organic bonding - isomerism Flashcards
(24 cards)
isomer definition
a molecule with the same chemical formula as another compound, but a different structural formula
stereoisomer definition
same molecular formula, different arrangement in space
constitutional/structural isomer definition
same molecular formula, different connectivity
enantiomer definition
stereoisomers that are non superimposable mirror images
diastereomer definition
stereoisomers that are not mirror images
why do some compounds have stereoisomers?
compounds with C=C bonds have stereoisomers because the π bond does not allow for rotation
cycloalkanes can also have stereoisomers
how are stereoisomers distinguished?
E/Z nomenclature
Z = when highest priority groups are on the same side
E = when highest priority groups are on opposite sides
how is priority assigned ?
cahn-ingold-prelog (CIP) priority rules
give the 3 CIP rules
1- of the atoms directly attached to the C=C bond, the one with the higher atomic no. takes priority (or highest atomic mass for isotopes)
2- if 2 substituents have equal rank (same atom), proceed along substituent chains until there is a point of difference, whichever has the closest connection to the double bond has higher priority
3- if the chain is connected to the same kind of atom multiple times, check if atom connected has a greater atomic mass, if so this has higher priority than the chain
chiral definition
objects that are not identical to its mirror image, cannot be superposed on its mirror image
achiral definition
objects that have planes of symmetry, therefore cannot exist as 2 non superimposable mirror images
how do the chemical properties of enantiomers differ?
the physical and chemical properties of pure enantiomers are mostly identical e.g. NMR, b.p, m.p
they only differ in their reactions with other chiral substances and with plane polarised light
racemic definition
a 50/50 mixture of 2 enantiomers
- no optical activity as opposite rotation effects cancel out
stereogenic centre definition
a chiral centre - an atom/carbon carrying 4 different groups therefore has no plane of symmetry/centre of inversion
if a molecule has many stereogenic centres, does this mean it is chiral?
not necessarily, can still have a plane of symmetry
how are the configurations of stereogenic centres distinguished?
R/S nomenclature
how is R/S nomenclature assigned?
- assign a priority number to each substituent using CIP priority rules
- arrange molecule so lowest priority group is facing away
- move from substituent 1 to 2 to 3 in order of decreasing priority and determine direction of movement
clockwise = R, anticlockwise = S
what is the association between R/S isomerism and rotations of plane polarised light?
there is no association
enantiomerically pure definition
if a sample contains only 1 enantiomer
enantioenriched/non-racemic/scalemic definition
if a sample contains both enantiomers but 1 predomniates
if starting materials are achiral, and products are chiral, how can you determine what enantiomer will be produced?
always be a racemic mixture - as energetically it makes no difference for the substituents to come from the front or from the back
meso compound definition
molecules that are achiral but have multiple stereogenic centres - this means there is a plane of symmetry
axial chirality definition
seen in compounds with 2 perpendicular planes containing substituents which aren’t symmetry planes (as opposed to a centre of chirality?
spiro compounds definition
compounds in which the planes containing rings aren’t symmetry planes, these are chiral compounds with no stereocentre
- some can have chirality due to restricted rotation
- these compounds are examples or axial chirality
