CHEM301 Ch.5 Stereochemistry

Allenes

-Compounds that contain the C=C=C unit, with two C=C double bonds meeting at a single carbon atom. -No chiral carbon, but if different substituents are added to the two sides the molecule can be chiral.

Configurations

The two differing (mirror-image) spatial arrangements of a chiral atom.

Stereochemistry of Molecules with Two or More Asymmetric Carbons (2n Rule)

-A compound with n asymmetric carbon atoms might have as many as has 2^n stereoisomers. -This formula is called the 2^n rule, where n is the number of chirality centers (usually asymmetric carbon atoms). -The 2^n rule suggests we should look for a maximum of 2^n stereoisomers. We may not always find 2^n isomers, especially when two of the asymmetric carbon atoms have identical substituents. -Rule will not apply to compounds w/ internal plane of symmetry.

Racemic mixture

-A mixture that contains equal amounts of the (+) and (-) enantiomers. Results in a net zero rotation for the mixture. - A reaction that uses optically inactive reactants and catalysts cannot produce a product that is optically active. Any chiral product must be formed as a racemic mixture (can't make optically active product from non-active reagents).

Chirality of Conformationally Mobile Systems

-A molecule cannot be optically active if its chiral conformations are in equilibrium with their mirror images. We consider such a molecule to be achiral. -To determine whether a conformationally mobile molecule can be optically active, consider its most symmetric conformation.

Absolute and Relative Configuration

-ABSOLUTE CONFIGURATION: The detailed stereochemical picture of a molecule, including how the atoms are arranged in space. Alternatively, the (R) or (S) configuration at each chirality center. -RELATIVE CONFIGURATION: The experimentally determined relationship between the configurations of two molecules, even though we may not know the absolute configuration of either.

Optical activity

-Activity observed when polarized light passes through a solution containing a chiral compound, the chiral compound causes the plane of vibration to rotate. -Enantiomeric compounds rotate the plane of polarized light by exactly the same amount but in opposite directions. -We cannot predict which direction a particular enantiomer [either (R) or (S)] will rotate the plane of polarized light. -Dextrorotatory (clockwise) rotations are (+) or (d). -Levorotatory (counterclockwise) rotations are (-) or (l).

Meso compounds

-An achiral compound that has chirality centers (usually asymmetric carbons). -Will have internal plane of symmetry. -A meso compound with two chirality centers will be (R,S) or (S,R) because the chirality centers must be mirror images of each other, reflected across the internal mirror plane.

Internal mirror plane

-Any molecule that has an internal mirror plane of symmetry cannot be chiral, even though it may contain asymmetric carbon atoms. -When we cannot find a mirror plane of symmetry, that does not necessarily mean that the molecule must be chiral.

Chiral/Asymmetric Atom

-Chiral center of a molecule -Must have 4 different bound groups -Most often carbon, others possible (ex. nitrogen) -Type of stereocenter

Physical Properties of Diastereomers

-Enantiomers have identical physical properties except for the direction in which they rotate polarized light, BUT diastereomers, on the other hand, generally have different physical properties.

Conformational Enantiomerism

-Highly bulky/strained molecules can be conformationally locked, or unable to interconvert forms. -Each conformation must be assessed individually for chirality. -If two of its conformations form nonsuperimposable mirror images, they are optically active and can be said to be conformational enantiomers.

Fischer projection

-Horizontal lines represent wedges (projecting outward). -Carbon chain, if more than one carbon, should be vertical. -Vertical lines represent dashes (projecting inward). -Rotation by 180° is allowed. -A 90° rotation is NOT allowed (usually produces enantiomer)

Chromatographic Resolution of Enantiomers

-In some cases, enantiomers may be resolved by passing the racemic mixture through a column containing particles whose surface is coated with chiral molecules. -As the solution passes through the column, the enantiomers form weak complexes, usually through hydrogen bonding, with the chiral column packing. -The solvent flows continually through the column, and the dissolved enantiomers gradually move along, retarded by the time they spend complexed with the column packing.

Cahn-Ingold-Prelog Convention Overview

-Most widely accepted system for naming the configurations of chirality centers. -Each asymmetric carbon atom is assigned a letter (R) or (S) based on its three-dimensional configuration

Assigning (R) and (S) Configurations from Fischer Projections

-Normal priority rules apply, but because lowest priority will typically be on horizontal line (projecting outwards), we must reverse our direction (i.e. clockwise on Fischer is [S]).

Chemical Resolution of Enantiomers

-Resolution=Separation of individual enantiomers. -Diastereomers have different physical properties, and can be separated relatively easily (distillation, recrystallization, chromatography, etc). -A chiral probe is necessary for the resolution of enantiomers; such a chiral compound or apparatus is called a resolving agent.

Drawing Mirror Images of Fischer Projections/Testing Chirality

-Rotate 180° -If nonsuperimposable, molecule is chiral and the two enantiomers are represented

Diastereomers

-Stereoisomers that are not mirror images. -Most diastereomers are either geometric isomers or compounds containing two or more chirality centers. -Geometric/cis-trans isomers are an example.

Optical purity (o.p.)

-The ratio of a mixture's rotation to the rotation of a pure enantiomer. -Ex. if we have some [mostly (+)] butan-2-ol with a specific rotation of +9.72°, we compare this rotation with the +13.5° rotation of the pure (+) enantiomer to calculate O.P.

Enantiomeric excess (e.e.)

-To compute the enantiomeric excess of a mixture, we calculate the excess of the predominant enantiomer as a percentage of the entire mixture. -For a chemically pure compound, the calculation of enantiomeric excess generally gives the same result as the calculation of optical purity, and we often use the two terms interchangeably.

Cahn-Ingold-Prelog Convention Steps

1) Assign a priority to each group (1 highest, 4 lowest). Atoms with higher atomic numbers receive higher priorities. In case of ties, use the next atoms along the chain of each group as tiebreakers. Treat double and triple bonds as if each were a bond to a separate atom (note that when you break a bond, you always add two imaginary atoms). 2) Rotate molecule so that lowest priority group is facing away. Draw arrow from first priority group through second and to third. Classify R or S.

Chirality Rules

1. If a compound has no asymmetric carbon atom, it is usually achiral. 2. If a compound has just one asymmetric carbon atom, it must be chiral. 3. If a compound has more than one asymmetric carbon, it may or may not be chiral.

Constitutional Isomers

Differ in the order of attachment of atoms

Stereoisomers

Have the same bonding sequence, but differ in the orientation of their atoms in space.

Plane-Polarized Light

Light composed of waves that vibrate in only one plane. Useful for distinguishing between enantiomers via polarimetry.

Chiral

Molecules that have non-superimposable left-handed and right-handed forms

Specific Rotation [alpha]

The rotation found using a 10-cm (1-dm) sample cell and a concentration of 1 g/mL.