Isomers

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Alem
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190948
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Isomers
Updated:
2013-01-03 18:00:01
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MCAT OCHEM
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MCAT OCHEM ISOMERS
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  1. Isomers
    Chemical compounds that have the same molecular formula but differ in structure-- that is, in their atomic connectivity, rotational orientation, or the 3-dimensional position of their atoms. 
  2. Structural Isomers
    Structural isomers are compounds that share only their molecular formula. Because their atomic connections may be completely different, they often have very different chemical and physical properties (such as melting point, boiling point, solubility, etc.)
  3. Stereoisomerism
    Stereoisomers are compounds that differ from each other only in that their atoms are oriented in space. 

    Stereoisomers include:- 

    • 1. Geometric Isomers: 
    •                  A. Enantiomers
    •                  B. Diastereomers
    •                  C. Meso Compounds
    • 2. Conformational Isomers:
  4. 1. Geometric Isomers
    Geometric isomers are compounds that differ in the position of substituents attached to a double bond 

    - Cis and Trans

    - Z (zame zide: same side)  and E (epposite: opposite side) 
  5. Chirality
    An object that is not superimposable upon its mirror image is called chiral.

    A carbon must have four different substituents to be a chiral center. 

    Achiral objects are mirror images that can be superimposed. For example the letter A.
  6. A. Enantiomers

    Stereoisomers include:- 
    1. Geometric Isomers:                  
        A. Enantiomers               
        B. Diastereomers               
        C. Meso Compounds

    2. Conformational Isomers:
    Chiral objects that are non-superimposable mirror images are called enantiomers.

    Enantiomers have identical chemical and physical properties with one exception: optical activity.
  7. Optical activity
    A compound is optical active if it has the ability to rotate plane-polarized light. 

    If plane-polarized light is passed through an optically active compound, the orientation of the plane is rotated by an angle alpha. The enantiomer of this compound will rotate light by the same amount, but in the opposite direction. 
  8. Optical activity
    A compound that rotates the plane of polarized light to the right, or clockwise is dextrorotatory (+). 

    A compound that rotates light toward the left, or counterclockwise, is levorotatory (-).
  9. Optical activity 
    Rotations measured at different concentrations and tube lengths can be converted to a standardized specific rotation (alpha) using the following equation: 

    • Specific rotation(alpha) =
    • (observed rotation alpha) / (concentration g/ml)x(length dm) 
  10. Racemic mixture
    A mixture of equal concentration of both the (+) and (-) enantiomers. The rotations cancel each other and no optical activity is observed.
  11. B. Diastereomers

    Stereoisomers include:- 
    1. Geometric Isomers:                
         A. Enantiomers                 
         B. Diastereomers                 
         C. Meso Compounds

    2. Conformational Isomers:
    Stereoisomers are isomers (different compounds with the same chemical formula) that differ only in the spatial orientation of their component atoms. 

    Diastereomers are stereoisomers that are not mirror images.

    Diastereomers may have different physical properties (such as solubility) and can therefore be separated by physical means.

  12. Diastereomers
    For any molecule with n chiral centers, there are 2^n possible stereoisomers.
  13. C. Meso Compounds 

    Stereoisomers include:- 
    1. Geometric Isomers:                      
        A. Enantiomers                      
        B. Diastereomers                      
        C. Meso Compounds

    2. Conformational Isomers:
    A molecule with two or more chiral centers is not optically active if it has a plane of symmetry. Such a compound is known as meso compound. 

  14. 2. Conformational Isomers

    Stereoisomers include:- 
    1. Geometric Isomers:                         
         A. Enantiomers                         
         B. Diastereomers                         
         C. Meso Compounds

    2. Conformational Isomers:
    Conformational isomers are compounds that differ only by rotation about one or more single bonds.

    Essentially, these isomers represent the same compound in a slightly different position-- analogous to a person who may be either standing up or sitting down.

    These different conformations can be seen when the molecule is depicted in a Newman projection. 
  15. 2. Conformational Isomers 

    Newman Projection

    a. Straight-Chain Conformation
    eg. n-butane
    • 1. Staggered (anti)
    • 2. Gauche
    • 3. Eclipsed 
    • 4. Totally Eclipsed
  16. 1. Staggered (anti)
    The most stable conformation is when the two methyl groups are oriented 180 degree from each other. This conformation is known as staggered or anti conformation. 

  17. 2. Gauche
    Gauche occurs when the two methyl groups are 60 degree apart. 

  18. 3. Eclipsed
    Eclipsed conformation is when the two methyl groups are 120 degree apart. 

  19. 4. Totally Eclipsed
    When the two methyl groups overlap with each other, the molecule is said to be totally eclipsed and is in its highest energy state. 

  20. 2. Conformational Isomerism 

    b. Cyclic Conformations

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