hey guys professor Dave here, I wanna talk to you about some more nomenclature
this time the nomenclature of alkenes, alkynes
and cyclic compounds.
so alkenes, like alkanes, are hydrocarbons
however alkenes are molecules that have at least one double bond present.
now just like alcohols as we saw before, having a double bond present in the molecule
sufficiently changes the reactivity of the molecule that we need to change the
parent name of the molecule. so an alkene
will bear the suffix "ene" to let us know that there is a double bond
present in the molecule. now fortunately most of the rules remain the same.
we're going to identify the longest carbon chain
just as with the presence of the hydroxyl group, the presence of the double bond
means that we're going to number the parent chain so as to give
the double bond specifically occurring soonest
even if there is some alkyl or halogen substituent occurring sooner if we
number from the other side
so here we're gonna need to number left to right
let's identify
that there are two methyl groups over here on carbon 4
so we are going to list that first in the molecule
that's 4,4-dimethyl
we do need to list both those otherwise there would be
ambiguity as to where the second methyl group would be so 4,4-dimethyl
and then we have to list the location of the double bond
it begins at carbon 1 so
this is 1-pentene
so 4,4-dimethyl-1-pentene
we do need to list where the double bond occurs in the molecule
so as to remove ambiguity as to its location
so let's take a look at alkynes now
an alkyne is a hydrocarbon that contains at least one
triple carbon-carbon bond. now alkynes can be a little bit tricky to draw
properly because
any carbon atom that is participating in a triple bond
is necessarily sp hybridized, this means that it exhibits
linear molecular geometry, we remember from gen-chem
and that means that the two carbons that
it is attached to must be 180 degrees apart
so this looks a little funny but bear in mind that
these are carbon atoms and these are the two carbon atoms that are participating
in the triple bond
so let's go ahead and circle the longest chain, now we have to number it and as you
expect just as with the double bond
or the hydroxyl group we must number this so as to give
the triple bond occurring soonest even if there is some other alkyl or halogen
substituent occurring sooner from the other direction
so that means that even though a fluorine is on carbon 2
coming from the right side, we must number this from the left
because that will give the triple bond occurring soonest.
so we do have a 7-carbon alkyne
once again bearing in mind that these are carbon atoms right there
and this is the appropriate way to draw an alkyne
a very common error I'll see is alkenes
sorry alkynes, drawn like that. while you may still be able to provide or
interpret the correct IUPAC name for the molecule
this is geometrically incorrect so we prefer
not to draw it that way. so now we just have to identify the substituents, we have
a fluoro group on carbon 2
sorry, carbon 6
6-fluoro
and then we must state the location of the triple bond, it begins on carbon 3
so we have a 3-heptyne
so up until this point we've been looking at some pretty
simple examples
but in reality a lot of organic molecules have multiple
different kinds of substituents and different functional groups on the
molecule so let's look at a slightly trickier example
and see what happens when we have multiple different kinds of substituents
on the molecule
so the first thing that we have to do as always is identify the longest carbon chain
what we have to keep in mind is that there is an order of prioritization
of these functional groups that we need to consult so
of the groups that we've learned so far this is the priority
the hydroxyl takes top priority after that would be an alkyne so a
triple bond
then alkene or any double bond and then alkyl groups and halogen substituents have
the same priority so in this case because hydroxide takes the highest priority
it is the location of the hydroxyl group that is gonna determine the direction
that we number
this parent chain and so because the hydroxyl is over here we must number this
left to right
we have a seven carbon chain with a hydroxyl on carbon 1
now here's another tricky thing
on this molecule have two substituents that are both attempting to modify
the suffix of the molecule because this is an alcohol
but it is also an alkene, so that's going to make things a little bit tricky to
name but we do have a way to do it
so let's see what we have, we have got a chloro group
we've got a methyl group and we've got an ethyl group
so we're going to list the chloro group first because
C for chloro, so that's 5-chloro
and then we have
3-ethyl
and then we have four methyl and then here's the kicker
we're gonna go 3-hepten
1-ol
so this seems a little bit strange but it has to be done this way because both
this E is very important because it is indicative of
a double bond and the OL is important because
it is indicative the hydroxyl group but both of those are suffixes at the end of
the molecule
so they both have to be there. the convention is to drop the E
in heptene to indicate that there is more of the molecule to list
and then after that list the presence of the hydroxyl group last
because that does take the top priority, so once again
the simpler substituents your halogens and your
alkyls go first in alphabetical order and then
always last are the functional groups
that are modifying the suffix of the molecule, in this case
the presence of the double bond and the hydroxyl, and we do
have to list numbers indicating where they occur
otherwise it would be ambiguous as to their location on the molecule
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