Chapter 5 Stereoisomers

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

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

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

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

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

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

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