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 and 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 five carbons is pentane seven is heptane eight is octane nine is nine ten 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 account it so how can we name it we have a method group on carbon two and there's five carbons in the parent chain so this is going to be called 2-methyl pentane 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 two and three are lower than four and five 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 method 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 two comma three dash dimethyl dies for two tries for three tetras of four so it's dimethyl hexane 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 one and two 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 two and we have two methyl groups on carbon three so we have to write three three you don't wanna write the three just once you need to write it twice because there's two of them on carbon three and then we 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 dash 4-ethyl and the parent chain has seven carbons so that's heptane or should we say for ethyl dash three 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 could 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 the senate 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 name in this molecule based on the way it's counted it's going to be called 5 ethyl dash 4-propyl and there's eight carbons in the parent chain so that's going to be octane now for this molecule at the bottom the ethyl is on carbon four so it's four ethyl and then the propyl is on carbon five so five 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 five and four and four and five 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 decrease in order it's five then four 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 two and an ethyl on carbon four from right to left we have an ethylene carbon three two methyls and carbon five so two two four is less than three three i mean three five five so we want to number it from left to right now let's go ahead and name it so we have a ethel on carbon four and two methyls on carbon two so that's a dimethyl now how should we name this should we say it's four ethyl dash 2 comma 2 dimethyl hexane or should we say it's 2 comma 2 dimethyl dash 4-ethyl hexane which is the right way to name it now looking at the second example the numbers appear to be an 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