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Current time:0:00Total duration:9:52

Video transcript

let's say we're asked to draw all the structural isomers that have the molecular formula c5h12 the word isomer means same parts and so we're talking about the same number of atoms all of our structural isomers are going to have five carbons and 12 hydrogens our isomers are going to differ and how those atoms are connected to each other so they differ in terms of their structure and that's why we call them structural isomers you can also call them constitutional isomers so we need five carbons so for our first isomer we could just draw five carbons in a chain so here are my five carbons in a chain and you should have already seen the video on bond line structures before you watch this one so let's draw out those five carbons and let's double check and make sure we have the correct number of hydrogen's the carbon on the far left has three hydrogen's so here we have our three hydrogen's next carbon has two same with the next carbon so two for this one two for the next carbon and finally three hydrogen's for the last carbon so let's count up everything and make sure we have the correct molecular formula so we have 1 2 3 4 5 carbons so that's c5 and then we should have 12 hydrogen's here's 3 plus 2 gives us 5 plus 2 gives us 7 plus 2 gives us 9 and then we need we have 3 more for a total of 12 so c5h12 is the molecular formula for this compound all right let's draw another structural isomer that has the same molecular formula so instead of drawing five carbons in a chain now we have to draw four so let's start by drawing four carbons we need a total of five carbons so we need to show the fifth carbon branching off of our chain so we could show the fifth carbon branching off of our chain here so let's draw in those five carbons so here we have our five carbons let's count up hydrogen's carbon on the left has three so three hydrogen's here three hydrogen's on this top carbon there's only one hydrogen on this carbon two hydrogen's on this one and finally three high on this carbon so let's count up our atoms so let's use red for this one we have 1 2 3 4 5 carbons so that's c5 and then for hydrogen's we have 3 here plus 3 gives us 6 plus 1 gives us 7 plus 2 gives us 9 and 3 more for a total of 12 so c5h12 is the molecular formula for this compound so these two these two drawings represent two different molecules both these molecules have the molecular formula c5h12 but they differ in terms of how those atoms are connected they turn they differ in terms of their structure so we call them structural isomers of each other all right to draw another structural isomer some students might say oh well we could start with four carbons in our chain again and this time instead of showing instead of showing a branch off of this carbon we could show a branch off of this carbon and so a student might draw this structure and say okay there's a different structural isomer but actually these just two different ways to represent the same molecule if you analyze that second structure that we just drew all right the connections are the same we have a CH right here bonded to a ch3 bonded to a ch3 and bonded to a ch2 and the ch2 is bonded to a ch3 that's the same that's the same structure as what we drew out over here so it looks like it's a different structure all right it's a different drawing than the one up here but actually this is just two different ways to represent the same molecule all right so we have two structural isomers so far let's think about one more so we can no longer do four carbons in our chain so we go down to three carbons so we start with three carbons in our chain we know we need a total of five carbons so we need to show two more carbons added to our chain and we would have to add those two carbons to our central carbon like that let's draw out all of our carbons here and let's add in our hydrogen's so this carbon would have three hydrogen's same with this carbon and the same with this one and finally the same for this carbon the carbon in the center alright this carbon the center here already has four bonds so it doesn't have any hydrogen's on it let's count up everything let's count our carbons first 1 2 3 4 5 carbons so c5 and then we have three hydrogen's plus 3 is 6 plus 3 is 9 plus 3 is 12 so c5h12 is the molecular formula for this compound and this is a another structural isomer so it's a different molecule from the other two so we have a total of 3 structural isomers that have the molecular formula c5h12 now let's draw all the structural isomers that have the molecular formula c3h8 oh and we'll start with the molecule we talked about in the bond line structure videos so that molecule looked like this we had 3 carbons and then we had an OHA off of the central carbon alright let's expand that out and make sure that this has the correct molecular formula we have our 3 carbons and on the middle carbon we have an OHA so an oxygen bonded to a hydrogen I'll go ahead and put lone pairs of electrons on this oxygen how many hydrogen's do we need to add to the carbon on the left well we need to add 3 hydrogen's so go ahead and drawn those 3 hydrogen's the carbon in the center already has three bonds so it needs one more so we add one hydrogen to that carbon the carbon on the right needs 3 hydrogen's so let's count everything up now so we'll start with our carbons so we have one two three carbons so that's c3 we have three hydrogen's here and 3 here so that's 6 plus 1 is 7 and don't forget about the hydrogen on the oxygen for eight so we have eight hydrogen's and obviously we have one oxygen here so I went ahead and put in lone pairs of electrons on that oxygen so the molecular formula for this for this molecule is c3h8 oh if i numbered this if i said this was carbon 1 and this was carbon 2 and this was carbon 3 that helps us to draw the next structural isomer because we can think about instead of that Oh H group coming off of carbon - what if that Oh H group came off of carbon 1 and so let's draw out our 3 carbons here and now we put our H group coming off of carbon 1 and let's expand this out and draw the Lewis dot structure and make sure that this has the correct molecular formula so we have 3 carbons again in a row and then the carbon on the left is bonded to the oxygen the oxygen is bonded to a hydrogen I'll put in lone pairs of electrons on the oxygen now we need to add in carbon hydrogen bonds so this carbon needs to the next carbon also needs to and the carbon on the end would need 3 so that's 1 2 & 3 when we add everything up let's use blue for that that's 1 2 3 carbons so we have C 3 we have 3 hydrogen's here plus 2 is 5 plus 2 is 7 and 1 here is 8 so C 3 H 8 and then of course our oxygen so C 3 H 8 o is the molecular formula alright next some students might think okay well we put an O H coming off of carbon 1 but what if I put an O H on the other side so over here on the other side so let's see what would that give us if I put an O H coming off of that carbon hopefully it's obvious that these two represent the same molecule right there's no difference in terms of how those two are connected structurally so this is the same molecule so two different ways to draw the same one so this is not a new structural isomer just a new way of looking at this molecule all right now let's draw one more so we can't we can't put the O H on the other carbon so now we have to figure out something else that we can do well we could this time put two carbons in a row and put an oxygen in between so putting an oxygen to break up our carbon chain so now this would be carbon bonded to carbon bonded to oxygen bonded to carbon and then we fill in our hydrogen's so there'd be three on this carbon there would be two on this carbon there would be three on this carbon and I could put in lone pairs of electrons on oxygen like that and count everything up so we have one two three carbons so that's c3 we have three hydrogen's plus two is five plus three is eight so we have the H H and then of course the one oxygen so this is another structural isomer again some students might say well we could go like this and this would be yet another structural isomer like that but really this is just another way to draw this molecule so it's not a new structural isomer it has the same connections so we have a total of three structural isomers that have the molecular formula c3h8 oh and as you go further in organic chemistry you'll learn that the first two isomers we talked about so this one and this one the ones that have an H on it those are called alcohols and the last structural isomer is called an ether so we'll worry about that more later in in other videos