The 3-D Structure of Molecules
-
Stereochemistry
For additional help, check out the Organic Chemistry section of the following website:
http://www.khanacademy.org/#browse
Scroll down to the items on “Chirality”, etc.
Isomerism
Consitutional isomers - Atoms are bonded to different atoms - different IUPAC names
& structural formulas
Structural
Positional
Functional group
Stereoisomers
Atoms are oriented differently
Cis-Trans - orientation around a double bond
Tetrahedral C atom mirror images
Chiral vs. Achiral
Chirality
Chirality is based on whether a molecule has a “chiral” center.
Carbon atom with FOUR DIFFERENT attached (tetrahedrally) groups
Chiral molecules = mirror images that are NOT superimposable
vs.
•(left
right handedness)
Achiral molecules = mirror images that are superimposable
vs.
•(no left
right handedness)
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 atoms
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 Stereoisomer
Enantiomers - molecules that are nonsuperimposable mirror images
Ex.: Left & right handed with single chiral center
Diastereomers - molecules that are not mirror images
Ex.: Cis-trans (possible in some rings and around double bonds
Thalidomide - an example of a chiral center in a cyclic compound
Fischer Projection - 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
Carbon chain is positioned vertically, with the carbonyl group at or near the top.
Ex.: glyceraldehyde (2,3-dihydroxypropanal)
Latin:
–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 Compounds
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 Compounds
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
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
repsonse to the L form.