Transcript for:
Organic Molecule Representations

hello class in organic chemistry there are many ways to represent molecules and we need to be able to do all of them and so I want to go through an example of one molecule and look at the various ways you can draw that molecule and what we're going to look at is isopropyl alcohol that is the rubbing alcohol that you buy from the stores to clean up a wound or to clean an area of skin before you get a shot or anything like that so the molecular formula of let's see is this going to be a good marker let's check it out here C3 h8 oh so that I got a focus here okay this is going to be the molecular formula there's our molecular formula of isopropyl alcohol now that does not that just the only information that tells us is how many carbons hydrogens and oxygens like it doesn't tell us the shape or the structure or the three-dimensional features of this molecule it just says boom there are the atoms in that molecule now could you look at that molecular formula and say isopropyl alcohol no you couldn't you could only say that there's three carbons eight hydrogens and one oxygen but if you go to What's called a condensed structure is going to start showing us the connectivity and what that molecule looks like so we have a ch3 and we're going to have two of them and so I'm putting it in parentheses and doing a subscript too just to tell me that I have two of these ch3s and then we have a CH oh now if I did this correctly we should have the exact same amount of atoms in the condensed structure versus the molecular formula so if this carbon here counts twice there's two three I have three carbons and how many hydrogens do I have well there's three times two that's six seven eight so eight hydrogens and one oxygen so it all has to match that's called a condensed structure and then we have something called a partially condensed a partially condensed structure is one where we start drawing out the the bonds you see how that there's no bonds or Sigma bonds so I'm going to run out of space there so I'm going to draw it below it here so partially partially condensed structures I'll take like this the auction has a lone pair two lone pairs and ch3 H3 and then one hydrogen that also is isopropyl alcohol now pay attention see how this is ch3 and then this is on the left side of the molecule so why don't I why don't I write it like this foreign because we read from left to right right the reason why I don't like drawing it like this is because this Sigma Bond right here the way it's drawn is implying that you're forming a bond with that one of those hydrogen atoms and that's one of the Cardinal rules that we cannot break and that is a hydrogen atom can only form one Bond if I took that hydrogen and said hey let's form it to another carbon like this do you see how that hydrogen right there has two bonds that's not possible and so when I draw it like this what's happening is this hydrogen here one of these hydrogens is bonded to this carbon and then it looks like it's bonded to that carbon as well and you cannot have two bonds per hydrogen atom only one so that's why I draw it like this because we're going to have a carbon bonded to a carbon and then H3 now these whenever you see a ch3 like that it has a special name and we call those methyls and it looks like I got cut off a little bit from the screen so these are called methyl groups so we could just say methyl groups now we could even get some more information by drawing the Lewis structure and the Lewis structure is when we expand out all the atoms and show all the all the bonds so what we have is this methyl group right here I will represent it like this see how there's three hydrogens three hydrogens one carbon that carbon is bonded to another carbon so I'll draw that and that carbon is bonded to an oxygen atom like so it has one hydrogen and then another methyl group so there's isopropyl alcohol in its Lewis structure completely just drawn out so when you take a look at these structures here you can see that the Lewis structure gives us the most information the molecular formula gives us the least amount of information when you take molecules however and draw them out using the Lewis structure it takes a lot of time a lot of time because you're drawing all the bonds you're drawing all the atoms you're drawing all the lone pairs just a lot of work and so what has happened in order to give us all the information in this Lewis structure but draw it more simply we've come up with something called a zigzag structure a zigzag structure uh we could it also goes by the name of a line structure or your textbook refers to it as a bond line structure a bond line that is a horrible way a bond line structure and the bond line structure for isopropyl alcohol looks like this and I'll draw in a different color and put it right here so the bond line structure for isopropyl alcohol looks like this and you can see where the name zigzag it's just boom boom it's zigzagging so that is another way to represent isopropyl alcohol and depending on the question or what you're trying to do each structure has its pros and cons but more often than not organic chemists