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C-I-P rules - Cahn—Ingold-Prelog Rules for assigning...

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Unformatted text preview: Cahn—Ingold-Prelog Rules for assigning configuration to a stereogenic carbon atom: 1. The groups around the carbon are assigned priorities. The higher the atomic number of the atom bonded directly to the stereogenic (optically asymmetric) carbon, the higher the priority of the substituent. e.g., C1>O>N>C>H 2. If two atoms of the same atomic number are attached to the stereogenic carbon, the next atoms in the chains are taken into account until some difference is identified. A decision is made at the first point of difference: lowest priority ' r, .- stereogenic C - r H H H , ‘ Cl H I I I , ' I | Cl—C-C—C C; C—H g .‘ | I ' I H : H H‘_ C "" be H second highest , ’ I ‘2 priority has 2 H -‘ ‘ ’ H -- C -H H 'whighest priority has Cl bound and a C bound I it, H “second lowest priority has 3 H bound 3. Double bonds are counted as two single bonds for both the atoms involved. The same principle holds for triple bonds. \ / I I \ I (:20 6—» ——c—c-— and pc==o <—-—> -—-—c—0 / \ I I / I I C C O C lilC I I ~‘CEN H ——C-N C I N 4. The molecule is then viewed with the lowest priority nibstituent pointed away from the viewer (into the page). Examine the three remaining groups. If the order of priority (from lst to 2nd to 3rd priority) increases in a clockwise manner, the stereogenic carbon is assigned the R configuration. If the order of priority (from lst to 2nd to 3rd priority) increases in an anticlockwise manner, the stereogenic carbon is assigned the S configuration. Note: The designation of a compound as R or S has nothing to do with the direction in which the compound will rotate the plane of polarization of a beam of plane polarized light. Therefore, both types of designation may appear in the name. e. g, (S)-(d)-alanine and (R)-(l)—lactrc acid. ...
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