Assigning Stereochemistry

RULES
TIPS & EXAMPLES
Look for stereoisomerism especially in these three circumstances:
1. When there are multiple substituents on a ring.
  • Characterize two groups as being
    cis = same side of the ring or
    trans = opposite side of the ring
The easiest way to see this is to either draw in all the hydrogens or draw the ring flat and use wedges and dashes for each substituent.

molecule
2. Whenever both carbons in a double bond contain different groups
  • When there are two hydrogens you may characterize those as being cis or trans.
  • When 0 or 1 hydrogen is attached to the double bond you must describe the highest priority groups (see below) as being:
    Z = zusammen = "zame zide" or
    E = entgegen = opposite sides
If either C atom in a double bond contains the same two groups there is NO stereoisomerism possible.

Divide the double bond as shown to determine cis/trans and E/Z. If both of the groups are in the same color region then they are said to be cis. If the two grous are in opposite color regions then it is trans.

molecule
relationship
groups
a&b a&c a&d b&c b&d c&d
cis or Z
box
box
checked box
checked box
box
 box
trans or E box checked box box box checked box box

I circle the higher priority group on each carbon to quickly see E from Z.
3. Whenever there are four different groups on a carbon atom this is called a stereogenic center.
  • each group is prioritized (see below)
  • the lowest priority group is placed in back
  • if tracing a path from highest to lowest priority leads in a
    • clockwise (right) direction then you use an R (rectus = right)
    • counterclockwise (left) direction then you use an S (sinister = left)
An odd number of stereogenic centers (*) requires that the molecule be chiral. If there is an even number of stereogenic centers you must look for internal planes of symmetry.

Any molecule that can be drawn to show an internal plane of symmetry is NOT chiral.

molecule
Prioritizing (otherwise known as "The Cahn-Ingold-Prelog System")
  • Each group is assigned priority based on the connecting atom. A higher atom number (mass number is used for isotopes) means a higher priority.
  • When two substituents have the same first element, the attached atoms break the tie.
    • double bonds are treated as two bonds to the same element (triple bonds as three)
  • Keep going out, one atom at a time, until the tie is broken.
Examples:

-I is higher than -Br

-D (2H) is higher than -H (1H)

An aldehyde group, -(C=O)H, is higher priority than a methanol group, -CH2OH

  • -(C=O)H has 2 C-O bonds
  • -CH2OH has 1 C-O bond

BrCH2CH2- is higher priority than
ICH2CH2CH2-

  • The C atoms are tied: 1C and 2H bonds each.
  • C attached to Br, C, and 2H wins versus
  • C attached to 2C and 2H