when they want to discuss and show you a structure they're going to use this zigzag structure because did you notice if you took like a stopwatch how long did it take me to draw this guy versus this guy do you see how fast I can draw isopropyl alcohol it's just boom boom boom done compared to having to draw all this out and drawing in the lone pairs so this is the this is where we want to get to we want to be able to go between all of these structures except for the molecular formula because that one doesn't tell us the structure it just tells us how many atoms there are so what we want to be able to do is if I gave you this condensed structure could you draw me the zigzag or if I gave you the zigzag could you go and draw the Lewis structure so we want to be able to use all of these interchangeably but when we're in when we're trying to be quick the best way is to use our zigzag structures so how do I know this guy right here is representing this well we will get to that after I discuss with you some major major rules that you need to be aware of and need to be reminded of some of these you have learned from General chemistry and we just need to remind ourselves that these ideas here so let's just enumerate them rule one carbon atoms have four bonds carbon atoms need four bonds now what does that mean a carbon atom with four bonds directly attached to it has to have four now this rule right here the first one that I'm talking about for right now that's the law that you're living by four bonds now that law is going to change as we progress through the course but for right now remember carbon needs to have four bonds the second rule is hydrogens can only have one Bond and I already discussed an example of one bond for every hydrogen atom you cannot do double bonds or multiple Bonds on a hydrogen atom but you need to be aware of is oxygen atoms oxygen atoms can have two or three bonds there's another very important rule is your halogens and halogens are simply rep more often than not represented by just a capital X and so X would equal fluorine chlorine bromine and iodine those are the halogens halogens also are like the hydrogen atoms only one Bond so if I had a molecule that looked like this you see how fluorine is bonded to a carbon atom by a sigma Bond only one Bond I want to contrast that what if you tried to draw a molecule like this what is wrong with this molecule it's breaking the fourth rule you cannot have a halogen with two bonds only one and then the fifth rule it is a very very important rule to remember and it kind of goes along with a rule number one is that four bonds sometimes people like to form what's called a Texas carbon and the Texas carbon is simply a carbon that has five bonds to it you cannot have five Bonds on carbon the max is four and so we're going to go through some examples then look at what a Texas carbon would look like so let's let's maybe take this molecule right here right here and just draw a Texas carbon so we can get a better visualization of what's going on here all right let's draw a Texas carbon so what if I put another carbon here and then I go one two [Music] three [Music] four so let's just start from the left so I'm just looking at the carbons here's a carbon how many bonds are around this carbon one two three four perfect this carbon one two three four done this carbon one two three four perfect come to this last carbon one two three four five that right there is a Texas carbon and does not form it cannot form because carbons can only have a Max of four carbons so if I wanted to fix that I would just erase these hydrogens right here and just draw in only one of them so now that is a beautiful carbon and not a Texas carbon okay so those are the rules so now let's show you how to look at a Lewis structure and convert that into a zigzag all right and the way you do that is oh let's get this back to isopropyl alcohol shall we what you do is you count how many carbons there are in like a long in the longest chain so do you see here if I just considered this a chain of carbons going from left to right you see that I can count one two three there's three carbons in that longest chain and so what you do is you have to draw three carbons in the zigzag and every point on the zigzag represents a carbon atom so if I color code or number these if that's carbon one carbon two and carbon three every point on this zigzag represents a carbon and so that's carbon one carbon two and carbon three and on carbons one two and three do you see in the zigzag I don't draw any of the hydrogens on zigzag structures you don't draw hydrogens they are just implied now let me restate that hydrogens attached to carbon atoms are not drawn they are implied but if the hydrogen atom is attached to a heteroatom like this then you have to draw on the hydrogens so I use the word heteroatoms heteroatoms are all atoms except carbon and hydrogen foreign so halogens so your chlorines and your bromines what if you want to put phosphorus or sulfur or a metal like magnesium whenever you have heteroatoms which are non-carbon non-hydrogen elements you have to draw them out in the zigzag structure now I could draw it like