Isomerism +Steroechemistry Flashcards
what are the 3 types of isomerism
-structural
-conformational
-stereoisomerism
define structural isomerism
molecules that have the same molecular formula but differ in the sequence in which the atoms are bonded together
what differs for structural isomers and why
-physical properties
-chemical properties if they have different functional groups
what’s the general rule for conformational isomers
the isomer with the least number of eclipsed bonds is the lowest in energy
how many degrees are there between the H in eclipsed conformation
0 degrees
how many degrees are there between the H in staggered conformation
60 degrees
why is the eclipsed conformation higher in energy than the staggered conformation
due to stereo electronic (electrons repel each other)repulsion between C-H bonds
how does staggered conformation minimise stereo electronic repulsion
-the electrons are further apart
-so not repelling one another as much
-so lower in energy
why is the chain conformation of cyclohexane strain free
-as eclipsing is completely prevented
-bond angles are very nearly tetrahedral
-there’s no stereo electronic repulsion
what’s the lowest energy conformer of butane+ why
-anti-conformation
-due to minimisation of steric hinderance (2 methyl groups are far apart) +stereo electronic repulsion
what’s the highest energy conformer of butane+ why
-eclipsed isomer
-due to steric hinderance of 2 methyl groups+ stereo electronic repulsion of all bonds
what are the 2 types of positions for H bonds
-axial (vertical, up and down)
-equatorial (horizontal, up and down)
how are more stable conformations made
when bulkier/bigger groups are put in the equatorial groups
what happens when a conformer ring flips
a H which appears equatorial in one conformer will be axial in the ring flipped conformer
describe the boat conformation of cyclohexane
-stereo electronic repulsion between the flagpole H’s
-H’s in the base of the boat
describe the twist boat conformation of cyclohexane
doesn’t have total eclipsing of H bonds at the base of the bond so relieves some of the stereo electronic repulsion
which conformer does cyclohexane mainly exist in + why
chair conformation as it’s the most stable
what are the 2 chair conformations of mono substituted cyclohexanes
equatorial and axial
why is the equatorial conformer more stable
as it doesn’t have 1,3-diaxial strain
what’s the general rule for mono-substituted cyclohexanes
the cyclohexane will adopt the conformation which has the most substituents in the equatorial position
what happens when there are groups on the hexane ring
the biggest group adopts the equatorial position
what are the 2 types of stereoisomerism
-enantiomerism
-geometric isomerism
when do molecules exhibit enantiomerism
when they lack all 3 symmetrical elements
-a centre of symmetry
-an alternating axis of symmetry
-a plane of symmetry
what’s a chiral molecule
-have an asymmetric carbon
-are non-superimposable
-compounds of enantiomers of each other
what are the properties of enantiomers
-physical properties are the same
-rotation of plane polarised light differs
-chemical properties differ
what’s a racemic mixture+describe a property
-an equimolar mixture of 2 enantiomers
-it isn’t optically active
what are the priority rules of Cahn-Ingold-prelog Convention
- atom with the highest atomic mass attached to the stereocentre, is the highest priority
- look at groups attached
- higher oxidised atoms(double bonds) have higher priority
what are the assignment rules of absolute configuration
- make sure the lowest priority group (usually H)is pointing away (on the dashed line). may have to redraw the molecule
- assign a sequence of priority to the atoms or groups of attached to the stereo centre
- draw an arrow away from the highest priority group to the lowest priority group
what do you assign if the arrow rotates anti-clockwise
S
what do you assign if the arrow rotates clockwise
R
what are diastereomers
stereoisomers which aren’t related as mirror images
what are the properties of diastereomers
-differ in physical +chemical properties
what’s diastereomeric resolution
converting inseparable enantiomers into diastereomers then using chromatography or recrystallisation to resolve the enantiomers
what are the 3 steps involved in diastereomic resolution
- racemic mixture are reacted with a single enantiomer of a different chiral compound giving diastereomers
- new diastereomers are separated by crystallisation or chromatography
- separated diastereomers are then chemically converted back into the original enantiomers
what’s a mess compound
