NAMING CIS - TRANS ISOMERS for ALKENES

CHEMISTRY 102

NAMING CIS - TRANS ISOMERS for ALKENES


Last Modified: Wednesday, 22-Mar-2006 00:05:32 EST

REVIEW: NAMING CIS - TRANS ISOMERS for ALKENES

The following discusses the issue of cis-trans isomers in alkenes.  Take a look
at this and if you still have any questions bring them with you to the review
on Sunday.

	The cis-trans is in relation to the PARENT chain (as indicated in the
	book).  We are not using the E-Z method, a different method of naming
	the arrangement of groups around alkene carbon atoms.  This is not even
	mentioned in the book.

	This means there are certain times we can not name something as cis
	or trans.  In class I said if there are 4 totally different groups we
	wouldn't worry about naming as cis-trans.  What I was refering to was
	something like the following:

	    CH3     Cl
	     \     /
	      C = C
	     /     \
	    F      Br

	For this molecule there is really no way to use cis and trans
	unambiguously.  We would have to use E-Z and we are not using that
	system.  So we won't name it as cis or trans but should still realize
	there are cis and trans isomers.  We would simply name this as
	1-bromo-1-chloro-2-fluoropropene (don't need the 1 indicating the
	C=C since it has to be between C1 and C2).

	We will see below there are times when there are 4 "different" groups
	but we can still use cis and trans.  Let's look at some easier
	situations first.

 	Look at the following molecule.  This one has two different groups on
	each C of the C=C so it will have cis-trans isomerism.  All together
	there are only two different groups, methyl and ethyl (one of each on
	each C atom of the C=C).

	 CH3CH2     CH2CH3
	     \     /
	      C = C			cis-3,4-dimethyl-3-hexene
	     /     \
	    CH3     CH3

	This one's easy since the groups on each C of the C=C are the same.
	This is the cis isomer.  You can see this by looking at either the
	two methyl or ethyl groups.  Technically it goes by the two ends of
	the parent chain.

	Look at the following molecule that has a total of 3 different groups.

	 CH3CH2     CH2CH2CH3
	     \     /
	      C = C			cis-3,4-dimethyl-3-heptene
	     /     \
	    CH3     CH3

	Even though there are 3 different groups all together this one isn't
	too bad since there is a CH3 on each C of the C=C and that makes it
	easy to see this is "cis"  (the two ends of the parent chain are also
	cis and this is how you should view it).  Again, even though you can
	see that this is cis from the CH3 groups, technically it is cis
	because the two ends of the parent chain are cis to each other.

	There are times even with 4 "different" groups when we could still
	name the compound as cis or trans according to the two ends of the
	parent chain.

	Look at the following molecule.  It has 4 different groups on the
	C=C carbon atoms.

	 CH3CH2     CH2CH2CH3
	     \     /
	      C = C			cis-4-chloro-3-methyl-3-heptene
	     /     \
	    CH3     Cl

	In our simple world this would still be refered to as 'cis' since the
	two ends of the PARENT chain are on the same side of the double bond.
 	The two ends of the parent chain are not the same group.
	However,  the two ends of the parent chain are kind of the "same".
	They are both alkyl groups and part of the parent chain.  The two ends
	of the parent chain are cis to each other.

	Look at the next two molecules.  In each case there aren't alkyl groups
	coming off each C of the C=C.  You can still name them.  Even though
	there aren't two "ends" of a parent chain to use for naming as cis-trans
	you can use something else (other atoms or groups) and get unambiguous
	names or be able to draw the structure from the name.

 	Look at the following molecule.  It has a total of 3 different groups.

	 CH3CH2     Cl
	     \     /
	      C = C		trans-1,2-dichloro-1-butene
	     /     \
	    Cl      H

	This is different than the one above with 3 different groups.  There
	is no alkyl group coming off the right-hand C of the C=C (just an H).
	We can still use cis and trans for this.  We can look at the Cl atoms
	and use those (trans to each other in this case).

	What if you have something like the following.  This is similiar to
	the one immediately above but with an H on each C of the C=C rather
	than Cl atoms.

            Cl      H
             \     /
              C = C		trans-1-chloropropene
             /     \
            H       CH3

	In this case you don't really have a "parent" chain.  Each C of the
	C=C does not have an alkyl group attached to it to give two ends of
	a "parent" chain.  In this case you name according to the H atoms
	and this would be trans.  Note that I didn't need a number to indicate
	the position of the double bond since in propene the C=C has to be
	between C1 and C2.

	How about the following case.  It is a little tricky.

	 CH3CH2     H
	     \     /
	      C = C		trans-3-methyl-2-pentene
	     /     \
	    CH3     CH3

	Is this cis or trans?  If you just look at the CH3 groups this looks
	like it is cis (the CH3 groups are on the same side of the C=C).
	According to the book this would be trans.  Why?
	You are supposed to go by the parent chain.  The parent chain is a
	pentene and the two ends are TRANS to each other.

	We could not do this for the following molecule that has 4 different
	groups because of how the groups are situated on the C=C atoms.

	 CH3CH2     H
	     \     /
	      C = C		1-chloro-2-methyl-1-butene
	     /     \
	    CH3     Cl

	We can not use cis or trans because it really isn't unambiguous.
	We might say the "H" is cis to the "CH2CH3" group and treat it
	as part of the "parent" chain.  However, that is not quite the same
	or at least not as easy to see as the case above where there is a
	Cl on each atom.  We could just as easily say the H is trans to the
	CH3 group.  I can't name it as either cis or trans so I might just
	give this name with the understanding that we could have cis or trans
	isomers but not be able to name it using cis or trans.

	There are many times that we just have a condensed formula not showing
	whether the molecule is cis or trans or we really aren't concerned
	about whether we have cis or trans (just the fact it is an alkene).
	Then we might just show the name as immediately above w/o cis or trans
	in the name.  We would understand after drawing it that there would be
	cis and trans isomers.

I've gotten most of this information from Dr. Oullette himself (the author of
the book), the answer manual to the book and the test bank for the book, which
was originally put together by Dr. Oullette.