ok guys for today's problem we're going to look at this reaction, we have two
stereoisomers, these two molecules both have a bromo group and a tert-butyl group
but on this compound those are cis and on this compound those are trans. now
what we're looking at is that those both will undergo E2 elimination with
tert-butoxide to form these two products, however from the cis compound
the reaction is very fast and from the trans isomer the reaction is very slow
and the question is simply, why? so if this is confusing we're going to want to take
a look at E2 elimination reactions as well as, hint hint cyclohexane chair conformations
to answer this question there are two key points that we have to recall, the first is that in order for an elimination
to occur the leaving group and the beta proton being extracted in the
elimination, they must be antiperiplanar and if we're doing this on a cyclic
system, if we're doing this on a cyclohexane ring they must be both in the axial
position in order to be anti to one another so what that means is that over
here on this chair we're saying that for the cis
isomer this chair is exclusively the one from which elimination can
proceed, this one is not going to work because the the leaving group is
equatorial, so when the leaving group is equatorial it's actually anti to the
rest of the ring and so it's not going to be able to proceed from that
conformation so this chair is going to work for cis and then likewise for
the trans, this one is not going to work because there it's equatorial and
then over here we've got these beta protons
so these two chairs, this chair for the cis and this chair for the trans are the
conformations from which E2 can proceed so that's part one. number two is
to look at the stability here, so if we look at the stability we understand that
the tert-butyl group is a very bulky group, it provides a lot of steric
hindrance and so there's a pretty huge premium on that tert-butyl
group being in the equatorial position so it more or less
tethers the molecule in a particular conformation because of the extreme
thermodynamic favorability of that group being equatorial over over axial so E2
can occur from both molecules, from the cis we could say, grab that proton
and then we do the elimination and that would lead to one of those and we could
also do that over here, let's take this all the way down, let's say we got that
proton, thats a beta proton, and we do this elimination and that could lead to
one of those products. but the thing is, in solution, the the distribution of
conformations reflects the thermodynamic favorability and so almost all of the
time this isomer is going to look like that
conformation because of the favorability of putting the tert-butyl in the
equatorial group so that means that if if a tert-butoxide collides with this
molecule it almost always is gonna look like this and so it's probably gonna be
able to extract a proton and have the elimination go, however for the trans
isomer the molecules are almost always gonna look like
this because this is the one where the tert-butyl group is an equatorial
position and so that means tert-butoxide might collide with a molecule of
substrate a million times before it finally finds one that is in the correct
conformation for elimination to occur because it is going to have to overcome
that overwhelming
energetic unfavorability to do the chair flip and get into this position
because the tert-butyl group does not want to be in the axial position, there's too
many diaxial interactions so both of these can do E2
they will do so from specifically these chair confirmations but because the
trans isomer, the the chair from which E2 can occur is so much more
unfavorable energetically, the end result on the macroscopic scale is that this
reaction is much much much slower than that one
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