Hey guys, Professor Dave here. I want to talk to you a little bit about IUPAC nomenclature of alkanes. He knows a lot about the science stuff, Professor Dave explains. So 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.
So 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 because in organic chemistry we're going to be dealing with very large molecules and scientists, chemists are lazy. We don't want to write every atom.
So we're going to be using from now on something called line notation. So what we see here where we would be explicitly writing out every single 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 hydrogen atoms are implied. And So you can see how this 5-carbon hydrocarbon is going to actually be notated like this. So 1, 2, 3, 4, 5 carbons, and the hydrogens are implied.
So 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 given carbon will be implied. So that's something that we're going to have to get used to. We're going to have to be able to look at something like this and realize that there are 3 hydrogens here, 2 hydrogens, etc. Now... it may look a little funny in this kind of zigzag pattern but if we remember our molecular geometry stuff from general chemistry that's actually for a very important reason right because each carbon here in an alkane 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 that is 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 these hydrogens. so we're not going to draw them. this is what we're going to draw. but this is what that really means. and if you haven't seen this kind of notation yet with the dash and the wedge lines, what this means, each wedge bond implies that that atom is coming towards you.
it is coming out of the plane of the board of the page. so this hydrogen is extended this way. and the dash bonds are going into the page or into the board. So this hydrogen is further away from you than the plane of the board.
And 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. So 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. so 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.
so an organization called IUPAC, this is 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. so the first kind of molecule that we want to talk about is alkanes. so alkanes are a type of 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 hydrogen's possible, in other words no double or triple carbon-carbon bonds.
we have only single carbon-carbon bonds. so any alkane is going to bear the suffix"-ane". so that is the suffix that will be present in the molecule.
the prefix is going to be determined by how many carbons are in the molecule. so depending on what your particular chemistry class is like, you may have to memorize more or fewer prefixes, but the very least we're all gonna have to know these ten. so meth eth prop but pent hex hept oct non dec. these would be the prefixes of a given molecule.
so let's take our five carbon alkane that we were looking at before right one two three four five 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. so what we need to do when we are naming a hydrocarbon is we need to 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.
so now that we have identified the longest chain in the molecule we're going to have to number it. now 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're going to follow we will all be able to assign the same name to the molecule so 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. so this is going to be one two three four like that and the one carbon substituent is going to be on carbon two. if we had numbered from this direction it would have been a number three.
so now we have to name the substituent. we're going to use the same prefixes that we already know, but we're going to change the suffix. so this is a one carbon substituent so we're going to use the prefix meth, but this is not a methane group because methane would be a completely separate one carbon molecule this is a substituent coming off of the main chain so we are going to call this a methyl group. if you had a two carbon substituent it would be an ethyl group three carbon substituent 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, right? 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 two methyl butane. so 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 us 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, professordaveexplains at gmail.com.