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Organic chemistry
cis-trans and E-Z naming scheme for alkenes
Cis-Trans and E-Z Naming Scheme for Alkenes. Created by Sal Khan.
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- So (E)-but-2-ene and (Z)-but-2-ene are diastereomers, right?(20 votes)
- Can you please explain cis and trans a little bit more? Are they only involved in alkenes or can they be involved in alkanes as well? And when should I use E and Z vs. cis and trans?(6 votes)
- Cis and Trans isomerism only happens around an alkene because they're locked into the positions thanks to the pi structure. You don't get any rotation around a pi bond whereas you do around a sigma bond so theres no cis and trans for alkanes
So if you had two methyl groups in the 2,4 positions of butane, one above and one below the chain it wouldn't be trans-2,4-dimethyl-butane because the main chain can rotate and essentially bring them into the cis arrangement so it just doesn't work.
E and Z comes in when you have and alkene with more than two substituents such as CFBR-CClH where you can't say exactly say which is cis and which is trans: The rules for this are, the higher its atomic mass the more priority the atom has so if Br and Cl are on the same side across the double bond the molecule is Z where as if they're on opposite sides across the bond the molecule is E :)(11 votes)
- The priority order of functional groups for naming hydrocarbons(4 votes)
- carboxylic acid > ester >aldehyde >ketone >alcohol >amine(7 votes)
- In the video Sal says the CH3 group is a functional group. What does he mean by this because I thought hydrocarbon chains aren't functional groups?(3 votes)
- Perhaps a better word to use would be "substituent". We normally do not consider alkyl groups as functional groups.(3 votes)
- Which is correct:
cis - 2, 4 - dichloro - 3 - propylhex - 2 - ene or
2, 4 - dichloro - 3 - propyl - cis - hex - 2 - ene ?(2 votes)- Cis would come first.(5 votes)
- What is a functional group?(1 vote)
- It is the part of the molecule that determines the characteristic properties and reactions of the compound. The functional group is used for the naming and classification of a compund(4 votes)
- is it necessary for cis/trans to have the same functional group on either side of double bond(2 votes)
- No. There is no reason to have the same functional group on both sides because you will never say "cis" or "trans" for an alkene except in conversation. Organic chemists use E or Z exclusively. The ones who don't are....still stuck in freshman biology.(2 votes)
- So why we use syn and anti naming system when we already have cis - trans and E-Z naming system?(2 votes)
- Alkenes are usually named using cis/trans or E/Z. Syn and anti are reserved to indicate the geometry of addition across a double bond when it undergoes reactions: if two atoms add from the same side of the double bond then it is called syn, and if they add from opposite sides of the double bond, then it is called anti addition. Anti and syn are sometimes used in common names, such as in oximes to indicate the orientation of the OH group; there's no particular reason why oximes can be named that way, that's just how the (non-systematic) naming emerged.(2 votes)
- Why don't we have exercise about organic chemistry?(2 votes)
- Why does 1,2-dimethylbenzene does not have geometric isomery?(2 votes)
- Simple answer: There is no way to make geometric isomers of 1,2-dimethylbenzene.
The molecule rotates, but no more than any other. A geometric isomer (outdated term) is now know as an E/Z double bond. Can you make the double bond between the two methyls anything other than "Z". It's impossible to place an E double bond inside a ring with fewer than 8 carbons...unless the ring gets bigger. That's why. The prior response doesn't include any explanation at all.(3 votes)
Video transcript
If I were to come over to your
place and say, name this molecule, the first thing you
would do is you'd say, well, look, we have a double
bond, So we're dealing with an alkene. We have four carbons in
the longest chain. There's actually
one chain here. So it's one, two, three,
four carbons. So the prefix will be but-, and
regardless of which end you count from, the double bond
starts at the number two carbon, so this is but-2-ene. I'd say, well, that's very
good, and that is correct based on everything
we've learned. But what about what about
this molecule? What about the molecule
that looks like this? And you'd go through
the same process. You'd say it has
a double bond. It's an alkene. Our longest chain has
four carbons: one, two, three, four. Regardless of which end you
start counting from, the double bond starts at the number
two carbon: one, two, three, four. So once again, you say, hey,
Sal, that's also but-2-ene. And I'd point out that these
are each fundamentally different molecules. And the reason why they're
fundamentally different molecules is because this double
bond can't rotate. It's got that pi bond there
for that second bond that keeps this carbon-carbon
bond rigid. So it's not like the molecule
can switch configurations between this-- it's not like
it can switch conformations between this and that. It cannot rotate around this
double bond, so these are different molecules. These are different molecules
that will behave different chemically. And because they behave
different, we have to have different names for them. Now, there's two different
naming conventions. One is the simpler naming
convention, and it works when we have one functional group on
each of the carbons in the double bond. And then in the next two videos,
we'll talk about the slightly more advanced naming
scheme that will work when we have more than one
functional group. So the first way to name
it is if you have your functional groups. So let me circle the
functional groups. So in this top one, our
functional groups are on opposite sides. They're apart. They're on opposite sides of the
carbon-carbon double bond. This is on the top, This
is on the bottom. In this molecule right here, our
functional groups are on the same side. So when our functional groups
are on different sides, we could call it trans. We could call it trans,
literally, I believe, Latin for, apart, or Latin
for opposite. I'm not a Latin scholar,
so forgive me. But the functional groups
are apart, so we call it trans-but-2-ene. The other convention to use
comes from the German forepart and there we call it entgegen. And instead of writing out
entgegen, we write E in parentheses. So we would call them-- let me
just use different colors since this molecule
down here is blue. So we would call this
(E)-but-2-ene. These mean the same thing. (E)-but-2-ene. But this is now kind of the
standard convention. We'll see this notation where
you use entgegen, or (E), is actually more powerful. It can extend to when we have
more than one functional group on each carbon. Let me just make it clear that
the (E) stands for entgegen, which is German for apart, or
at least I believe it is. I can't speak German so I'll
just have to take people's words for it. Now, in this situation where our
functional groups are on the same side, you could call
this cis-but-2-ene. Cis, I believe, comes from the
Latin for together, or you could call it (Z)-but-2-ene. And you could imagine
this (Z) comes from the German for together. And it stands for zusammen,
which is German for together. And that's all you have to do. And in the next few videos,
we'll do a bunch of examples here so we can really figure out
whether to label something with either an (E) or a (Z). But I wanted expose you to the
cis- and trans- naming scheme, because that's sometimes used
for simpler molecules where you only have one functional
group on each side. But I want you to realize that
they're really trying to make the same differentiation. They're really just trying to
decide whether the functional groups on one side of a double
bond and on the other side of a double bond, whether they're
on the same side, cis-, or (Z), or zusammen, or on
different sides, trans- or (E), or entgegen.