CHEM 3331 1nd Edition Lecture 11 Outline of Last Lecture I. Stereoisomers redefinedII. SymmetryIII. EnantiomersIV. Chiral centersV. NomenclatureVI. Optical activityOutline of Current Lecture I. Optical activityII. Racemic mixture/ racemateIII. Optical purityIV. Chirality in cyclohexanesV. Chirality without chiral centersVI. Fischer projectionsCurrent LectureI. Optical activityCircular polarized light is a chiral stimuli. Specific rotation is [α]. It distinguishes chiral and achiraland which enantiomer we have. [α]= α(observed)/[c]*LII. Racemic mixture/ racemateWith a 1:1 mix of r and s configuration, α=0. S-2-butanol has α=13.5. R-2-butanol has α=-13.5.An achiral molecule plus an achiral molecule results in an achiral molecule. Butane and bromineare achiral. When added together they create equal proportions of r and s configuration enantiomers.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.All biomolecules exist as a single enantiomer in the body. Where did original excess of single enantiomer and chirality rise from? III. Optical purityYou take the rotation of the observed enantiomer and compare it to the pure enantiomer. Op= [α]/[α]pureEnantiomeric excess (ee) = absolute value of [+]-[-]/[+]+[-]*100%A chiral catalyst can cause chirality from two achiral reactants.IV. Chirality in cyclohexanesThe chair conformations of cyclohexane would be thought to be chiral but they are not. Since the chair conformation is continually flipping between axial and equatorial. To determine chirality we use the planar cyclohexane. Newman projections can also be used. However they quickly equilibriate so chirality no longer exists only in individual conformations. For example anti periplanar of butane is achiral. We examine the most symmetrical configuration and if it contains symmetry then it is achiral.V. Chirality without chiral centersWhen determining chirality symmetry is the most important. Some chiral molecules don’t center around sp3 hybridized carbon. For example two benzene joined by a singe bond possess free rotation so it is achiral. However adding an iodine atom and a bromine atom to two carbons stops the free rotation and makes the molecule chiral. This is known as axial chirality.Besides benzene rings there are allenes. H2C=C=CH2. The middle carbon is sp hybridized and the two outer carbons are sp2 hybridized. If we replace two hydrogens on the ends with CH3. The molecule is chiral.VI. Fischer ProjectionsUsed specifically for chiral molecules.This example shows the transition from a Fischer projection to a sp3 configuration. Horizontal lines= molecules in front of the plane and vertical lines represent molecules behind the plane. You cannot rotate the Fischer projections by 90 degrees it must be 180. You can assign r and s configurations using the
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