Chemistry 106 You need: Text & Lab Book (+ all lab safety stuff) Online Notes access:

The 3-D Structure of Molecules -Stereochemistry- For additional help with stereochemistry, check out the Organic Chemistry section of the following website:

Isomerism 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

Chirality 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 The mirror image of the right hand is the left hand. Conversely, the mirror image of the left hand is the right hand.

Chirality

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 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)

Enantiomers Diastereomers

Thalidomide - introduced as a sedative and antiemetic in later 1950s and withdrawn in 1961 due to its teratogenicity.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) 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 = 2 n (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 Thalidomide 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.