molecules which contain stereogenic centres but aren’t optically active, they’re achiral or meso
what does geometric isomerism occur from
due to restricted rotation about a C=C double bond
what’s geometric isomerism
a diastereomer
what are the 2 types of absolute configuration around C=C double bond
E(opposite side)
Z(same side)
wha’s a stereospecific reaction
a reaction where a stereoisomer reacts to give a single product which is a single stereoisomer (concerted)(good)
what’s a stereo random reaction
a reaction where a stereoisomer reacts to give a product which is a mixture of stereoisomers (non-concerted)(bad, as loose stereo info to make enantiomers)
what’s a concerted reaction
bond formation+bond cleavage occur at the same time
where does attack of the nucleophile take place from
the opposite side to the leaving group
what’s walden inversion
if a chiral molecule undergoes an SN2 reaction then the absolute configuration is inverted
when are sulfonates commonly used in chemistry
used as intermediates during the conversion of alcohols into other functional groups
what’s made when 2s orbitals + 2p orbitals are hybridised on a sp3 carbon
4 sp orbitals, arranged in a tetrahedral shape
what are the 3 types of p orbitals
px
py
pz
what are the 2 key stages in the use of sulfonates in SN2 reactions
- formation of the tosylate or mesylate from the alcohol
- SN2 reaction of the nucleophile with either the tosylate or mesylate
why are sulfonates great leaving groups
due to resonance stabilisation of the anion
when do SN2 reactions of cyclohexanes take place
with a strong leaving group +strong nucleophile
what are the 2 occurrences that happen during a SN2 reaction of cyclohexanes
-equatorial leaving group is replaced by axial nucleophile
-axial leaving group is replaced by equatorial nucelophile
what occurs during a SN1 reaction
2 steps:
1. slow formation of the carbocation (rds)
2. attack of the nucleophile (fast step)
what type of process is a SN1 reaction
a non-concerted, stepwise process of bond breaking +bond formation
why is a SN1 reaction unimolecular
as the rate equation is dependant on only one reagent
what type of reaction is a SN1 reaction?
a stereorandom reaction
why is optical activity lost during a SN1 reaction
as a racemic mixture is made
what does a planar chiral intermediate result in
the nucleophile can attack from both faces to give both enantiomers
what are elimination reactions used for
to synthesise alkenes
what does a E1 reaction result in
formations of a mixture of isomers
what is the rate dependant on for a E2 reaction
the concentration of 2 reagents
what must occur for the E2 reaction to take place
overlap of the sp3 orbitals on both C atoms for formation of the new pi-bond
what are the 2 conformations that allow for overlap of the sp3 orbitals on both C atoms for an E2 reaction
- syn-coplanar
2.anti-periplanar
why does the anti-periplanar conformation always occur
-as the sun-coplanar results in eclipsing interactions
-resulting in electrostatic repulsion between the base+leaving group
Describe the positioning of an H atom and a leaving group in an E2 reaction
they’re anti-periplanar
for elimination reactions, what happens if 2 H’s are axial
2 products are formed
what are the 2 types of reaction processes addition of alkenes can occur by
-stereospecific
-stereorandom
when is a reaction stereospecific
when the reaction is concerted, bonds are broken +formed at the same time
what are the 2 types of addition reactions
- syn-addition
- anti-addition
explain a syn-addition reaction
as the molecule is flat, attack can take place from either face producing 2 stereoisomers
due to a sin-addition being concerted what does this mean
cis-relationship of R group is retained
explain an anti-addition reaction
-attack takes place from the one face+then the other
-if an intermediate is formed where C-C bond rotation is possible, then geometry of alkene can be lost
what compound is used during dihydroxylation of alkenes
mainly OsO4 (osmium tetroxide) but can be done using KMnO4
what’s OsO4 role during dihydroxylation of alkenes
OsO4 attacks in sun concerted fashion(both hydroxyls are added from the same face)
what is formed in a dihydroxylation of alkenes reaction
a racemic mixture
what does the sun-addition of oxygen to alkenes result in
the formation of epoxides
what does the epoxidation of alkenes reaction involve
an alkenes+ a per acid
why is epoxidation a useful reaction
as ring strained epoxides react quickly with nucleophiles to produce substituted alcohols
what does the anti-addition of Br2 to alkenes result in
a racemic mixture
