Hey guys, Professor Dave here.
I want to talk to you a little bit about IUPAC nomenclature of alkanes.
If you're going to be learning organic chemistry
it's going to be very important for us to be able to discuss nomenclature
how we're going to refer to molecules by name.
The first thing that we have to understand is, what we were maybe familiar with in general chemistry
this condensed formula notation, that's no longer what we're going to use in organic chemistry.
In organic chemistry we are going to be dealing with very large molecules
chemists are lazy, we don't want to write every atom
so we are going to be using something called line notation.
So what we see here, where we would be explicitly writing out every atom
we're going to have this new convention here, and what it is, is that every vertex and endpoint
is a carbon atom, and all of the hydrogens are implied.
So you can see how this five-carbon hydrocarbon is going to actually be notated like this.
1, 2, 3, 4, 5 carbons, and the hydrogens are implied.
What I mean by saying that the hydrogens are implied is that they are there
and the maximum number of hydrogens that can be bound to any carbon will be implied.
So that's something that we're going to have to get used to, to look at something like this
and realize that there are three hydrogens here, two hydrogens here, et cetera.
Now, it may look a little funny in this zig-zag pattern
but if we remember our molecular geometry stuff from general chemistry
it's actually for a very important reason.
Each carbon in an alkane, a fully saturated where there's no double or triple bonds
each carbon is going to be sp3 hybridized
so it must be bound to four different atoms
and anything sp3 hybridized is going to have tetrahedral molecular geometry.
So this is what this actually looks like, but we want to save the time, we don't want to draw all the hydrogens
so we're not going to draw them, this is what we're going to draw, but this is what that really means.
If you haven't seen this kind of notation yet, with the dash and the wedge lines
each wedge bond implies that that atom is coming toward you, it is coming out of the plane of the board
so this hydrogen is extended this way
and the dash bonds are going into the board, so this hydrogen is further away from you
so that obeys tetrahedral geometry, we can see that all of the atoms are accounted for
and this is the kind of notation we are going to use from now on as we learn organic chemistry.
As chemists, we're going to want to be able to communicate with each other efficiently.
This means that when we want to discuss a particular molecule, it may be easier to refer to it by name
but that name is going to have to be of a type that every scientist wherever they are in the world
is going to agree about what molecule we're talking about.
We're going to have to learn a set of rules that are completely arbitrary
but if we all follow them, we will all be able to name molecules the same way.
An organization called IUPAC
the International Union of Pure and Applied Chemistry
has come up with these rules that we can follow to assign names to any organic molecule.
The first kind of molecule that we want to talk about is alkanes.
Alkanes are a type of a hydrocarbon
a hydrocarbon being a molecule consisting only of carbon and hydrogen
and alkanes are hydrocarbons that are fully saturated.
This means that they contain the maximum number of hydrogens possible
inotherwords, no double or triple carbon-carbon bonds.
We have only single carbon-carbon bonds.
Any alkane is going to bear the suffix "ane".
The prefix is going to be determined by how many carbons are in the molecule.
Depending on what your particular chemistry class is like, you may have to memorize more or fewer prefixes
but at the very least we're all going to have to know these ten.
meth, eth, prop, but, pent, hex, hept, oct, non, dec.
These would be the prefixes of a given molecule.
Let's take our five-carbon alkane we were looking at before, 1, 2, 3, 4, 5
that is going to be pent for the prefix, and it is an alkane, so ane.
That makes this molecule pentane.
So while it would be wonderful if every molecule we had to name were as simple as a straight-chain alkane
that's not going to be the case. Molecules have a variety of substituents which are smaller groups
branching off of a main chain.
So let's look first at a very easy example of a branched-chain alkane.
What we need to do when we are naming a hydrocarbon
is identify the longest chain of carbons in the molecule
Right now that's going to be pretty simple, we can see that there is a four-carbon chain here
and there is a one-carbon substituent branching from the main chain.
Now that we have identified the longest chain in the molecule, we have to number it.
There are two options, you can go left to right, or right to left. We are going to choose the direction
that gives the substituent occurring sooner or on the lower numbered carbon.
Once again, this is a completely arbitrary rule, it has nothing to do with nature
but if we all agree that these are the rules we are going to follow
we will all be able to assign the same name to the molecule.
Here we are going to number the molecule left to right
and not right to left, because if we number from the left
the one-carbon substituent will occur sooner.
This is going to be 1, 2, 3, 4 like that, and the one-carbon substituent is going to be on carbon 2.
If we had numbered from this direction, it would have been on number 3.
Now we have to name the substituent.
We are going to use the same prefixes that we already know, but we are going to change the suffix.
This is a one-carbon substituent so we are going to use the prefix meth
but this is not a methane group, methane would be a completely separate one-carbon molecule.
This is a substituent coming off of the main chain
We are going to call this a methyl group.
If you had a two-carbon substituent it would be an ethyl group, three carbons would be propyl, and so forth.
So now we know that we have a main chain of four carbons making this a molecule of butane
four for butane
and we have a methyl group on carbon 2
so now we're going to go ahead and name this molecule, and we have to list the substituents.
We only have one so it's going to be easy
we have to state the location, so we are going to call this 2-methylbutane.
Anybody anywhere around the world could read 2-methylbutane and be able to draw this molecule
because it is describing unambiguously everything about the molecule.
Butane is telling is that it is a four-carbon hydrocarbon
and 2-methyl is telling us that there is a methyl group
and it is telling us where it occurs on the molecule.
Thanks for watching, guys. Subscribe to my channel for more tutorials
and as always feel free to email me with questions