Chemistry 104

Lecture #13: Stereoisomers

Read: pg. 473-492 and 497-500

HW: pg. 493-496 (1a,b,d,g,i,k,l,m, 2, 5, 6, 9, 11a, 12, 13b, 17a, 28, 25b,e,26a,d, and 42 or 43) and pg. 500 (1,2,3)

Optional Reading: Mirror-Image Molecules, Science News, vol 143, pg. 348, 1993.

!!Great Job on Posters !!

Objectives:

Geometric stereoisomers

  1. Be able to draw cis and trans stereoisomers for alkenes and cycloalkanes (e.g., 2-butene and 1,2-dimethylcyclobutane).
  2. Know that when light converts cis-retinal to trans-retinal the change in shape of the molecule initiates a nerve impulse so we see something.

    3. Enantiomers

  3. Recognize enantiomers (nonsuperimposable mirror image isomers).
  4. Be able to identify chiral carbons.
  5. Know that enantiomers have the same physical properties except that they rotate polarized light in opposite directions (know what polarized light is). The enantiomer that rotates light to the right is the (+) enantiomer, like (+) lactic acid. The enantiomer that rotates light to the left is the (-) enantiomer. A mixture of both the (+) and (-) enantiomers is called a racemic mixture and is designated as (± ) like (± ) lactic acid. The direction of rotation is determined in the lab. There is no way to predict which way a chiral compound will rotate light.
  6. (+) Enantiomers are sometimes called dextrorotary (because they rotate light to the right and are designated as d-. d-Lactic acid is the same as (+) lactic acid. (-) enantiomers are also called levorotary (because they rotate light to the left) and are called l-. l-Lactic acid is the same as (-)lactic acid.
  7. Capital letters D and L distinguish between enantiomers BUT DO NOT indicate the direction of rotation of polarized light.
  8. Know that enantiomers have very different biological activities (because enzymes and receptors (gigantic polymers of aminoacids; i.e., big beautiful 3-D protein molecules) are like a glove and will fit only the right or left handed enantiomer. Enzymes and receptors are "stereospecific". In other words, they fit one hand better than the other.