hey guys, professor Dave here, and today I wanna talk about E and Z alkenes.
were probably familiar with cis and trans terminology as it relates to alkenes so
for example we would know that
this is a trans 2-butene and this is cis
2-butene, and we're able to use this terminology because
these are only disubstituted alkenes
so there's no ambiguity when we say cis or trans
with regards to those alkenes however if we're looking at a tri
or tetrasubstituted alkenes cis and trans become ambiguous
because then it's a question of what is cis to what and what is trans to what
so we're gonna need a form of absolute terminology
some absolute configuration
to be able to label any given alkene and so
we're gonna need to understand E and Z
as a way of labeling alkenes so these are German words
E means entgegen and Z means
zusammen, and this is referring to
a group being on this,
the highest priority groups on each side
of an alkene being on the same or opposite sides
the way we're going to do, so basically we're looking at an alkene like let's say
this one
this is one of the carbons and this is the other carbon
participating in the double bond
so each of those two carbons has to have its own
substituents so for example on the left we have a carbon with a methyl group
and we have a hydrogen, on the right
we have OH and then on the bottom we have over here, this
this stuff, so we need a way to prioritize
one of those groups over the other for
each carbon participating in the double bond. the way we're gonna do that
is using the Cahn-Ingold-Prelog convention
so hopefully we remember that one from assigning R and S for centers
so we're gonna use the same convention and just as with
the R/S stuff we're gonna assign the priority of one group over the
other
by going one atom at a time. so sometimes we only have to go one atom
along
so basically here this carbon is attached to a carbon
and the implied hydrogen so remember that we're going by atomic mass here
so it's simply that carbon is heavier than hydrogen
is the reason that this group wins
so for the left carbon we've decided that it's the carbon that takes priority
down here
over the implied hydrogen. okay to now on the right side
this is where we have to recall that we must put on our blinders and go one
atom at a time because if we didn't do that we might look at this side and say
well clearly this group takes priority because it's
it's bigger and bulkier it's heavier
that's going to take priority however that's not going to be the case because
recall we're going one item at a time so actually
this carbon is bound to oxygen and it is bound to carbon
and so this group takes priority actually
and for no other reason other than
oxygen is heavier than carbon so we must go on and
one atom at a time so once we have determined
which two groups take the priority now all we're doing is looking at the plane
the plane of the double bond here and we're seeing that the two
the two top priority groups are on opposite sides
of the double bond and so opposite sides
means that this is an E
alkene so whereas
for something like this we couldn't use cis and trans
because what would be cis and what would be trans, well this group is trans to
that but this group is cis to that so
that becomes ambiguous terminology but everyone could use the
Cahn-Ingold-Prelog convention and agree
that this is an E alkene. so now let's look over here
so same thing on the right so here's the two carbons
each one has two groups and we must decide which has priority over the other
so over here bromine is heavier than chlorine
so bromine takes priority. now over here
this carbon is bound to a carbon on this side and a carbon on that side
now here we have a tie so that means that we have to continue until there is
a point of difference
so we must see what
these carbons are attached to so on the top we have a carbon
and that is attached to this carbon this carbon and the implied hydrogen
so carbon carbon hydrogen down here
this carbon is attached to you one carbon
and then two hydrogens and so we compare
carbon carbon hydrogen ought to beat carbon hydrogen
hydrogen right so that means that this group is going to take priority
so once again if we didn't bear in mind that we have to go one at a time
our eyes might go directly to this iodine, realize that's a heavy atom
and decide that this is going to be the higher priority but it's
actually not the case
if we're using the convention properly so
now that we've decided that the two high-priority groups
are on the same side the plane of the double bond
then we're realizing that they are zusammen and this is
Z and if you want a very easy way to remember which one is E in which one is Z
we just have to know
"zee zame side" so if you know that Z means zee zame zide
it's a little bit silly but you'll never forget what E and Z mean
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and as always feel free to email me with questions