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Video transcript

in the previous video we start with the molecular formula c3h8 oh and we looked at one of the possible Lewis dot structures that you can draw that has that molecular formula from this Lewis dot structure we look to other ways to represent the same molecule however we didn't have time to talk about bond line structures so let's start this video by taking this Lewis dot structure and turning it into a bond line structure if you look at the drawing on the left it implies that these three carbons aren't perfectly straight line but the drawing on the right does a little bit better job of showing what the molecule looks like in reality those carbons are not in a perfectly straight line you can see there's a bend to them like that so when you're drawing a bond line structure and you have a carbon chain you want to show that carbon chain in a zigzag pattern next let's think about the carbon hydrogen bonds if you were to draw every carbon hydrogen bond in an organic chemistry class it would take you forever so we leave those out in bond line structures carbon is still bonded to these hydrogen's but we're going to ignore them for our bond line structure just to simplify things so I'm going to draw this around the carbon hydrogen bonds so we're going to ignore them for the time being and now we have our three carbons drawn like that and this carbon is bonded to an oxygen and this oxygen is bonded to a hydrogen I'll put in lone pairs of electrons on that oxygen next we can simplify this even further we can leave out those carbons alright so the carbons are still there I'm just talking about not drawing these C's in here because they can get kind of confusing so we take out those C's and I'll leave off the lone pairs of electrons on the oxygen and we have our bond line structure so this is our bond line structure it contains the same information as our Lewis dot structure does but it's obviously much easier to draw takes less time it's going to help you out throughout your course when you're looking at chemical reactions so let's focus in on some carbons here so this carbon in red right that's this carbon so the carbon is still there we're just not drawing in the C and let's look at our other carbons the carbon on the right is the one in magenta so that's this carbon right here and the carbon on the left is in blue so that's this carbon the carbon in blue is still bonded to three hydrogen's right this carbon in blue is still bonded to three hydrogen's we just leave them off in our bond line structure the carbon on the right is still bonded to three hydrogen's all right but again we leave those off when we're drawing a bond line structure and the carbon in the middle this red carbon here is bonded to an OHA right that's already shown in our bond line structure and it's bonded to one more hydrogen so there still is a hydrogen bonded to that carbon all right we just leave them off - to make things easier to see so let me go ahead and erase what I just did here all right so let's take off those let's take off those hydrogen's so those hydrogen's are still there all right we just know that they are there so this just takes some practice all right this just takes some practice to figure out what these bond line structures mean so let's do let's do several examples of understanding bond line structures and the information that they contain let's start by analyzing this bond line structure and we'll start with this carbon right here in magenta so that carbon in magenta I'll represented over here is bonded to another carbon I'll use light blue for that so that's this carbon right here so there's a bond between those two carbons and let me draw in that bond now we have another carbon I'll use red right here so the carbon in red is up here I'll show the bond between those two carbons and then let's use green for the next carbon so we have a carbon right here in green and finally there's one more carbon to think about so let me let's see what color do we need to use here let's use dark blue so we have one more carbon right here in dark blue and I'll show that bond so we have five carbons in this molecule right so five carbons so let's write the molecular formula so BC five next let's figure out how many hydrogen's now to do that you need to remember that a neutral carbon atom forms four bonds so let's see how many bonds we already have we'll start with the car in magenta the carbon in magenta already has one bond and a neutral carbon atom forms four bonds so if that carbon already has one bond it needs three bonds to hydrogen so one bond hydrogen two bonds to hydrogen and three bonds to hydrogen next let's go with this top carbon here so the one in red this carbon already has one bond so it needs a total of four so it needs three more bonds and those bonds are two hydrogen right so it's implied those bonds are two hydrogen next we'll go for the light blue carbon in here so how many bonds does this carbon already have well here's one here's two and here's three that carbon already has three bonds so it only needs one more so we can draw in one hydrogen we know that carbon is bonded to only one hydrogen next we'll do the green carbon so the green carbon right here already has two bonds here's one and here's another one so if we think about a neutral carbon atom forming four bonds that carbon needs two more bonds and those bonds are two hydrogen's we draw in those hydrogen's there and