this with the oxyhydrogen bond I could expand it out like that if I'd like or to save time and ink can just represent it like that because I'm looking at this auction right there and it's right there as well now on the zigzag structures you can put in the lone pairs if you'd like I think it's a good idea to put them on the zigzag structures because you're going to need these lone pairs in the future and so it's a good idea to know how many there are and so in order to know how many there are you need to go back to General chemistry and review formal charges and how many lone pairs are around atoms and we will we will touch on that uh throughout the course but right now the point of this lesson is to show you that hydrogens attach to heteroatoms need to be drawn whereas hydrogen atoms that are attached to carbon are not so if we so that's how you draw the zigzag structure and we've numbered the carbons so you can see that you don't draw on the hydrogens but as you're learning this you need to be able to look at a zigzag structure and know exactly where the hydrogens are and how many there are and so if you have to draw them in until your brain can start seeing what's Happening Here so this is carbon one how many hydrogens are attached to carbon one well if we go to the Lewis structure it's easy to see one two three but in the skeletal structure you don't you you don't draw them in but remember the rule that carbon has to have four bonds so if I represent this carbon right here like this and that carbon is then bonded to this carbon and then that carbon is bonded to that carbon and I've ran out of my board space here let's let's do it down here shall we so carbon one could be represented like this carbon two and then bond is a carbon three and then to carbon two which is the one in the middle what do we have an o h like so so this zigzag structure is saying hey carbon one when I draw it just like this and not drawing any hydrogens how many bonds does this carbon have it only has one drawn but we know Carbon has to have four bonds so this is carbon one and I if I draw in the hydrogens there those three hydrogens are still there they're just implied and then here carbon two look at Carbon two how many bonds are there one two three but I've said there has to be four so that means there is a hydrogen that's implied we go to carbon three we see that there's only one Bond but a has to have four so you fill them up with hydrogens so you could in theory take a zigzag structure and this is for your own learning ability or this is for you to make sure you understand what's going on is you could in theory draw in your hydrogens and why do I want you to draw in your hydrogens because there will be a situation where I will give you a zigzag structure just like this isopropyl alcohol and I will ask you how many hydrogens are in this molecule and you'll be like okay well I gotta count them and you'll be like okay there's one so let's do it in this blue structure one two three four five six seven eight there's eight hydrogens but you've got to be able to do it in the zigzag so you'll get really good at this you'll be like there's one two three four five six seven eight but until you get really good at it it's okay to take a zigzag structure and draw in your hydrogens okay so let's so now let's look at another example here on what I expect you guys to know so what we have here is a condensed structure of a molecule and I want you to be able to look at that condensed structure and convert that into a zigzag structure the way I do this is I start numbering the carbons that are not in parentheses so one two well let's see here not only the carbons but I'm also going to number this oxygen because it's not in a parenthesis so 4 5 6. and 7. now remember that when we convert this to a zigzag structure each point on that zigzag represents a carbon and so when I go like this just draw a line like well let me start off let's see here so when I look at this structure here I see that whenever you have parentheses that parenthesis is going that group inside the parentheses is going to be attached to the carbon that is right before it so here's the parentheses so right before it it goes on this carbon foreign once again now we see carbon 5 has a hydrogen and a parenthesis so this methyl group because it's a ch3 is going to be attached to carbon 5. whenever you have parentheses on the outside of the carbon of the molecule like these two right here those methyls are going to be attached to carbon one so what I can see here is I have a carbon carbon one and I have a a ch3 I have a methyl group directly attached to carbon one and another methyl group and then carbon one is bonded to carbon two and carbon two if we've numbered these carbon one carbon two what is directly attached to carbon two this methyl group so I'm going to draw it up here and so that the parentheses is attached to carbon two but this carbon right here carbon two is still bought is also bonded to this oxygen now let's let's keep going here and we'll come back to this so that's oxygen three then that is bonded to carbon four and carbon four has two hydrogens on it those two hydrogens right