S20-Lectures-Week2

S20-Lectures-Week2 - Chemistry S-20ab Chemistry E-2a:...

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Symmetry, Chirality, and Enantiomers N.B. Many of these slides have been borrowed from Prof. Garry Procter. Reading : Section 6.1 Chemistry S-20ab Week 2 81
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Some Important Vocabulary Words Any molecule that has a plane of symmetry will be achiral : Any molecule that lacks a plane of symmetrty will be chiral : Enantiomers are molecules that are nonsuperimposable mirror images . For each of the following molecules, determine whether it is chiral or achiral. If it is chiral, draw its enantiomer. H F Cl Br Reading : Section 6.1 Chemistry S-20ab Week 2 82
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Naming Enantiomers: The R , S System Here are the rules for naming enantiomers: Assign R or S configurations to each of the following: H F Cl Br H Cl Reading : Section 6.2 Chemistry S-20ab Week 2 83
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Are Enantiomers Different? Roald Hoffmann (who shared the Nobel Prize for frontier molecular orbitals) wrote a wonderful book about chemistry titled The Same and Not The Same . His point was that much of chemistry involves substances that are the same in some respects, and not the same in others. Enantiomers are an excellent example: A pair of enantiomers will generally have identical physical properties : - color, boiling point, melting point, solubility, etc. A pair of enantiomers will generally have different biological properties: - taste, smell, toxicity, therapeutic action, etc. A pair of enantiomers can exhibit the same chemical reactivity OR different chemical reactivity , depending on what they are reacting with (more on this later). For instance: O H O H R -carvone spearmint S -carvone caraway ("rye") As an excercise, can you assign the R and S designations to these enantiomers of carvone? Do you remember the rule for multiple bonds? Chemistry S-20ab Week 2 84
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Optical Activity: Interaction with Polarized Light Enantiomers do differ in one important physical property: they rotate a plane of polarized light equally in opposite directions: Reading : Section 6.3 Chemistry S-20ab Week 2 85
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Mixtures of Enantiomers: Racemates and Purity What would happen to polarized light if it passed through a mixture composed of equal amounts of both enantiomers of a compound? Such a 50-50 mixture of enantiomers is called a racemic mixture or a racemate . For any other mixture of enantiomers, we can characterize the mixture by measuring its optical rotation. We define the optical purity or enantiomeric excess (e.e. ) as: optical purity = e.e. = observed specific rotation specific rotation of pure enantiomer = (% major enantiomer) – (% minor enantiomer) Example: What is the optical purity of the sample? What is the composition of the mixture? Reading : Sections 6.3, 6.4 Chemistry S-20ab Week 2 86
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What do we notice about enantiomers and diastereomers when we look at their R , S - designations? How can we determine the relationship between two molecules: "the same and not the
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S20-Lectures-Week2 - Chemistry S-20ab Chemistry E-2a:...

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