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UH CHEM 3331 - Molecules with multiple stereocenters
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CHEM 3331 1nd Edition Lecture 12Outline of Last Lecture I. Optical ActivityII. Racemic mixture/ racemateIII. Optical PurityIV. Chirality in CyclohexanesV. Chirality without Chiral CentersVI. Fischer ProjectionsOutline of Current Lecture I. DiastereomersII. Meso CompoundsIII. Absolute and Relative ConfigurationsIV. Physical PropertiesV. Separation of enantiomersCurrent LectureI. DiastereomersDiastereomers are all stereoisomers that are not mirror images of each other. In the class stereoisomers we divide it into two groups: enantiomers and diastereomers. Diastereomers contain three groups: cis/trans alkenes, cis/trans cycloalkanes, and compounds with multiple stereocenters. We will be focusing on the last group of diastereomers. 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.This picture shows the relationship between enantiomers and diastereomers. If you have an S,R compound then you will hve an R,S enantiomer and an S,S and R,R diastereomers. For a compound with n stereocenters/chiral centers, there can be up to 2^n diastereomers. This is not always the case. The first two compounds are enantiomers that are chiral. The third is the diastereomer to the first two compounds. As we can see this compound is achiral because it possesses a line of symmetry. The enantiomer to this diastereomer is also achiral and if we rotate the compound 180 degrees they are superimposable. Making these two diastereomers equal to each other so no longer being enantiomers. Therefore the first compound will only have 3 diastereomers sincetwo of them are identical.II. Meso CompoundsMeso compounds are compounds that have multiple stereocenters but possess a plane of symmetry so it is achiral. R,S=S,R. this was shown in the example above.III. Absolute and Relative ConfigurationLooking at R-2bromobutane we know the absolute configuration. Absolute configuration means we are absolutely certain this is how the compound is structured. We know this through x-ray crystallography. X-rays possess high energy and low wavelength. In crystal form the molecules are very ordered and positioned the same way over and over. Therefore, we can determine the configuration of the molecule from its crystal form. This was not always the case since this was not available until 1951. However, Fischer got his Nobel prize for his configurations in 1902. He used relative configurations. To do this we compare our molecule with a similar molecule with aknown configuration. The original compound used to use for comparison was L- glyceraldehyde,the simplest sugar. Luckily for them all of their configurations were correct and they didn’t have to change any of the literature on the subject.IV. Physical PropertiesDiastereomers are not mirror images so physical properties will be very different. For example melting points and boiling points.V. Separation of enantiomersTwo enantiomers can crystallize separately. Crystals will become chiral and enantiomers so we cannot distinguish the enantiomers from each other using physical properties. Louis Pasteur did an experiment using potassium tartarate from the bottom of wine bottles. He scraped out the crystals, he looked at them under a microscope and separated enantiomers. Then he dissolved the crystals in a solution. One solution turned the polarized light one way and the other solutionturned it the other way. A more general method is to add a chiral center that is R creating diastereomers that are R, R and S,R. then you r can separate the diastereomers and cleave off the added chiral center to obtain the pure R and S enantiomers. We find this pure R that is added from a chiral pool such as R amino acids. We can do this method two ways either by a chemical reaction or by


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UH CHEM 3331 - Molecules with multiple stereocenters

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