Chemistry 106You need:

Text & Lab Book (+ all lab safety stuff)

Online Notes access:

http://learn.roguecc.edu/science/ploozen/

The 3-D Structure of Molecules-Stereochemistry-

For 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• 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

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 Stereoisomersa) 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Thalidomide - introduced as a sedative and antiemetic in later 1950s and withdrawn in 1961 due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

Thalidomidebeneficial 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.