there and carbon four is still is bonded to carbon five which has a hydrogen on it which also has a methyl group attached and then that carbon right here so that's carbon four that's carbon five and that is bonded to carbon six and look at this this is kind of tricky the way this is drawn it's it kind of implies that it looks like this right that does not work why does that not work because hydrogen has two bonds we've broken that rule can't do it we've also in those list of rules I said oxygens forms two to three bonds that only has one so that cannot work so what happens is we look at a different Lewis structure what if we draw it like this and that would be oxygen seven that does work why because now oxygen has two bonds like it wants to have hydrogen only has one Bond and carbon 6 has how many bonds one two three four has four bonds it's happy so now as we look at this Lewis structure here are there any carbons without four bonds four four oxygen atom has two bonds it's happy carbon how many bonds does this carbon have one two three so that doesn't work how many bonds does this carbon have one two three but when we look at Carbon one and carbon two here in the formula are there any hydrogens attached to it there's not so what we have to do is add a pi Bond or a double bond and now that double bond right there now look at Carbon one it has one two three four carbon two one two three four so it is satisfied and now what we're going to do is [Music] now that we can kind of visualize this now we can convert this into a Lewis structure or a zigzag structure so the way I like to do this is I just start drawing my zigzag here so if I start at this carbon right here and draw a line this carbon that I'm pointing to is that carbon and that carbon is directly bonded to that carbon which is that carbon and then we can go zigzag now to carbon two so we could call that carbon zero 0 1 and 2. now Carbon 2 is directly bonded to oxygen so we just have to draw the heteroatom and then we're bound to carbon four carbon five carbon six so if we number these four five and six now if I take a look at Carbon one and carbon 2 what is missing in my zigzag the double bond right there and then attached to carbon two is what a methyl group so we just have to put a line right there so this line or right there is this methyl group right there attached to carbon one is this methyl group that we haven't worried about yet so I'll just draw in a line so that point right there represents that methyl group so I'm just going from carbon one to making sure everything matches methyl methyl methyl methyl double bond double bond carbon two directly attached to an oxygen oxygen and then carbon two has this methyl which is right there and I just keep going down the chain okay this is carbon four what what's the test of carbon for two hydrogens do I draw them in nope carbon five ah carbon 5 has what a methyl group so I have to draw a methyl group do I draw in this hydrogen on carbon five no I don't it's just implied and then carbon six is this guy and with carbon 6 we have to draw in all our hetero atoms so I'm going to do our double bond o and so that's carbon six and oxygen seven now this hydrogen right there is attached to carbon six do I draw it in like this do I have to no you don't have to because hydrogens that are directly attached to carbons are implied now we don't need to draw them but do you need to know that a hydrogen is there yes you do you just don't need to draw them so that right here is the zigzag structure of the condensed structure so you're going to get a lot of practice going from these condensed structures to the zigzags but you also have to get good at going from the zigzag to the condensed you have to do it in both directions and you will find that in organic chemistry when you're learning something in the forward Direction also learn it in the reverse make sure you can do things in both directions okay let's see here time wise here I think we have time to do another example okay in this example you just number the carbons like any last time except the ones in the parentheses so one two three four let's see here to do what are my let's double check something here okay so well that's telling me to do is draw a zigzag with four carbons like this two three four to see if we number that now I have one two three and four and now I just need to make sure the connectivity is right and that I've accounted for these groups that are in parentheses when you have a ch2 and a ch3 that is called a ethyl group and ethyl stands for two carbons and here we have a second ethyl so what what kind of terminology have we learned so far this group right here is called a methyl when you have two carbons ch2 and a ch3 that's called an ethyl and so I'm seeing carbon one is a methyl here carbon two is bonded to carbon three but what's also attached to carbon two well this group The ethyl group in parentheses is going to be attached to the previous carbon which is carbon two so it's just two carbons so I have to just draw in two carbons off of carbon two so one carbon two carbon so if I start that guy that would be that guy right there and if I put a little