Stereochemistry Quiz: Isomers and Structures

Stereochemistry

• Stereochemistry is the study of the three-dimensional structure of a molecule.
• Isomers are different compounds with the same molecular formula. Only the 3-D arrangement of atoms differ.
• Two main classes of isomers are constitutional (structural) isomers and stereoisomers.
• Constitutional isomers differ in how atoms are connected.
• Stereoisomers differ only in the way atoms are oriented in space.

Thalidomide

• Used as a sedative and anti-nausea drug for pregnant women.
• Sold as a mixture of two enantiomers.
• Enantiomer 1 had the desired therapeutic effect.
• Enantiomer 2 caused catastrophic birth defects in children born to women who took the drug during pregnancy.

Starch and Cellulose

• Recall that stereochemistry is the three-dimensional structure of a molecule.
• Starch is a main carbohydrate found in seeds and roots of plants.
• When consumed, starch is hydrolyzed to glucose.
• Cellulose is nature's most abundant organic material, providing rigidity to tree trunks and plant stems.
• Complete hydrolysis of cellulose also forms glucose. Humans cannot metabolize cellulose to glucose.

Starch and Cellulose (Repeating Units)

• Starch and cellulose are both composed of the same repeating unit.
• The repeating unit is a six-membered ring with an oxygen atom.
• Three OH groups are attached to the ring.
• They differ by the position of the oxygen atom joining the rings together.
• In cellulose, the joining oxygen atom uses equatorial bonds.
• In starch, the joining oxygen atom uses one equatorial and one axial bond.

Isomers, Constitutional and Stereoisomers

• There are two kinds of isomerism - constitutional and stereoisomers.
• Constitutional isomers differ in the way their atoms are connected.
• Stereoisomers differ in the spatial arrangement of their atoms.

Configuration

• Configuration refers to a specific three-dimensional arrangement of atoms.
• Configuration is important to isomers. Isomers have different configurations.

Chiral and Achiral Molecules

• Chiral molecules are not superimposable on their mirror images
• Examples of chiral molecules are your hands. The left hand and right hand are mirror images, but they are not superimposable on each other
• Achiral molecules are superimposable on their mirror images. Examples of achiral molecules include socks.
• To test for chirality, draw a molecule and its mirror image and see if superimpose on each other.

Stereogenic Centers

• Stereogenic centers are atoms in a molecule where the interchange of groups leads to a stereoisomer.
• To identify a stereogenic center, examine each tetrahedral carbon atom (not part of a ring). Count the groups attached to that carbon. If all four groups are different, it is a stereogenic center.

Drawing a Pair of Enantiomers

• To draw a pair of enantiomers, draw the first molecule and use a mirror image to draw a second molecule (the mirror image).

Enantiomers and Stereogenic Centers

• A molecule with one stereogenic center exists as a pair of enantiomers.
• Any molecule with a tetrahedral stereogenic center is chiral and exists as a pair of enantiomers.

Labeling Stereogenic Centers

• Rule 1: Assign priorities to groups bound to stereogenic center in order of decreasing atomic number. Highest atomic number = priority 1
• Rule 2: If two identical atoms are on the stereogenic center, prioritize based on the atomic number of atoms further bonded to identical atoms on the stereogenic center.
• Rule 3: If two isotopes bound to stereogenic center, assign priorities in order of decreasing atomic mass number.
• Rule 4: Multiple bonds treat equivalent numbers of singly bonded atoms.

Diastereomers

• Diastereomers are stereoisomers that are not mirror images.
• With two stereogenic centers, the maximum number of stereoisomers = 4 (some combinations might be superimposable/identical)
• To find the other stereoisomers, change the position of groups on one stereogenic center only and draw its mirror image.

Meso Compounds

• A meso compound has more than one stereogenic center, but the number of stereoisomers is less than the maximum.
• The mirror image of a meso compound is superimposable on the compound itself.

Stereoisomers in Compounds with Two or More Stereogenic Centers

• When a compound has more than one stereogenic center, assign R/S for each center.

Disubstituted Cycloalkanes

• If a cycloalkane has two substituents, the cis isomer is achiral; it is identical to its mirror image.
• The trans isomer is chiral; it is not superimposable on its mirror image.

Optical Activity

• Plane-polarized light passes through a polarimeter.
• Chiral compounds rotate the plane of polarized light.
• Optically active compounds rotate polarized light, while achiral compounds do not.
• Achiral compounds have no effect on the plane of polarized light.

Racemic Mixtures

• A racemic mixture consists of equal amounts of two enantiomers.
• Racemic mixtures are optically inactive; they do not rotate polarized light.

Specific Rotation

• Specific rotation equation = [α] = α / (l x c).
• α: observed rotation
• l: length of sample tube in decimeters
• c: concentration in g/mL

Enantiomeric Excess

• Enantiomeric excess is the percentage of one enantiomer - percentage of the other enantiomer .
• Enantiomeric excess tells us how much of one enantiomer is in excess compared to the other.

Physical Properties of Diastereomers

• Diastereomers have different physical properties compared to enantiomers.
• Thus, diastereomers can be separated from each other using common separation techniques like distillation.

Chemical Properties of Enantiomers

• Enantiomers have identical chemical properties except of their reaction with chiral nonracemic reagents.