Stereochemistry Terminology
Axis of symmetry: When an operation on an axis Cn, where n = 360°/rotation°, leads to a
structure indistinguishable from the original.
C2
R
R
R
180°
R
Plane of symmetry: (σ) A plane that divides the molecule into two identical halves. Also
visualized as a mirror plane in which one half of the molecule reflects its enantiomeric image.
σ
R
R
Center of symmetry: a formal point in the center of the molecule, in reference to which each
atom present finds its equivalent upon extension of an imaginary line of similar length to that
joining it to i.
R
i - center of symmetry
R
Axis of rotation-reflection: (Sn) present in molecules that can be rotated in such axis by and
angle of 360°/n and then reflected through a plane perpendicular to the axis to provide a
structure identical to the original.
R
S4
R
R
90°
R
R
R
R
R
reflection R
R
R
R
Asymmetric molecule: A molecule that lacks all symmetry elements. Note that molecules can
be chiral and have an axis of symmetry - they are not asymmetric - they are dissymmetric.
Absolute configuration: A description for the absolute orientation of atoms in space about a
given stereogenic center.
Relative configuration: A description of the orientation of a stereogenic center as it relates to
another, rather than by its actual orientation in space.
CHEM 745 – Organic Synthesis Spring 2015
Fischer projection: A format for displaying 3-dimensional structures in two dimensions. The
carbon backbone is drawn vertically.. Stereogenic carbons lie in the plane of the paper. The
vertical bonds project back into the paper, and horizontal bonds project up away toward the
viewer.
CO2H
H
NH2
CH2 OH
CO2H
H
NH2
CH2 OH
=
D & L designation: A system of configuration nomenclature, historically based on glyceraldehyde,
stems from the Fischer projection in which the highest oxidation state carbon is placed on the
top. The position of the H and non-H groups in the horizontal bonds determines the D or L
assignment. If H is left and non-H is right it is D and vice versa for L.
CHO
H
OH
CH2 OH
CHO
HO
H
CH2 OH
D-glyceraldehyde
L-glyceraldehyde
Cahn-Ingold-Prelog designation: A system for prioritizing groups and define the absolute
configuration with the R and S designation.
1) Look at the atom directly attached to the stereogenic atom. The higher atomic number
takes priority.
2) If a distinction cannot be made at the 1st atom, continue out until you can make a distinction
and no further.
3) Multiple bonds are equivalent to the same number of single bonds for priority
determination.
4) Point the lowest group away from you making the other three groups define a circle. If you
travel from 1 to 2 to 3 clockwise, the configuration is (R), counterclockwise (S).
Prochiral: A molecule in which a substitution, exchange, or other reaction leads to the formation
of a stereogenic center.
Homotopic: A molecule where the substitution of one or the other like groups generates the same
molecule.
homotopic
H
H 3C
H
CH3
CHEM 745 – Organic Synthesis Spring 2015
H
H 3C
A
CH3
=
A
H 3C
H
CH3
Enantiotopic: A molecule where the substitution of one or the other like groups generates
enantiomeric molecules.
enantiotopic
H
H
H3CH 2C
H
CH3
A
H3CH 2C
A
enantiomers
CH3
H3CH 2C
H
CH3
Diastereotopic: A molecule where the substitution of one or the other like groups generates
diastereomeric molecules.
diastereotopic
H
H 3C
H
H
H 3C
CH3
H OH
A
diastereomers
CH3
H OH
A
H 3C
H
CH3
H OH
Re & Si face designation: Used to specify the configuration of heterotopic faces - based on the
CIP priority rules.
O
Ph
2
si
1
H
3
O
H
3
1
Ph
2
re
P & M designation: Used to specify the configuration of helical chirality. As the helix comes out
of the paper toward the viewer, if it is clockwise then the designation is M, likewise P for
counterclockwise.
M
P
Cis & trans designation: Used to designate like groups on the same side (cis) or opposite sides
(trans) of a double bond or ring.
CHEM 745 – Organic Synthesis Spring 2015
E & Z designation: Used to designate the highest priority groups being in a cis (Z) arrangement
or a trans (E) arrangement.
Erythro & threo diastereomers: Used to designate the relative configuration of two stereogenic
centers with identical groups where a Fisher projection being the frame of reference. Erythro
has the groups on the same side, while threo has the groups on opposite sides.
CO2H
H
OH
H
OH
CO2H
erythro
tartaric acid
CO2H
H
OH
HO
H
CO2H
threo
tartaric acid
Syn & anti diastereomers: Used to designate the relative configuration of two stereogenic
centers comparing the groups being on the same side (syn) or opposite sides (anti) where the
fully extended staggered carbon chain is used as the frame of reference.
OH
HO2C
OH
CO2H or
HO2C
OH
OH
CO2H
OH
syn-tartaric acid
HO2C
CO2H
OH
anti-tartaric acid
Meso: A molecule which contains stereogenic centers, but is achiral. A meso compound has a
plane of symmetry.
HO
HO2C
OH
CO2H
meso tartaric acid
α & β designation: Historically used in steroid compounds - α being a down orientation and β
being an up orientation.
Stereoselective reaction: A reaction in which one stereoisomer is preferentially formed over
another.
Stereospecific reaction: A reaction is termed stereospecific if, in such a reaction, starting
materials differing only in their configuration are converted to stereoisomerically distinct
products.
CHEM 745 – Organic Synthesis Spring 2015
Asymmetric induction: Any factor contributing to the selective formation of a new stereogenic
center.
Internal asymmetric induction: The formation of a new stereogenic center with respect to the
simultaneous formation of another.
Relative asymmetric induction: The presence of an existing stereogenic center dictating the
selectivity for the formation of a new stereogenic center.
Syn & anti addition: Used to describe an addition reaction where two partners react on the same
face (syn) or opposite face (anti) of a reactant.
Antiperiplaner: An orientation of two groups on adjacent atoms that are in the same plane, but
180° apart.
Kinetic resolution: A process by which two stereoisomers are formed at different rates in a
reaction. Often referring to the formation of one enantiomer over another.
Racemization: A process which converts one enantiomer into the other.
Epimerization: A process which inverts the configuration of one stereogenic center in a molecule
and not other.
Gauche, eclipsed, and anti conformations: The orientation of groups based on the rotation
about a bond.
CH3
H 3CCH3
CH3
H
H
H 3C
H
H
H
H
H
CH3
anti-butane
H
gauche-butane
HH
H
H
eclipsed-butane
Ring strain: The extra strain associated with the constraints of ring compounds. Made up of
torsional strain, angle strain, and steric strain.
Allylic strain: The strain associated with the constraints of a double bond in the 1,3-relationships.
R
CHEM 745 – Organic Synthesis Spring 2015
R'
Anomeric effect: The stereoelectronic stabilization found when an electron withdrawing group is
placed in the axial position of an anomeric carbon. Due to filled pi orbital on oxygen
overlapping with unfilled sigma* orbital of the electron withdrawing group.
O
O
X
X
more stable
O
π
X
CHEM 745 – Organic Synthesis Spring 2015
σ*
π
O
σ*
X