stereoisomerism (2) Flashcards
enantiomers
pair of isomers - mirror images are not superimposable
molecules have chirality
example - ethanol
mirror able to rotate and is same as original therefore not enantiomers
and are achiral
example - lactic acid
rotate 180 - different to original as OH is above the plane while original is OH is below the plane
enantiomer - mirror image is not superimposable - chiral
example - erythrose
mirror image differ - enantiomer
chiral but have 2 stereogenic centres on both - opposite chirality in mirror image
chiral
not superimposable with mirror images
can derive from other sources and doesn’t always contain stereogenic centres
optical rotation
all physical properties of enantiomers are identical except optical rotation
chiral molecules in optical rotation
plane polarised light pass through solution and remains
achiral molecules in optical rotation
plane polarised light pass through solution - rotation in light
angle of rotation in polarimeter
angle alpha depend on concentration of solution (c), path length (l) and specific rotation [a] - characteristics of molecules
equation of characteristics of molecules
a = [a].D.c.l
temperature and wavelength dependent
racemic mixture
1:1 mixture of enantiomers
enantioenriched
ratio not 1:1 mixture
enantiopure/homochirality
no other enantiomer
Diastereomers
pair of stereoisomers - don’t bear mirror images relation
examples of diastereomers
threose and erythrose
glucose and galactose
they have same constitution but different arrangement of atoms and not mirror image
having >1 stereogenic centre in diastereomers
need at least one stereogenic centre that is different
epimer
diastereomer where only one out of some stereogenic centre is different
example of epimer
glucose and galactose - 5 SC and 1 is different
example of each diastereomer having corresponding enantiomer
D-erythrose and L-erythrose
mesoform
structure has SC but are achiral
example of mesoform - 2-tartaric acid diastereomer
rotate 180 horizontally and will be same
superimposable but one chiral and other is achiral
geometric isomers
stable entities and can’t easily interconvert
compounds with different chemical and physical properties
geometric isomers in alkene
focus on relative positions of substituents of C=C - cis/trans (same or opposite side horizontally respectively)
geometric isomer in ring structure
cis and trans - same and opposite side of ring plane
used to describe different substituent - 2 different on same side - cis
Fischer representation
uses horizontal and vertical lines
using highest priority at apical position located at top end
horizontal lines in fischer
located in front of plane
vertical lines in fischer
located behind plane - carbon chain
relation of sawhorse and fischer
substituent located on same side in Fischer = different sides in sawhorse
which representation used for carbohydrates and amino acids
Fischer representation
CIP nomenclature used:
alkene geometric isomer - E/Z
SC(differentiate enantiomers/diastereomer - R/S
CIP
Catin Ingold Prelog
absolute configuration
differentiated by unambiguous nomenclature
How CIP nomenclature works
- identify substituent C=C or SC
- assign priority sequence based on atomic mass
- from priority assign E/Z or R/S
E
entgergen - across
Z
zusammen - together
Nomenclature in SC - process
- see 4 atoms directly bonded to SC
- priority sequence based on atomic mass
- assign R/S - by placing lowest priority at back of plane and go from highest to lowest to see if it is going clockwise (R) or anticlockwise (S)
if 2 atoms are identical when looking at the nomenclature
look at next atom - what they are bonded to
atoms with double or triple bonds in Nomenclature
given single bond equivalences
phantom atom
new carbon representing double of triple bonds
when having 2 SC
2 enantiomers having S,S and R,R
R,R and R,S
S,R and S,S
diastereomer pair of compounds - determining absolute configuration of SC - compare molecules and see if they’re enantiomers/diastereomer/ just equal to each other
example of 2SC in 2 enantiomers
L/D-erythrose and L/D-threose
why this is important
most biological isomers - amino acids, nucleic acids, FA, cholesterol to proteins and other polymers
increase in complexity
example of important biological isomer
thalidomide - R - nausea-treatment but S- cause birth defects
absolute configuration
spatial arrangement of atoms of chiral molecule
