STEREOCHEMISTRY
ALEC KANAI
FIRST-YEAR ENGINEERING STUDENT
UNIVERSITY OF PITTSBURGH 2019
DASH AND WEDGE STRUCTURES
• Regular bond lines are in the plane of the page
Carbon chain
in plane of
board
CH3 going
into board
• Dashes represent bonds going into the page
• Solid wedges represent bonds coming out of the page
CH3 coming
out of the
board
PRACTICING DASH AND WEDGE STRUCTURES
Draw 2-chloro-2-methylbutane using dash and wedge structures. (hint: there is
more than one right answer, but draw only one)
PRACTICING DASH AND WEDGE STRUCTURES
Challenge: Draw isopropyl cyclohexane using dash and wedge structures.
CHIRALITY MEANS “HANDEDNESS”
We can label molecules as well as
individual carbons within the molecule.
Chiral molecules DO NOT have
superimposable mirror images.
Chiral carbons in a molecule are
called chiral centers.
DETERMINING CHIRAL MOLECULES
Superimposed configuration
Mirror image is superimposable on the original,
so chlorofluoromethane is ACHIRAL .
ACHIRAL MOLECULES
The simplest way to determine if a
molecule is achiral, is to find an
internal plane of symmetry.
A molecule only needs one plane of
symmetry to be considered achiral.
DETERMINING CHIRAL MOLECULES CONT’D
Enantiomers
Non-superimposable mirror images
Bromochlorofluromethane is a CHIRAL molecule.
If the mirror image is not superimposable,
they are deemed enantiomers.
DETERMINING CHIRAL CENTERS
chiral center = stereocenter = asymmetric center = asymmetric carbon = chiral carbon
1.
Chiral centers must have FOUR different
substituents.
2.
Chiral centers MUST be sp3 hybridized.
HOW DO WE DISTINGUISH
BETWEEN STEREOISOMERS
THROUGH NOMENCLATURE?
2-flurobutane
R-2-flurobutane
S-2-flurobutane
PRIORITIZING SUBSTITUENTS
Rule 1: Look at the atoms directly bonded to the stereocenter. The higher
atomic number takes precedence over lower atomic number. Hydrogen will
always be the lowest priority.
Elements bonded directly
to carbon stereocenter:
• Bromine at#35
• Hydrogen at#1
• Carbon at#6
• Oxygen at#8
a
d
c
b
DETERMINING R OR S
Look down the lowest-priority bond:
a
a
d
c
b
Clockwise: R
d
b
c
Counter clockwise: S
a
c
d
Clockwise, so this is the R conformer of
1-bromoethanol.
b
IUPAC name: (R)-1-bromoethanol
WHAT IF THE ATOMS ARE THE SAME?
Rule 2: If two or more atoms directly bonded to the stereocenter share
atomic numbers, advance to the next atom(s) out from the center until a
point of difference is found
Elements bonded directly
to carbon stereocenter:
• Fluorine
a
• Hydrogen
d
c
• Carbon 1
b
• Carbon 3
We know Fluorine will be of highest
precedence, and Hydrogen will be
of the lowest.
Carbon 1 will be lower in
precedence than in carbon 3.
a
c
d
b
Clockwise, so this is the R conformer of
2-fluorobutane.
IUPAC name: (R)-2-flurobutane