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90º rotation inverts stereogenic center 2 180º

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90º rotation inverts stereogenic center 2. 180º rotation in the plane of the board (paper) produces the same molecule 3. Holding one group in place while rotating the other 3 in order produces the same molecule
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6 Determining R and S with Fischer Projections If the lowest priority group (often a hydrogen) is on a vertical bond, the configuration is given directly from the relative positions of the three higher-ranked substituents. If the lowest priority group is on a horizontal bond, the positions of the remaining groups give the wrong answer (you are in looking at the configuration from the wrong side), so you simply reverse it. (We’ll come back to compounds with more than one chiral center in a bit.) Fischer Projections The carbon chain is written vertically usually with the most highly oxidized carbon atom at the top. Horizontal lines show groups projecting above the plane of the paper. Vertical lines show groups projecting behind the plane of the paper. O H H OH CH 2 OH OH H CH 2 OH O H = CH 2 OH OH H H O = O H HO CH 2 OH H = R-(+)-glyceraldehyde also called D-glyceraldehyde 1 2 3 Fischer arbitrarily assigned (+)-glyceraldehyde to the R configuration. 60 years later, Bijvoet used X-ray diffraction to show that his guess was correct!
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