SET 5 - Maria Ballester PDF

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Maria Ballester...

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SET 5  Starch and Cellulose  1. In starch, the O atoms joins two rings using one equatorial and one axial bond.  2. In cellulose, the O atom joins two rings using two equatorial bonds.  Starch and cellulose are isomers. Indeed, stereoisomers.  Just this minor difference in the 3D arrangment confers very different properties on starch and cellulose.  Constitutional Isomers  Differ in the way the atoms are connected to each other. They have different IUPAC names, the same or different functional groups; different physical properties; and different chemical properties. They have the same molecular formula though.  Stereoisomers  Differ only in the way atoms are oriented in space. Identifcal IUPAC names except for prefix like cis or trans. Same functional groups. Particular 3d arrangement is called a configuration.  Chiral  A molecule that is not superimposable on its mirror image. Ex. Human hand, CHBRClF  Achiral  Molecule that is superimposable on its mirror image. Ex. Socks, H20,  Enantiomers  Mirror images that are not superimposable.  Tetrahedral stereogenic Center  Carbon atom bonded to four different groups. Most chiral molecules contain one or more stereogenic centers.  General principles regarding stereogenic centers  1. With no stereogenic centers a molecule generally is not chiral.  2. With one tetrahedral stereogenic center, a molecule is always chiral.  3. With two or more stereogenic centers, a molecule may or may not be chiral.  Achiral molecules usually contain a plane of symmetry but chiral molecules do not, T or F?  True.  These C atoms cannot be tetrahedral stereogenic centers  1. CH₂ and CH₃ groups (more than one H bonded to C).  2. any sp or sp² hybridized C (less than four groups around C). i.e. double or triple bond.  ***Organic Molecules can contain two, three, or even hundreds of stereogenic centers.  Any molecule with one tretrahedral stereogenic center is a chiral compound and exists as a pair of enantiomers.  T  Stereogenic Centers for Cyclic Compounds  Find the possible stereogenic center (Not CH2 or CH3, or anything with a double or triple bond). Then compare the sides that the possible stereogeneic center is bonded to an figure out if the groups are different (i.e. a double or triple bond, other functional groups).  Thalidomide

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 An example of an active compound that contains one or more stereogenic centers on ring carbons. Sucrose  Has nine stereogenic centers. How to label stereogenic centers with R and S  1. Assign priorities (1,2,3,4) to the atoms directly bonded to the stereogenic center in order of decreasing atomic number. The atom with the highest atomic (Z) receives the designation one.  2. If two atoms are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher atomic number determines a higher priority.  3. IF two isotopes are bonded to the stereogenic center, assign priorities in order of decreasing mass number.  4. To assign a priority to an atom that is part of a multiple bond, teat a multiply bonded atom as an equivalent number of signly Assigning the letters  1. orient the molecule with the lowest priority group back (on a dash), and visualize the relative positions of hte remaining three groups. visualize as a newmann projection.  2. Trace a circle from priority group 1→2→3. If tracing the circle goes in a clockwise direction- the isomer is named R.  * If tracing the circle goes in a counterclockwise direction- to the left from the noon position-the isomer is named S. 3. The letters R or S precede the IUPAC name of the molecule. Ex. (2R)-2-butanol. ***If the lowest priority gropu is oriented in front, not behind, switch the position with the group located behind the page, determine R or S as usual and then reverse the answer (ex. S to R). Diastereomers  May or not be chiral. For N stereogenic centers, the maximum number of stereoisomers is 2ⁿ. *With one stereogenic center there are always two stereisomers and they are entantiomers. With two stereogenic centers, the maximum number of stereoisomers is four, although sometimes there are fewer. How to find and draw all possible stereoisomers for a compound with Two Stereogenic Centers  1. Draw one stereoisomer by arbitrarily arranging substituents around the stereogenic centers. Then draw its mirror image. See if they align- Place b directly on top of A and rotate B 180 degrees and place it on top of A to see if the atoms align. If the images are nonsuperimposable mirror images they are enantiomers.  2. Draw a third possible stereoisomer by switching the positions of any two groups on one stereogenic center only. Then draw its mirror image. Then you have four stereoisomers and A and B are diastereomers of C and D and vice versa. Diastereomers are stereoisomers that are not mirror images. *Only two types of stereoisomers.

 Meso Compounds  Achiral compound that contains tetrahedral stereogenic centers. It contains a plane of symmetry. They generally have a plane of symmetry.  R,S configurations being used to determine if two compounds are identical, enantiomers, or diastereomers.  1. Identical compounds have the same R, S designations at every tetrahedral stereogenic center.  2. Enantiomers have exactly oposite R, S designations  3. Diastereomers have the same R, S designation for at least one stereogenic center and the oposite for at least one of the other stereogenic centers.  Summary of Isomers  Isomers → Constitutional or Stereoisomers→Enantiomers (non super imposable mirror images) or Diastereomers (Not mirror images).  Two nonidentical molecules 1. Do they have the same molecular formula  N- Not Isomers  Y- Isomers  2. Are the molecules named the same except for the prefixes such as cis, trans, R, or S?  N- Constitutional isomers  Y- Stereoisomers  3. Are the molecules mirror images of each other?  N- Diastereomers.  Y- Enantiomers....