triangle there it would be that carbon there then I go to carbon three which is attached to carbon four and now look at this carbon 4 is attached sorry carbon 4 is attached to two ethyl groups so ethyls are two carbons so one two but there's this subscript right there that says I have two of them so I just draw two more one two so if we color code this here we could let's just say that's a DOT so that would be we could represent that one there and then it X right there but then there's two of these so there's the other dot there's the other X so that right there is the zigzag structure for this condensed formula now I want to make sure that when you look at a zigzag you can tell me and yourself how many hydrogens there are so let's look at a few carbons here on this molecule here so if I looked at this carbon how many hydrogens are on this carbon well in my mind or I could say hey that's a carbon and that carbon is bonded to this carbon so there has to be four Total Bonds so that carbon has to have three hydrogens this carbon right here is bonded to one carbon two carbon three carbon it has three bonds so it has to have one hydrogen this carbon right here is bonded to one carbon two carbons so there has to be a total of four bonds so there are two hydrogens so now you can see this carbon right there in the center has one two three four how many carbohydrogens does this carbon have do you think pause the video and answer that question you will see that if you do your counting correct that there are two hydrogens okay now what we have here is when you have a molecule that is just carbon and just hydrogens do you see that there's no heteroatoms no oxygen sulfur or anything like that when you have just carbon and hydrogen we call that a alkane and more importantly to understand is an alkane is a molecule with only carbon and hydrogens and only have single bonds you see everywhere in this molecule it is just single bonds there's no double bonds no triple bonds so so that would be single bonds okay let's take a look at Rings we can have molecules in the form of rings let's take a look at a ring called cyclo texting and the prefix hex tells us that there's going to be six carbons so if I have a hexane ring it would look something like this so if I did that right there's one two three four five six carbons in a ring but each carbon has to have four bonds so if I look at this starred carbon right there what's so important about this guy well there already has two bonds there so it is implied that there's two hydrogens attached and so every carbon is going to have hydrogens coming off do you see how labor intensive this is to draw on all the hydrogens and to draw on all the carbons so it's just easier to draw a cyclohexane ring like this boom done we don't have to worry about it anymore but when you look at cyclohexane do you see that these carbons right here are all going to have two hydrogens attached because you have to have four bonds per carbon now we could have another molecule called benzene and we could take Benz benzene we could draw like kind of doing the same thing as with cyclohexane we could Benzene is also a six-membered ring like that but what's different in Benzene is that it has alternating double single bonds so this carbon right here at top how many bonds does it have that's shown one two three so that means there has to be one hydrogen on every carbon now that's very labor intensive that's benzene but a better way to draw benzene and an easier way to drop Benzene is draw a six-membered ring and then draw on your double bonds like that oftentimes that right there is a little too labor intensive for some people and instead of drawing Benzene like that you can also draw it like this kind of looks like a donut so that little circle there in the middle there is telling me and telling you that you have a double single double single double could you have drawn Benzene like this do you see the difference between the two both are correct because all you're looking for is double single double single double single if you have a six-membered ring with that three double Bonds in there then you will um have drawn Benzene correctly or if you just draw it like this that is also benzene okay so we can we can look at all different sizes of rings you can have three carbons in a ring so it looks like a triangle you could have four carbons in a ring you could even have you can have five and then drawing The Seven membered Rings and the eight member Rings get a little bit more difficult but you can draw them so if we take a one of these let's just do this for fun let's take a look at this molecule right here the five-membered ring one and that is called a cyclo pentane tent meaning five like the Pentagon five sides how many hydrogens are in cyclopentane we'll just look at each carbon how many bonds are there four are drawn so that means it's implied that there are two hydrogens on each carbon and so how many hydrogens are on this cyclopentane ring there's going to be what one two three four five times two so that's 10 hydrogens right you have to be able to look at the skeletal structures and count the hydrogens essentially essential