Organic Ch 5

• Constitutional Isomers
• Stereoisomers

different compouynds with same molecular formula

isomers whose atoms hae a different connectivity

isomers that have the same connectivity but differ in spatial arrangement of their atoms

• Enantiomers
• Diastereomers

stereoisomers that are nonsuperposable mirror images of each other

stereoisomers that are NOT mirror images of each other

an object is achiral if the object and itsmirror image are identical

a chiral object is one that cannot be superposed on its mirror image

One enantiomer causes birth defects, the other cures morning sickness

one enantiomer is a bronchodialator, the other inhibits platelet aggregation

For a chiral molecule is usually only favorable in one way

chiral

NO

enantiomeric

molecules that contain a carbon atom bonded to four different groups

with an asterisk (*)

Any atom at which an interchange of groups produces a stereoisomer

chirality centers

• (+)-limonene is found in oranges
• (-)-limonene is found in lemons

to alleviate the symptoms of morning sickness in pregnant women

caused horrible birth defects in many children born subsequent ot the use of the drug

An imaginary plane that bisects a molecule such that the two halves of the molecule are mirror images of each other

achiral

• Assign proproties to the four different groups directly attached to the stereocenter from highest to lowest (proproty is based on atomic number, the higher the atomic number, the higher the priority)
• When a proprity cannot be assigned on the basis of the atomic number of the atoms that are directly attached to the chirality center, then the next set of atoms in the unassigned groups is examined. this process is continued until a decision ca be made
• visualize the molecule so that the lowest proprity group is directed away from you, then trace a path from the highest to lowest proprity. If the path is a clockwise motion, then the configuration at the asymmetric carbon is (R). If the path is a counter0clockwise motion, then the configuration is (S)
• For groups containing double or triple bonds, assign priorities as if both atoms were duplicated or triplicated

identical physical properties (melting point, boiling point, refractive index, solubility, etc)

Only when they interact with other chiral substances

turn plane-polarized light on opposite direction

• The electric field of light is oscillating in all possible planes
• When this light passes through a polarizer (plarized lens), we get plane-polarized light (oscillating in only one plane)

A device for measuring the optical activity of a chiral compound

• o
• Specific rotation [α]
• [α]=α/c*l

• [α]=specific rotation
• α=observed rotation
• c=concentration of the solution (g/mL)
• l=length of the cell in decimeters

Two enantiomers should have the same value of specific rotation, but the signs are opposite

An equimolar mixture of two enantiomers

A racemic mixture causes no net rotatiom of plane-polarized light

A sampe of an optically active substance that consistus of a single enantiomer

Enantiomeric excess

% enantiomeric excess=((mol of one enantiomer)-(moles of another enantiomer))/total moles of both enantiomers

% enantiomeric excess= observed specific rotation/specific rotation of the pure enantiomers * 100%

Reactions that lead to a preferentil formation of one stereoisomer over other stereoisomers that could possible be formed

If a reaction produces preferentially one enantiomer over its mirror image

If a reaction leads preferentially to one diastereomer over others that are possible

2^n; where n= the number of tetrahedral stereogenic centers

no

yes

Because enantiomers have very similar properties, and diastereoisomers dont, and are therefor easy to separate

• Achiral despite the presence of stereocenters
• not optically active
• superposable on its mirror image
• has a plane of summetry

a 2-dimensional representation of chiral molecules

bonds that project behind the plane of the paper

Bonds that project out of the plane of the paper

• both cis- and trans- are achiral and optically inactive
• the cis- and trans- forms are diastereomers

• cis-1,3-Dimethylcyclohexane has a plane of symmetry and is a meso compound
• trans-1,3-Dimethylcyclohexane exists as a pair of enantiomers
• has 2 chirality centers but only three stereoisomers

• trans-1,2-Dimethylcyclohexane exists as a pair of enantiomers, cannot be interconverted by ring flip
• cis-1,2-Dimethylcyclohexane the situation is complicated
• -(I) and (II) are enantiomers to each other
• -(II) can rapidly be interconverted to (II) by a ring flip
• -rotation of (III) along the vertical axis gives (I)

The product will of necessity have the same general configuration of groups around the chirality center as the reactant

• Chirality centers in different molecules have the same relative configuration
• If they share three groups in common and if these groups with the central carbon can be superposed in a pyrimidal arrangement

The absolute configuration of a chirality center is its (R) or (S) designation, which can only be specified by knowledge of the actual arrangement of groups in space at the chirality center

separation of two enantiomers

Conformational isomers that stable and can be isolated

Planes of the pi bonds are perpendicular to each other