Chirality, Stereoisomers, Enantiomers

Posted on October 12th, 2012

Hello Orgo Students!

Here at StudyOrgo.com we like to simplify things. You know that whole big chapter in your textbook on Chirality, Stereoisomers and Enantiomers that’s so confusing that you’re just lost?! Well, let’s see if we can simplify things.

Here is our summary on the topic:

This is some of the information presented in Part 9 of our Study Guide: one of the many resources available to StudyOrgo.com members. Learn more about it here: http://www.studyorgo.com/how-it-works.php

 

  1. Chiral
    1. Definition: a plane of symmetry does NOT exist in a given structure
    2. Two objects are chiral if they are NOT superimposable on one another
  2. Achiral
    1. Definition: a plane of symmetry DOES exitst in a given structure
    2. Two objects are achiral if they are superimposable on one another
  3. Enantiomers
    1. Definition: Two isomers that are mirror images of one another
    2. Enantiomers are chiral and they are NOT superimposable on one another
    3. Properties of Enantiomers
      1. A pair of enantiomers almost have identical properties. They differ in one aspect:
        1. Two enantiomers rotate plane-polarized light in opposite directions
          1. Plane polarized light are light waves that travel only in one plane
          2. This is referred to as optical activity
          3. Clockwise rotations are abbreviated by a (+) or the letter “d”
          4. Counterclockwise rotations are abbreviated by a (-) or the letter “l”
    4. Racemic mixture
      1. Definition: When equal amounts of each enantiomer of a pair are mixed together
      2. While each individual enantiomer on its own is optically active, when the two equal amounts are mixed together, the mixture is NOT optically active
    5. Meso compound
      1. Definition: A structure that possesses both a plane of symmetry and at least one asymmetric carbon. Meso compounds are achiral.
    6. Chiral carbons
      1. A chiral carbon has four different items attached to it.
      2. A chiral carbon is also known as asymmetric carbon
      3. The chiral carbon is the chirality center of the molecule
      4. Enantiomers possess chiral carbons.
  4. R and S nomenclature
    1. Used to assign an identifying label to each chiral carbon in a given enantiomer
    2. How to determine R and S nomenclature
      1. Step 1: Find the chiral carbon
      2. Step 2: Assign the numbers 1 through 4 to each item attached to that carbon.
        1. Assign 1 to the item that is of highest priority through 4 to the item of lowest priority
        2. Hierarchy of priority (in order from most important to least important)
          1. a. Halogens (I>Br>Cl>F)
          2. O
          3. N
          4. C
          5. H
        3. Some important notes:
          1. a. Often you will have a tie. In that case, go to the next item after that atom to break the tie and determine the priority.
          2. b. When encountering double or triple bonds, pretend they are single bonds and duplicate the items attached to that bond. Duplicate twice for triple bonds.
      3. Step 3: Draw a circle from 1 to 2 to 3 to 4.
        1. Before starting, look and see where item #4 is. If item #4 is within the plane of the screen or paper: redraw the image to have item #4 going into the plane of the screen or paper before continuing.
          1. If item #4 is going into the screen or paper or coming out of the screen or paper, continue as follows.
        2. Determine if the circle was drawn clockwise or counterclockwise.
        3. If the circle was drawn clockwise: assign the letter “R”
        4. If the circle was drawn counterclockwise: assign the letter “S”
      4. Step 4: Check and see where the item #4 is
        1. Usually this item is a hydrogen. If item #4 is in the plane positioned behind you or is going into the plane of the screen or paper then leave the assigned letter as is. However, if item #4 is positioned in front or is coming out of the plane of the screen or paper, then assign the opposite letter from what you assigned in Step 3.
        2. If item #4 is coming out of the plane of the screen or paper? assign the opposite letter
          1. If R was chosen: new assignment is S
          2. If S was chosen: new assignment is R
        3. Remember, if item #4 is within the plane of the screen or paper: redraw the image to have item #4 going into the plane of the screen or paper before determining clockwise or counterclockwise (see step 3 above).

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