Chemistry 106 You need: Text & Lab Book (+ all lab safety stuff) Online Notes access: http://learn.roguecc.edu/science/ploozen/ The 3-D Structure of Molecules -StereochemistryFor additional help with stereochemistry, check out the Organic Chemistry section of the following website: http://www.khanacademy.org/#browse 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 • 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 Chirality 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around double bonds) Enantiomers Diastereomers Thalidomide - introduced as a sedative and antiemetic in later 1950s and withdrawn in 1961 due to its teratogenicity.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 = 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 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.