skill but once you guys get really really good at looking at skeletal structures and being able to count how many hydrogens and how many carbons there are we are going to use that skill set to be able to look at a molecule and name it we need to be able to look at a molecule and say hey that thing that I just drew or that thing that I just made or that thing I'm looking at what do I call it and so we have a rather long list of different types of molecules that you just have to memorize what they are and get flash cards write them out as you're walking between classes look at your flash cards organic chemistry is learning another language and it's going to just take a lot of time and effort to get this down so one thing that you need to be able to look at a molecule and identify it as such is something that looks like this that is called a that's not an acid that is called a alkyl halide and alkyl halide is when you have carbons a carbon chain in this case has one two three carbons and you have a halogen directly attached to it so this halogen right here is chlorine in this case but it can be any of the halogens so if I represent it with an X and that X could be fluorine chlorine bromine and iodine now there are millions and millions of molecules and an alkyl halide doesn't just look like one two three an alkyl halide could be something that looks like this do you see how I put the X on the middle carbon instead of the N carbon I could even have something that looks like this look at that carbon chain how many carbons is that one two three four five six a six carbon chain that is also an alkyl halide I could even tackle on some other groups I could even tack on let's say a ring so this molecule right there is a alkyl halide just for good effects here what if I put that halide right there that is an alkyl halide so how can you represent basically an infinite amount of alkyl halides how can you memorize it well it's the generic form of a alkyl halide looks like this so it's r X so that r is telling us anything that is alkyl so this word right there and alkyl is simply just carbon chains so this alkyl halide here can be represented like this and so the R Group in this particular alkyl halide is going to be oh wow that was squeaky let's get a different marker so what I just put in that rectangle in pink is the R Group and this in green right there that's the R Group so this massive Beast right here what we would do is be like all of that is the R Group so this alkyl halide right here can be represented like that so that right there represents a alkyl halide and when we look at molecules and say boom that's an alkyl halide what we're doing is we're classifying its functional group but I'm running out of space here functional groups so we are going to go through a list of the different types of functional groups that you need to be aware of and be able to identify when you see it so I'm going to just show you a couple more and then we will conclude this uh lesson here but there are tables in your textbook that talk about and show you all the different functional groups that you need to be aware of okay so let's do a couple more to wrap this up so what we have is an alkane and we've already seen that already that is just simply a molecule with carbon hydrogen bonds only okay just anything anything with this carbon and hydrogens is a alkane we have something also called a alkene do you see how the suffix is just a little bit different we replace an a with an e and alkene has a generic representation like this and this is where the R's in this generic formula can be hydrogen or alkyl remember alkyl is just adding carbons and hydrogens or alkylos adding a carbon chain so if we look at this generic alkene we could take it and replace let's say this R let's replace it with the hydrogen this R let's replace it with a methyl this R let's replace it with an ethyl so ch2 let's see here how do I want to do it let's do on this side let's replace that with a FL ch2 ch3 and a hydrogen and let's just draw in three carbons okay that is a alkene if you have a molecule with only carbon and hydrogens but you have one double bond anywhere in that molecule you have a alkene and the last one that I want to share with you for this video is a alkyne and an alkyne is a carbon hydrogen molecule molecule with only carbons and hydrogens with a triple bond so those are groups right there those R's they can be hydrogens or alkyl okay so let me show you uh a subclass of alkynes that are that you need to understand and then we will be done we have what's called a terminal alkyne versus a internal so a terminal alkyne would look something like this we will go ch3 carbons double bond or triple bond ch3 do you see how that the sorry I actually Drew nope so this is a terminal and we call it a terminal because this R right here is a hydrogen when you have a hydrogen for one of these r's you have a terminal alkyne an internal alkyne looks like this three carbon like that so when both are groups and this is the r group and that's the R Group when both of the r groups are alkyl for carbon then we call that a internal alkyne okay that's where we will stop for today