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because of that lack of rotation about
carbon-carbon double bonds, that means that when you have a double bond in the
middle of an alkyl chain
like in 2-butene there's two possible isomers that can form
and they have the same relationship as when we had
methyl groups that could be on the same side or the opposite side of a
cyclohexane ring
like when we are discussing the dimethyl cyclohexane rings back in chapter
three
and at the time we used cis and trans to distinguish those two possibilities
and so those terms pop up again. cis means that
we've got two groups on the same side of the double bond like these two methyl
groups
or for that matter these two hydrogens are cis relative to one another
whereas in the trans isomer the two methyl groups are on opposite sides of
the double bond
and also the two hydrogens on opposite sides for that matter
the 1,2-dichloroethenes have that same type of relationship
where we can have the chlorines that are said to be cis relative to each other
again the same side relative to the double bond or those chlorines can be
trans and the hydrogens are trans so this is used oftentimes whenever we have
a
double bond somewhere in the middle of alkene chain
If this were 1-butene for example there would be
no possibility of a cis or trans isomer
because you have two identical hydrogens at one and the double bond
and this works fine as long as the two
groups around that double bond can can be put in pairs
in each of these molecules on this page we've got
two hydrogens that we can compare to one another or the two methyl groups to
the two chlorines
but I made up a slide here to show how
cis and trans doesn't work in all cases
and then I'll show you that next slide that shows an alternative to
naming things as sis or trans. take a look at these two molecules
I just made up the slide a few minutes ago. we've got four different
atoms attached to the carbon-carbon double bond
and so if you look at the structures it really wouldn't be possible to call
either one of them cis or trans
and yet they are different isomers and we need to have a different name for them
so the alternative the more formal alternative to cis and trans
is to use a capital E or a Z in front of the name
and what we are doing here as it says in blue is we're ranking the atoms attached
to the carbons with that double bond
and we're ranking them by atomic number so we're comparing bromine and fluorine and
bromine has the high ranking because it's got an atomic number of 35
compared to 9 for fluorine and on the other side were comparing hydrogen
with atomic number of just one to carbon atomic number six
so the carbon outranks the hydrogen and that's the basis for this High and low
and so when the two high-ranking substituents are on the
same side the double bond you can say they're cis
relative to each other then we call that isomer Z
It stands for the German word zusammen, in which means "together"
and this other isomer's designated E
the letter E comes from the German word for "opposite" which is
entgegen and what's opposite is the location of the two high-ranking
substituents
and the two lower ranking ones, so this kinda thing still works with
anything that we would call cis and trans but cis and trans are used
wherever thery're appropriate but when you start getting all four substituents
to be
four different things it's not possible really to use cis and trains
so getting back to
our slides here this next slide shows the same four molecules but now
they are
re-designated as either Z for together
or easy for opposite and it is true that things that are cis tend to correspond
to Z
and things that are trans tend to correspond to E, although that's not
always the case
but the nice thing about the E and Z is that any alkene that has
different isomers like these do
we can put an appropriate name with any of them
so you want to be able to either put a name with something labeled as E or Z
or provide that appropriate letter if your asked
to name a structure structure that you're looking at