYPES OF ISOMERISM
65
66 Biological role of stereochemistry (S)-amino acid (R)-amino acid Only one of the 2 amino acid enantiomers can achieve 3-point binding with the enzyme binding site
67 Importance of stereochemistry Thalidomide (Neurosedyn) S teratogenic and causes birth defects Ibuprofen (Ipren) S active form R effective against morning sickness R inactive
68 To recapitulate... chirality and optical activity plane of symmetry enantiomers and diastereomers R and S use Cahn-Ingold-Prelog priority rules meso compounds contain stereocenters, but are achiral stereoisomers can have significant biological effects
69 Web resources ISIS Draw Structure Drawing Software Chime Molecular Display RasMol Molecular Display Software Jmol Java viewer for 3D chemical structures General resource for organic chemistry Spartan computational chemistry
70 PROBLEM: ARE THESE IDENTICAL OR ARE THEY ENANTIOMERS? CH 3 C H C YOU CAN USE INTERCHANGES F H CH 3 F 1 CH 3 2 H H C C C C F H CH 3 ENANTIOMER F H CH 3 SAME F 3 CH 3 F ENANTIOMER
71 What is the best term to describe the relationship between A and each of the other molecules? Constitutional isomers? Conformations? Enantiomers? Diastereomers? Identical? Are any of these molecules optically active?
72 ANSWERS A-C and E are all the same constitutional isomer, 1,3,5-trichloro-, but F is 1,2,4-trichloro- F is a constitutional isomer to A and to any of the others. A (e,e,a) and B (a,a,e) are conformations A (e,e,a) and C (e,e,e) are diastereomers A and D are diastereomers (D is a conformation of C) A and E are diastereomers (E is identical to D, turned left-to-right) A and G are identical (G is the same as A turned left-to-right) Only F is optically active, all the rest are meso molecules! [α] D = 0 [α] D = 0
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