Stereoisomers

1
Stereoisomers

• Review
• Structural Isomers Compounds that have the same
  molecular formula, but differ in the structural
  arrangement of atoms.
• Examples 1-butene, 2-methyl-propene
• Geometric Isomers Compounds that have the same
  molecular formula, the same connectivity between
  atoms, but differ in the arrangement of groups
  attached to the double bond.
• Examples cis-2-butene, trans-2-butene
• Stereoisomers Isomers with the same molecular
  formula, same connectivity of atoms, but
  different arrangements of the atoms in 3-D space.
  
• Geometric Isomers are a form of stereoisomer.
• Enantiomers Chiral compounds that are
  nonsuperimposable mirror images make up a pair of
  enantiomers. Enantiomers behave the same in
  almost every aspect of physical and chemical
  properties. The only physical property that
  differs for a pair on enantiomers is optical
  activity. The only difference in chemical
  properties is in the way they react with other
  chiral compounds.
• Diastereomers Stereoisomers that are not
  enantiomers. Diastereomers do not have the same
  chemical and physical properties.
• Chirality Objects that have handedness are said
  to be chiral. They can not be superimposed on
  their mirror image. Objects that can be
  superimposed on their mirror image are said to be
  achiral (without chirality).
• If a molecule has an internal symmetry plane, it
  is achiral. A lack of an internal mirror plane
  indicates chirality. See the examples below, and
  determine which are chiral and which are achiral

2
Optical Activity

• Light travels in waves at all angles. Light can
  be plane polarized so that it travels only in one
  plane rather than at all angles. The light will
  appear the same to you and I.
• A chiral compound and its nonsuperimposable
  mirror image make up a pair of enantiomers.
  Enantiomers are often referred to as optical
  isomers because they are optically active.
• An optically active molecule will rotate plane
  polarized light either to the left (levorotatory)
  or the right (dextrorotatory). Enantiomers
  rotate plane polarized light in the exact
  opposite direction.
• If a molecule has two or more chiral centers,
  life gets more complicated due to the presence of
  meso compounds and diastereomers. In an attempt
  to make life a little easier, we will always draw
  these molecules as Fischer Projections.

3
The Significance of Enantiomers

• Enantiomers react differently with chiral
  compounds. Life is made up of chiral compounds.
  Amino acids used for
• biosynthesis in animals are always of the L
  conformation. We can obtain energy only from the
  D forms of
• carbohydrates. Enantiomers can be the difference
  between life and death. Enantiomers are
  important to
• Pharmaceutical companies
• Agriculture
• Food additive industries

4
Problems
Are the following pairs (1-3) Identical,
Enantiomers, Diastereomers, Unrelated, and/or
Meso 1. 2. 3. 4. Circle all
compounds above that will be optically active.