Chemistry 106
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Text & Lab Book (+ all lab safety stuff)
Online Notes access:
The 3-D Structure of Molecules
-StereochemistryFor additional help with stereochemistry, check out the Organic Chemistry section of the following
• Consitutional isomers
– Atoms are bonded to
different atoms,
therefore, different
bonding patterns different IUPAC names &
structural formulas
– Structural
– Positional
– Functional group
• Stereoisomers
– Atoms are oriented
differently, but same
bonding patterns
– Cis-Trans - orientation
around a double bond
– Tetrahedral C atom
mirror images
• Chiral vs. Achiral
The mirror image of the
right hand is the left hand.
Conversely, the mirror
image of the left hand is the
right hand.
Chirality is based on whether a molecule has a “chiral” center.
– Carbon atom with FOUR DIFFERENT attached (tetrahedrally) groups
(the atom + attached atoms which are attached to the center carbon)
– Chiral molecules = mirror images that are NOT superimposable
• (left vs. right handedness)
– Achiral molecules = mirror images that are superimposable
• (no left vs. right handedness)
Important Biochemical example:
– Left or right handedness of monosaccharides is determined by the position of OH on the chiral center.
– Naturally occurring monosaccharides are almost always right-handed.
– Plants produce only right-handed monosaccharides
Stereoisomerism - molecules that have the
same molecular AND structural formulas but different orientation of
In order for molecules to exhibit stereoisomerism
they must have:
A chiral center
Structural rigidity
This is the basis for cis-trans isomerism
Two types of Stereoisomers
a) Enantiomers - molecules that are nonsuperimposable
mirror images
Ex.: Left & right handed with single chiral centers
b) Diastereomers - molecules that are not mirror images
Ex.: Cis-trans (possible in some rings and around double bonds)
- introduced as a sedative and antiemetic in later 1950s and withdrawn in 1961 due
to its teratogenicity.
an example of a chiral center in a cyclic compound
Fischer Projections
• 2- dimensional structural notation showing the spatial
arrangement of groups around chiral centers (to
show handedness)
• Tetrahedral geometry:
– Vertical lines = bonds directed into the page
– Horizontal lines = bonds directed out of the page
Fischer Projections
Carbon chain is positioned vertically, with the carbonyl group at or near
the top.
Ex.: glyceraldehyde (2,3-dihydroxypropanal)
– Dextro = Right
– Levo= Left
– Determine “D” vs “L” by examining the
position of the functional group on the
chiral center
Compounds with multiple chiral centers
• Naming is complex
– Use the highest # chiral C atom in the chain to determine “D”
or “L”.
– If there are 2 or more “D”s and 2 or more “L”s, use different
common names for each pair.
– Ex. 2,3,4-trihydroxybutanal
Number of Stereoisomers possible for a
particular molecule:
• General rule*
– # of isomers = 2n (n = # of chiral centers)
• *Sometimes symmetry considerations make some mirror
images superimposable.
Properties of Isomers
• Constitutional - differ in most physical and
chemical properties
• Diastereomers - differ in most physical and
chemical properties
• Enantiomers - differ in only two properties:
– Interactions with plane-polarized light (ppl)
– Interactions with other chiral substances
Dextrorotatory & Levorotatory
• An enantiomer (chiral cpd) that rotates “ppl” in a
clockwise direction is dextrorotatory. (+)
• An enantiomer (chiral cpd) that rotates “ppl” in a
counterclockwise direction is levorotatory. (-)
• The handedness of enantiomers and the direction of
rotation are, unfortunately, not related.
Interactions Between Chiral
Enantiomers have the same FP, BP, density, etc.
– Properties depend on IMF
• IMF does not depend on Chirality
• IMF depends on functional groups
• Enantiomers have the same solubility in achiral solvents
(ethanol), but different solubility in chiral solvent (D-2-butanol).
• Rate & Extent of Reaction of Enantiomer is the same with an
achiral reactant but different with another chiral reactant.
• Receptor sites for molecules in the body have chirality, so
enantiomers generate different responses.
Enantiomers react differently to taste buds:
spearmint vs.carroway
beneficial vs. teratogenic
D-Epinephrine binds to the receptor at three points.
The human body exhibits a response to the D form that is 20 times greater than the response to
the L form.