Organic Ch 5
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What are the 2 types of Isomers?
- Constitutional Isomers
What is an isomer?
different compouynds with same molecular formula
What is a constitutional Isomer?
isomers whose atoms hae a different connectivity
What are stereoisomers?
isomers that have the same connectivity but differ in spatial arrangement of their atoms
What are the two types of stereoisomers?
What is an Enantiomer?
stereoisomers that are nonsuperposable mirror images of each other
What are Diasteromers?
stereoisomers that are NOT mirror images of each other
What does achiral mean?
an object is achiral if the object and itsmirror image are identical
What does chiral mean?
a chiral object is one that cannot be superposed on its mirror image
What is the significance of thalidomide?
One enantiomer causes birth defects, the other cures morning sickness
What is the significance of tretoquinol?
one enantiomer is a bronchodialator, the other inhibits platelet aggregation
What is the binding specificity of a chiral receptor site?
For a chiral molecule is usually only favorable in one way
Enantiomers occur only with compounds whose molecules are _______
Is a chiral molecule superposable on its mirror image?
what is the relationship between a chiral molecule and its mirror image?
what is the most common type of chiral compound that we encounter?
molecules that contain a carbon atom bonded to four different groups
how do we designate a chiral center?
with an asterisk (*)
What is a stereogenic center?
Any atom at which an interchange of groups produces a stereoisomer
What are tetrahedral stereogenic centers?
What enantiomers are found in oranges and lemons?
- (+)-limonene is found in oranges
- (-)-limonene is found in lemons
What was thalidomide used for?
to alleviate the symptoms of morning sickness in pregnant women
What were the effects of thalidomide?
caused horrible birth defects in many children born subsequent ot the use of the drug
What is a plane of symmetry?
An imaginary plane that bisects a molecule such that the two halves of the molecule are mirror images of each other
All molecules with a plane of summetry in their most symmetric conformation are ______
How to Assign (R) and (S) Configurations
- 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
How are enantiomers similar in characteristics?
identical physical properties (melting point, boiling point, refractive index, solubility, etc)
When do enantiomers show different behavior?
Only when they interact with other chiral substances
How do enantiomers react with plane polarized light?
turn plane-polarized light on opposite direction
What is plane-polarized light?
- 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)
What is a polarimeter?
A device for measuring the optical activity of a chiral compound
How do you calculate specific rotation?
- Specific rotation [α]
- [α]=specific rotation
- α=observed rotation
- c=concentration of the solution (g/mL)
- l=length of the cell in decimeters
How should the valvues of specific rotation of two enantiomers relate?
Two enantiomers should have the same value of specific rotation, but the signs are opposite
What is a racemic form?
An equimolar mixture of two enantiomers
How does a racemic mixture relate to polarized light?
A racemic mixture causes no net rotatiom of plane-polarized light
What is enantiomerically pure, or enantiomeric excess of 100%?
A sampe of an optically active substance that consistus of a single enantiomer
Whatis optical purity also know as?
How can you calculate % enantiomeric excess?
% enantiomeric excess=((mol of one enantiomer)-(moles of another enantiomer))/total moles of both enantiomers
How can you calculate % enantiomeric excess from optical rotations?
% enantiomeric excess= observed specific rotation/specific rotation of the pure enantiomers * 100%
What are stereoselective reactions?
Reactions that lead to a preferentil formation of one stereoisomer over other stereoisomers that could possible be formed
What is enantioselective?
If a reaction produces preferentially one enantiomer over its mirror image
What is diastereoselective?
If a reaction leads preferentially to one diastereomer over others that are possible
What is the maximum number of stereoisomers available?
2^n; where n= the number of tetrahedral stereogenic centers
Are enantiomers easily seperable?
Can diastereoisomers be separated?
Why can diastereoisomers be easily separated, but enantiomers cant?
Because enantiomers have very similar properties, and diastereoisomers dont, and are therefor easy to separate
What is a meso compound?
- Achiral despite the presence of stereocenters
- not optically active
- superposable on its mirror image
- has a plane of summetry
What is a Discher Projection Formula?
a 2-dimensional representation of chiral molecules
What do vertical lines represent in fischer projection formulas?
bonds that project behind the plane of the paper
What do horizontal lines represent in Fischer Projection Formulas?
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)
What happens if a reaction takes place in a way so that no bonds to the chirality center are broken?
The product will of necessity have the same general configuration of groups around the chirality center as the reactant
What is relative configuration?
- 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
What is absolute configuration?
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
What is resolution?
separation of two enantiomers
What are atropisomers?
Conformational isomers that stable and can be isolated
What are allenes?
Planes of the pi bonds are perpendicular to each other
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