In this video, we're going to focus on naming organic compounds, particularly alkanes. So let's start with a simple example. How can we name this alkane?
Well, we need to identify the number of carbons in the parent chain. And so because it has six carbons, this is called hexane. Now keep in mind, one carbon is methane, two carbons is ethane, three is for propane, four is for butane.
5 carbons is pentane, 7 is heptane, 8 is octane, 9 is nonane, 10 is decane. Now what about this example? How can we name this particular alkane? The first thing you want to do is count the number of carbon atoms in the parent chain, or the longest chain.
Now should we count it in this direction, or... should we count it in this direction. The methyl group that we see here is called a substituent and you want to count in such a way that the substituents have a lower number and 2 is lower than 4. So this is the correct way to count it.
So how can we name it? We have a methyl group on carbon 2 and There's five carbons in the parent chain, so this is going to be called 2-methylpentane. You need to use a hyphen to separate a number from a word.
Now what about this example? So clearly, we need to number it from left to right rather than right to left. Because we want the substituents to be 2, 3 instead of 4 and 5 if we counted it in this direction.
So 2 and 3 are lower than 4 and 5, so we're going to use this example. Now we have a methyl group on... Okay, this computer is having issues now. So we have a methyl group on carbon 2. and on carbon 3. So how can we name it in this case?
So the parent chain is hexane because it has six carbons and there's two methyl groups. So how can we write this? So you need to write 2,3-dimethyl, dyes for 2, tryes for 3, tetras for 4. So it's dimethylhexane. If you have multiple substituents, then you need to use a comma. to separate the numbers.
By the way, when you get a chance, check out the description section of this video. I'm going to post a few links, particularly my Patreon page, where you can access the Organic Chemistry 1 and 2 final exam preparation videos, if you're currently studying for that. So feel free to take a look at that when you get a chance. I'm going to post some other links to some other videos as well that you might find helpful too.
Now what about this example? Go ahead and name that particular alkane. So we're going to number it from left to right because it's going to give us the lower numbers. And this time, we have three methyl groups as opposed to two. So the parent chain is still hexane, and so let's put it all together.
So we have a methyl group on carbon 2, and we have two methyl groups on carbon 3, so we have to write 3, 3. You don't want to write the 3 just once. We need to write it twice because there's two of them on carbon-3. And then we'll need a hyphen, trimethyl, because there's three methyl groups, and then the parent name, hexane. So that's how we can name this particular organic compound. Here's another example for you.
Go ahead and try that problem. So we need to number it from left to right, and it looks like we have a methyl group on carbon 3, and on carbon 4, we have a substituent with two carbons, so that's an ethyl group. Now how should we name this? How can we put this together? Should we name it as 3-methyl-4-ethyl, and the parent chain has 7-methyl-4-ethyl?
carbons so that's heptane or should we say 4-ethyl-3-methyl-heptane. So which of these two names is correct? Now looking at the first example, the numbers are arranged in increasing order. Now looking at the second example, we can see that it's arranged in alphabetical order. E comes before M.
So which one should we give more priority to? Arranging the numbers in ascendant order or putting the letters in alphabetical order? It turns out in this situation, you need to alphabetize the substituents. So the ethyl group must come before the methyl group, regardless if the numbers are increasing or decreasing.
So this is the right answer. Now let's move on to our next example. So notice that we have a propyl substituent, there's three carbons, and we have an ethyl substituent.
How do you think we should name this particular molecule? If you want to pause the video and try the problem, feel free to do so. Now what direction should we count this molecule? So let's count it both ways. I'm going to redraw this structure.
So in the first example, let's count it from left to right. And for the second example, let's count it the other way. Now based on the last example, we know that we have to put the substituents in alphabetical order.
So ethyl comes before methyl. I mean not methyl, but, well it does come before methyl, but in this example, ethyl comes before propyl. So naming this molecule based on the way it's counted, it's going to be called 5-ethyl-4-propyl, and there's 8 carbons in the parent chain, so that's going to be octane. Now, for this molecule at the bottom, the ethyl is on carbon 4, so it's 4-ethyl, and then the propyl is on carbon 5. So 5-propyl and then octane. So looking at these two examples, it doesn't matter which way we number it in terms of the numbers that we have because we do get the same numbers.
Here we have 5 and 4 and 4 and 5. So going left to right or right to left, we still get the same exact numbers. But notice that the way the numbers are arranged when we name it. In both cases, the substituents are placed in alphabetical order.
But in the first example, the numbers are in decreasing order. It's 5 then 4. And in the second example, it's in ascendant order. So if you get to the situation where it's already placed in alphabetical order, then the first group should have the lower number.
Or in other words, you want to place the numbers in ascendant order, going from low to high. So this is the right way to name it. Now let's move on to our next example. Feel free to try that problem. So we need to number it from left to right.
If we do it the other way, notice that we won't get the lower numbers. Going from left to right, we have two methyls on carbon 2 and an ethyl on carbon 4. From right to left, we have an ethyl on carbon 3, two methyls on carbon 5. So 2, 2, 4 is less than 3, 3, I mean 3, 5, 5. So we want to number it from left to right. Now let's go ahead and name it. So we have an ethyl.
on carbon 4 and 2-methyls on carbon 2 so that's a dimethyl. Now how should we name this? Should we say it's 4-ethyl-2,2-dimethylhexane or should we say it's 2,2- dimethyl-4-ethylhexane, which is the right way to name it.
Now looking at the second example, the numbers appear to be in ascendant order, so that seems like a good sign. And also, D comes before E, so that looks like the second answer should be the right answer, but it turns out that it's not. And this...
is the right answer and the reason is that when you're placing the substituents in alphabetical order you don't take into account the prefixes so you need to ignore the prefix di tri tetra and things like that so you're comparing E and M so therefore ethyl comes before methyl so we need to put it in alphabetical order before looking at the way the numbers are arranged when you write your final answer. So placing it in alphabetical order has more priority than making sure that the numbers are in ascendant order.