Stereochemistry Flashcards
What are isomers?
compounds with same molecular formula but different structure
What is a constitution?
composition of molecule (can have different connectivity
eg. constitutional isomer
What is Configuration?
arrangement of bonds in space, geometric relationship between given set of atoms with same connectivity (constituition) but different configuration
eg.: stereoisomers like E/Z isomers
What is a conformation?
different spatial arrangment resulting from bond rotation
What is a Keilstrichformel?
perspective formula with bold and dashed bonds to show orientation (configuration)
What is an Eclipsed conformation?
What is a staggered conformation?
For ethane, is the staggered or eclipsed conformation favoured?
The staggered conformation is favoured for ethane
Why is the staggered conformation favoured for ethane?
- filled orbitals (C-H) repel each other in the eclipsed which causes destabilization
- hyperconjucation of the sigma(C-H) orbital with the periplanar sigma* which helps the staggered conf. to stabilize
- steric repulsion just between the two protons is small
Draw the 6 most significant conformers of butane and name them
How conformations in cyclic molecules influenced?
- angle strain (bayer strain)
- bond angle difference from optimal tetrahedral bond angle (109.5)
- Torsional strain
- repulsion of nearby substituents due to torsion of the bond angle and thereby a more eclipsed conformation
- steric strain
- spatial requirements by nearby substituents leads to repulsion
Why is cyclobutane slightly more stable than cyclopropane?
in cyclopropane, none of the C-C can be rotated, forcng all the C-H bonds to be eclipsed with neighbour. Making sigma bonds weaker
In cyclobutane, the ring disorts from planar conformation (into wing conformation) to reduce eclipsing interaction
Why is cyclopentane not completely strain-free? (eventhough in planar conformation angles are close to 109.5)?
- strain in planar conformation caused by eclipsing of adjacent C-H bonds, high torsional strain
- as in cyclobutane, the ring distorts to reduce eclipsing interactions but this increases the angle strain
- open envolope shape of 4 C atoms in plane and one above or below
name and draw the staggered conformation of cyclohexane
Name and draw the conformation of cyclohexane
Draw an energy diagram of cyclohexanes conformations
What are substituted cyclohexane derivatives? and which one is preferred?
Do bigger substituents stabilize the equatorial conformation?
- bigger susbstituents stabilize the equatorial conformation, more than smaller
*
When is a compound chiral?
if it is a non-superposable on its mirror image (eg. via roation)
- does not have inversion center or plane symmetry
What are chiral centers?
if a centre has 4 different substituents, there are 2 ways how they may be arranged (2 configurations)
What is a stereogenic center/stereocenter?
chiral center
what is a diasteromer?
two stereoisomers that are not always mirror images of each other
- should have at least 2 stereocentres
Are E/Z isomers diasteromers?
Yes. they have at least 2 stereo centres. Mirror image of E- isomer is still E isomer (same goes with Z) (BUT THEY ARE NOT CHIRAL)
can compounds with stereogenic centres be achiral?
Yes
What is a physical property of chiral compounds?
rotate polarized light (optical activity)
- 2 enantiomers show opposite rotation of plane polarized light
- if positive rotation (right), it’s (+) or d
- if negative rotation (left), its (-) or l
What is a racemic mixture?
1:1 mixture of both enantiomers
What is the enantiomeric excess (ee)?
What is an enantiomeric ratio?
What are properties of enantiomers?
- same geometry (distance between atoms)
- same melting/boiling point, density, spectroscopic
- show different optical rotation
- react differently in chiral environment (cells, enzymes)
What are properties of (chiral) diastereomers?
- have different geometry
- have different melting/boiling points
- show different optical roation when chiral (otherwise optically inactive)
How is it possible to separate enantiomers?
- conversion into diastereomers and physical separation
- crystallization (under certain circumstance)
- separation in chiral environment (whereboth enantiomers behave/react differently
