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Current time:0:00Total duration:7:29

Representing structures of organic molecules

Video transcript

the one thing that's probably causes some of the most pain in chemistry and in organic chemistry in particular is just the notation and the the nominal HR the naming that we use and what I want to do here in this video and really the next few videos is to just make sure we have a firm grounding in the notation and in the nomenclature or how we name things and then everything else will hopefully not be too difficult so just to start off and this is really a little bit of review of regular chemistry if I just have a chain of carbons an organic chemistry is dealing with chains of carbons let me just draw let me just draw a one carbon chain so it's really kind of ridiculous to call it a chain but if we have one carbon over here and it has four valence electrons it wants to get to eight that's the magic number we learned in just regular chemistry for all molecules that's the stable a valence structure I guess you could say so it would if it could partner to bond with is hydrogen so as it has it has four valence electrons and then hydrogen has one valence electron so they can each share an electron with each other they can each share an electron with each other and then they both look pretty happy I said it's the magic number for everybody except for hydrogen and helium both of them are happy because they are only trying to fit fit they're only trying to fill their 1s orbital so the magic number for those two guys is two so all of the hydrogen's now feel like they have two electrons the carbon feels like it has eight now there are several ways to write this you could write it just like this and you can see the electrons explicitly or you can draw a little lines here so I could also write this exact molecule which is methane and we'll talk a little bit more about why it's called methane later in this video I can write this exact structure like this a carbon bonded to four hydrogen's bonded to four hydrogen's and the way that I've written these bonds right here you could imagine that each of these bonds consists of two electrons one from the carbon and one from the hydrogen now let's explore slightly larger chain so let's say I have a two carbon chain now let me do a three carbon chain so it already looks like a chain so if I were to draw every explicitly it might look like this so I have a carbon I have a carbon it has one two three four electrons maybe I have another carbon here that has let me draw it let me do the carbons in slightly different shades of yellow I have another carbon here that has one two three four electrons and then let me do the other carbon in that first yellow and then I have another carbon so we have three carbon chain that has one two three four valence electrons now these other guys are unpaired and if you don't specify it it's normally going to be hydrogen so let me draw some hydrogen's over here so you're going to have a hydrogen there a hydrogen over there a hydrogen over here a hydrogen over here a hydrogen over there a hydrogen over here almost done hydrogen there and then a hydrogen there now notice in this molecular structure that I've drawn I have three carbons they they were each able to form four bonds this guy has bonds with three hydrogen's and another carbon this guy has a bond with two hydrogen's and two carbons this guy has a bond with three hydrogen's and then this carbon right here and so this is a completely valid molecular structure but it was kind of a pain to draw all of these valence electrons here so what we typically would want to do is at least this structure and we're going to see later in this video there's even simpler ways to write it so if we want to at least do it with this kind of these lines we can draw it like this so you have a carbon carbon carbon and then they are bonded to the hydrogen's so you'll almost never see it written like this because this is just kind of crazy hydrogen hydrogen at least crazy to write it takes forever and it might be messy like it might not be clear where these electrons belong I didn't write it as clearly as I could so they have two electrons there they're shared with these two guys hopefully that was reasonably clear but if we were to draw it with the lines it looks just like that so it's a little bit neater faster to draw same exact idea here and here and in general and we'll go in more detail on it this three carbon chain where everything is a single bond is propane let me write these words down because it's helpful to get this is methane and you're going to see the rhyme you're going to see the the reason to this naming soon enough this is methane this is propane propane and there's an even simple simpler way to write propane you could write it like this you could write it like this you could write it you could instead of explicitly drawing these bonds you could say that this part right here you could write that that part right there that is ch3 so you have a ch3 connected to a this is a ch2 that is ch2 ch2 which is then connected to a another ch3 and the important thing is no matter what the notation as long as you can figure out the exact molecular structure as long as you can so there's this last ch3 whether you have this this or this you know what the molecular structure is you could draw any one of these given any of the others now there's an even simpler way to write this you could write it just like this let me do it in a different color you literally could write it so we have three carbons so 1 2 3 and now this seems ridiculously simple and you're like how can this this thing right here give you the same information as all of these more complicated ways to draw it well in in chemistry and in organic chemistry in particular any of these this is called I call it a line diagram or line angle diagram it's the simplest way and it's actually probably the most useful way to show chains of carbons or to show organic molecules once they we could start to get really really complicated because then it's a pain to draw all of the H's but when you see something like this you assume that the end points of any lines have a carbon on it so if I if you see something like that you assume that there's a carbon at that end point a carbon at that end point and a carbon at that end point and then you know that carbon makes four bonds there there are no kind of charges here all of the comp carbons are going to make four bonds and each of the carbons here this carbon has two bonds so the other two bonds are implicitly going to be with hydrogen's if you they don't draw you assume that they're going to be with hydrogen's this guy has one bond so the other three must be with hydrogen this guy has one bond so the other three to be hydrogen's so just drawing that little line angle thing right there I actually did convey the exact same information as this this depiction this depiction or this depiction so you're going to see a lot of this this really simplifies things and sometimes you see things that are in between you might see someone draw it like this where they'll write ch3 and then they'll draw it like that so that's kind of combining this way of writing the molecule where you write the ch3 s for the endpoints but then you implicitly have the ch2 on the inside you assume that this endpoint right here is a C and it's bonded to two hydrogen so these are all completely valid ways of of drawing the molecular structures of these carbon chains or of these organic compounds
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