ok guys today's practice problem is about Newman projections, so what I've
done, I have drawn a molecule here and I've drawn three different confirmations that
occur as we rotate around this bond here so if we are rotating this carbon-carbon
sigma bond right there we can get these three different conformations for this
exact same molecule, so let's draw the three different Newman projections that
result from these three conformations and then let's rank those conformations in terms of increasing stability
ok so I've drawn these Newman projections for
you so go ahead, if we're right here in the board looking here for all of
these we're seeing the isopropyl group directly up so there's those and then
we're seeing this wedge methyl that's sticking out of the board that is down
and to our left so there's that wedge methyl and then the implied hydrogen on
the dash bond that is down and to the right, so the front carbon right here
notice that this portion of the molecule here looks exactly the same for all
three so the front carbon looks the same for all three Newman projections but it
is the back carbon, it is the rotation of this bond that is causing this to change
so let's see what's going on there, on the back carbon here for this first
one we've got this methyl down directly in the plane of the board so that's
going to be straight down, we've got this ethyl group that is up and to the left
because if I'm on this side that is going to be on the left side of me so
that's right there and then the implied hydrogen is going to be up and to the
right so there's that one, on this one here this is gonna be a little different
because here on this back carbon we've got that wedge methyl, that's down and to the
left and then we have the ethyl straight up and the implied hydrogen
down and to the right, and then for this last one we've got the
dash methyl that's past the board, that's going to be up and to our right because
it's on this side of the board so that's over there
the implied hydrogen is up and to the left and then the ethyl group is
straight down. so let's talk about the discrepancy in energy of these, so first
of all this one is eclipsed, these two have completely staggered interactions
which have dihedral angles of 60 degrees and so always all of the
all of the staggered conformations are going to be better than any of the
eclipsed conformations because there's so much steric hindrance associated with an
eclipsed conformation because those groups are right on top of each other so
this is certainly the least favorable and so it is the least favorable
energetically speaking so that's the least favorable but now let's compare
these two staggered conformations and take a look at what's going on
well we all would, not only do we want them to be staggered but we we also want
to minimize the gauche interactions because we have two bulky alkyl groups
the most favorable position for them to be in is anti to one another, that would
be one on the top one on the bottom, because that puts them as far away from each
other as possible, that minimizes steric hindrance but gauche is when it is staggered
but they are adjacent so two positions one on the front carbon one on the back
like that with a 60 degree dihedral angle those are said to be gauche to one
another, so let's demarcate all the gauche interactions, there's one
there, there's one there, there's one there, we're gonna mark gauche
interactions as alkyl groups that are gauche to one another so this
isopropyl and ethyl are gauche, this ethyl and this methyl are gauche, and these two methyls
are gauche, so that's three gauche interactions. over here we've got an
isopropyl and a methyl that are Ghosh and then we've got a methyl and ethyl that
are gauche, that's only two gauche interactions so certainly this one is
going to be the next most stable but this one is going to be the most stable
because not only is it staggered but it also has fewer gauche interactions than
number two. so these are the correct Newman projections in the correct order
of stability
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