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Chapter 5 Stereoisomers I.Introduction A.Mirror Images may not be Superimposable 1)Pair of gloves2) Shoes 3)Your image in a mirror 4) Screws B.Isomer Types.

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Presentation on theme: "Chapter 5 Stereoisomers I.Introduction A.Mirror Images may not be Superimposable 1)Pair of gloves2) Shoes 3)Your image in a mirror 4) Screws B.Isomer Types."— Presentation transcript:

1 Chapter 5 Stereoisomers I.Introduction A.Mirror Images may not be Superimposable 1)Pair of gloves2) Shoes 3)Your image in a mirror 4) Screws B.Isomer Types 1)Isomer = same molecular formula, different structure a)Constitutional Isomers: different order of connectivity b)Stereoisomers: same connectivity, different spatial orientation i.Diastereomers: stereoisomers that are not mirror images ii.Enantiomers: non-superimposable mirror images 2)Conformations = stereoisomers that can rapidly interconvert by rotation

2 II.Chirality = describes objects not superimposable with their mirror image A)Products of Radical Bromination of Butane 1)2 products are produced 2)Are they mirror images? Are they Superimposable? 3)Chiral, Enantiomeric products; each enantiomer is one of a pair 4)(Achiral = is superimposable with mirror image) 5)One carbon is bonded to 4 different: Me, Et, H, Br a)These carbons are always chiral b)Asymmetric carbon c)Stereocenter

3 B)Molecular Symmetry 1)Plane of Symmetry a)Bisects molecule to give mirror image halves b)Chiral molecules have zero planes of symmetry c)Methane has 6 planes of symmetry: Achiral d)CHFClBr has no planes of symmetry: Chiral 2)When is Chiral not Chiral? a)Rapid structural changes give an Average Symmetry b)A given structure must be room temperature isolatable c)Conformational changes i.Butane conformations are chiral, but Butane is achiral ii.cis-1,2-dimethylcyclohexane has achiral intermediate

4 III.Optical Activity A)How do you tell Enantiomers Apart? 1)Most Physical Properties are identical: mp, bp, density, etc… 2)Enantiomers interact differently with plane polarized light 3)Each enantiomer of a pair rotates the light in a different direction a)Dextrorotatory = clockwise rotation (+) b)Levorotatory = counterclockwise rotation (-) 4)Descriptions: Optical Activity, Optically Active, Optical Isomers

5 B)Using a Polarimeter to calculate Optical Purity , the rotation angle, depends on cell length, T, solvent, etc… a)L = length in dm (0.1 m) c = concentration in g/ml b)Specific rotation can be compared; constant for given molecule 2)Pure enantiomer has maximum  (+23 o or –23 o ) 3)Racemic Mixture = 50%/50% mixture of enantiomers has  = 0 4)Anywhere in between will have rotation between these values 5)Enantiomeric Excess = %Optical Purity = how much more of one enantiomer than the other 6)Exercise % optical purity, [  ] = 23.1 o, what is [  ] observed ? 7)How much of each enantiomer is present? NOT 75%/25% 12.5% (+) / 12.5% (-) and 75% (+) Total = 87.5% (+), 12.5% (-)

6 IV.R,S Rules A)Absolute Configuration 1)+/- tell us the interaction of light, not the exact structure of enantiomers 2)X-Ray Crystallography gives us Absolute Configuration a)Crystals are regularly arranged solid forms b)They Diffract X-rays regularly, so we can tell what atom is where c)X-Ray and Polarimetry lets us match +/- with a specific structure d)Similar molecules usually have same +/- correlation B)R/S Labels 1)Cahn-Ingold-Prelog System assigns name to each enantiomer 2)Arrange substituents with lowest priority in back a)Clockwise arrangement high-to-low = R (rectus = right) b)Counterclockwise = S (sinister = left) R enantiomerS enantiomer

7 3)Write the name: R-(+)-chloroflouorobromomethane 4)Racemic Mixture: R,S-(+/-)-chlorfluorobromomethane 5)Rules for assigning priority (Try Exercise 5-8 thru 5-10) a)Highest Atomic Number: Br > Cl > F > H b)For equal atomic number, look at what is attached i.Only look at one level at a time ii.If second level identical, then go to third level c)Treat double and triple bonds as 2 or 3 single bonds to that atom c a b Cl,2H > C,2H > 3H2C,H > C,2H > 3H a b c a b c a b c


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