stereochemistry Flashcards
what are constitutional isomers?
isomers with different connectivity
what are conformational isomers
isomers that can be interconverted by free rotation about single bonds e.g. staggered to eclipded
configurational isomers
differ in the spatial arrangement of their atoms, can only be interconverted by breaking bonds
facts about chair conformation
1) C-C-C bond angles = 111 degrees (no angle strain)
2) all bonds staggered (no tortional strain)
STERIC STRAIN
Repulsion between atoms trying
to occupy the same space
facts about boat conformation
1) high steric strain (short 1,4 distance)
2) there is tortional strain from eclipsing C-H bonds
3) 27kj mol more stable than chair
facts about twist boat conformation
1) less steric strain than boat (big 1,4 distance)
2) less tortional strain than boat (less eclipsing)
3) 6kj mol more stable than Boat
Why subsituents prefer to be equatorial
1,3-Diaxial interactions (steric strain) minimised
Ring-flip is relatively easy — allows equilibration
The larger the group, the greater the preference
enatiomers
stereoisomers which are non superimosable images of one another
diastereoisomers
- stereoisomers which are not enantiomers
- have different physical properties e.g. mp
- only possibe when there is more than 1 stereogenic centre in a molecule
stereospecific reactions
get 100% of one enantiomer product, mechanism related
stereoselective reactions
get tendancy to form 1 enantiomer but dont get 100% of it, structure related
Fischer convention
- Horizontal bonds are emerging
- vertical bonds are receding
assigning R/S to biaryls
assign a and b priority to the top aryl and (a = down b=up) assign c and d to the bottom and then check as normal
Epimers
diastereoisomers which differ in configuration at only 1 chiral centre
meso compound
contains chiral centres but is achiral as it has a plane of symmetry
enantiomeric excess
% more abundant isomer - %less abundant isomer
(X-Y)/100
what is torsional strain
Torsional strain or eclipsing strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom i.e. more than 1 bond away
what is syn addition
the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents. Generally the substrate will be an alkene or alkyne.
what is anti addition
An addition reaction in which two substituents are added to opposite sides (or faces) of a double bond or triple bond. Electrophilic addition of Br2 to 1,2-dimethylcyclohexene is an anti addition reaction, resulting in trans-1,2-dibromocyclohexane
methods of determining enantiomeric excess (e.e.)
1) polarimetry
2) sample derivistation
methods of separating enantiomers
1) salt formation (easiest)
2) covalent bond formation (need to be able to break covalent bond after)
3) kinetic resolution
4) chiral chromatography
separating enantiomers by salt formation
;used to resolve racemic acids, amines and bases
- separate by a crystalising salt
- get rid of resolving agent with acid/base extraction
separating enantiomers by covalent bond formation
- esterificaltion most important
- separate diastereoisomers by chromatography
separating enantiomers by kinetic resolution
- racemic substrate + homochiral reagent
- each enantiomer has a different transition state and different activation energies therefore different rates of reaction
- if rates are different enough the enantiomers can be separated using enzymes
homotopic
if replacing 2 hydrogens gives identical products both times
enantiotopic
if replacing a hydrogen on a carbon creates a chiral centre both times and the molecules formed are enantiomers
diastereotopic
if replacing a hydrogen on a carbon creates a chiral centre both times and the molecules formed are diastereoisomers
