1d Flashcards
when is a reaction called stereoselective
when the reaction can choose between which isomer it wants to make // make more of
in sn1,, what is formed and what option does this give the moecule
in sn1 theres 2 steps,, loss of lg and attack from nucleophile
loss of lg gives us a planar CC+ intermediate
and bc its planar,, we get to choose if the nuc is gonna react from the top or the bottom!!!
this gives us the posibility of making either isomer!!
this gives us enantiomers!!
whats an enantiomer
molecules that are non superimposable mirror images of eachother
all chiral centres change configuration
do enantiomers have the same energy
yes
they have the same energy,, esp if theyre made in a 1:1 ratio
if two molecules have the same energy what does this mean
they have the same activation energy
they have the same rate of reaction
they are formed in equal amounts
theyre enantiomers???/
think fot he energy diagram for enantiomers made dueing an sn1 reaction
u have an M shape energy diagram
with SM at the bottom,, low energy
then u have a transition state and an intermediate and then another transition state and then ur products.
the energy diagram for enantiomers is made up of one line which tells us that theyre both the same energy,, their synthesis has the same Ea,, ROR and therefore theyre mad in equal amounts.
when we have an enegry diagram for sn1,, whats the big Ea and small Ea
big Ea = grap between SM and first TS
this is the enrgy for loss of LG.
small Ea = gap between Ts and intermediate
this is the energy needed for the nuc to attack either side of the molecule.
this is sometimes the same for each side of the molecule,, which we can see if the energy diagram is made up of one line or 2.
if the Ea,, and ROR and everything is the same including the ratio the products are made to.. is the reaction a stereospecific one
nope!!!
its not stereoseletive bc its not selecting one isomer over the other,, which we can see bc same Ea,, same ROR and same ratio of synthesis.
what occurs when we have an SN2 reaction
u have inversion of stereochem
u go from S to R and etc
is an SN2 reaction stereospecific // strereoslective
yes its stereospecific bc ur going from S to R or R to S,,, u cant go from S to S in an SN2 reaction!!!!
what are diastereoisomers
not superimposable,, not mirror images of eachother
what happens to stereogenic centres of diastereoisoemrs
not all the stereogenic centres change!!! only some of them do.
think of SN1 again,, can it be stereoselective,,, like can the top attack soetimes be favoured over the bottom one
yes ofccccc
depends on the substituents and the moelcule but def!!
if theres a substituent pointing towards u,, the nuc will probs attack from th back to reduce the steric hinderance and prevent the extra energy if itwas to interact with it.
when u have an SN1 reaction that forms distereoisomers bc the back attack is favoured over the front attack,, what will our energy diagram look like
the SM is the same and so is the first TS
THE INTERMEDIATE IS THE FIRST POINT OF ENERGY DIFFERENCE
BC THIS IS WHEN THE NUC IS APPROACHING FROM A SPECFIC SIDE
the next TS state will then differ in eneryg and so will the product peaksssss
this is bc the reaction is stereoselective and one is def favoured over the other
from looking at an energy diagram,, how can we tell if a reaction is stereoselective or not
look at the products
see if theyre lines are overlapping or if one is higher than the other!!
the higehr one is higher energy and will probs be made less // at a lower yield than the one with lower energy.
when u have a tans alkene and do an epoxidation raction,, what 2 products do we get
its a concerted reaction meaning both products will be trans
u have trans alkane with dashed O
and u have trans alkane with wedged O
these are enantiomers!!! all the chiral centres switch
the energy diagram will therefore just be one peak with a TS mad eup of one line. bc theyre both the same energy.
when u have a 1: ratio of product,, what do we call this
racemic mix
racemate
whats different about enantiomers
they have different biollogical properties!!!
if we have enantimer products formed in a 1: 1 ratio,, how can we synthesise only one!! without forming diastereoisomers to separate the one we want from the one we dont want
we would find an oxidising agent,, if its for an epoxidation reaction and we will make sure it has a chiral substituent on it. aka the oxidising agent needs to be chiral!!
this makes the TRANSITION STATE diastereoisomers and allows one to be preferred!!!!!
when we want to form one enantiomer over the other even tho normally u get a 1:1 ratio what do we need to think to do
we need to make one have a faster ROR,, by reducing that ones Ea!!!!
ur SM and products have the same energy. only the TS is difff!!!!!
this is bc u still get enantiomers,,, with the same end energy,, but by making the oxidising agent // reagent chiral,, u allow the TS to be diastereoisomers,, allowing one of the enantiomers to form faster bc lower Ea and faster ROR, allowing u to form it and collect it before the other enantiomer