finally the carbon in dark blue the carbon in dark blue already has one bond so it needs three more so there's one there's two and there's three so now we've drawn out the complete lewis dot structure for this bond line structure over here and how many total hydrogen's do we have well if you count those up you'll get 12 so the molecular formula is c5h12 let's do another one so let's look at this next bond line structure here and let's focus in on our carbon so we'll start with this carbon right here in magenta so I'll draw that in right here the carbon in magenta is bonded to two other carbons so I'll make this top carbon here red so there's a bond to the carbon in red and there's a bond to this carbon here and light blue so let's draw in those bonds so we draw in those bonds here and let's just keep going with our carbons let's assign our carbons first and we'll come back to our hydrogen's so next let's make this carbon right here in green so we have another bond of a carbon to a carbon and then let's go with dark blue so we have dark blue over here for this carbon so let's show that bond and then we have another carbon over here so that carbon is right here and we can show we can show our last bond the carbon in red is bonded to a chlorine so let me go ahead and show that so there's our chlorine and now let's let's think about hydrogen's and let's start with the start with the carbon in red all right so this carbon in red how many bonds does it already have well one two and three it already has three bonds so it needs one more and so it's implied that that bond is to a hydrogen so that carbons bonded to one hydrogen next let's do the carbon in magenta so over here how many bonds is that carbon in magenta already have well here's one and here's two so it already has two so that carbon needs two more so we can draw in hydrogen here and a hydrogen here it's the same situation for all of the carbons around our ring if we go to this carbon here and light blue it already has two bonds so it needs two more bonds and that must mean two bonds to hydrogen so we go around the entire ring and add in two hydrogen's to all of these carbons so let me make sure I use the correct colors here so in blue and then we have this one over here so what's the total molecular formula for this compound well we have total of six carbons right 1 2 3 4 5 6 so C 6 and how many total hydrogen's we have two on five carbons and then we have another one here so 2 times 5 is 10 plus 1 is 11 so H 11 and then we have a chlorine as well so C 6 H 11 CL would be the molecular formula for this compound let's look at two more examples and we'll start with this carbon right here in magenta so I'll draw on that carbon that carbon in magenta is bonded to this carbon in blue but notice there are two bonds between those two carbons so the carbon in magenta is bonded - the carbon in blue but there's a double bond between our carbons this time and the carbon on the right here in red there's a single bond between the carbon in blue and the carbon in red so for the molecular formula so far we know there are a total of 3 carbons in this compound next we need to think about hydrogen's so we know a neutral carbon atom forms four bonds so carbon forms four bonds how many bonds is a carbon in magenta already have well here's one bond and here's another bond so the carbon in magenta needs two more bonds and those bonds must be two hydrogen's so let me draw in those carbon hydrogen bonds like that next we think about the carbon in blue how many bonds is a carbon in blue already have well here's one here's two and here's three so the carbon in blue already has three bonds it needs one more so we show one carbon hydrogen bond and finally the carbon in red already has one bond so it needs three more so we draw in three carbon hydrogen bonds so how many total hydrogen's do we have that would be six hydrogen's so the molecular formula is c3h6 you can start to think about hybridization States here too because if you look at this carbon and this carbon you know both of those carbons are sp2 hybridized and if those carbons are sp2 hybridized we're talking about trigonal planar geometry around those atoms and we try to show that in our dot structure as best we can right approximately approximately 120 degree bond angles around here so hybridization can come into it as well next let's look at this one right here which has a triple bond and triple bonds often confuse students on bond line structures so let's let's assign our carbons again let's start with this one right here in magenta so let me draw on that carbon in magenta the carbon in magenta is bonded to this carbon in blue and there's a single bond between those two carbons next there's a bond between the carbon in blue and this carbon right here in red so that car in red there's a single bond between those the carbon in red is bonded to one more carbon than the opposite side of our triple bond so that carbon in blue is right there there's a triple bond between the carbon in red and the carbon in blue so now we have our carbons drawn out that's four carbons so this would be c4 so far for the molecular formula next we need to think about hydrogen's so the carbon in magenta already has one bond so it needs three more bonds so we draw in three bonds of carbon to hydrogen the carbon in blue here already has two bonds there's one and there's two so the carbon in blue needs two more what about the carbon in red well the carbon in red has one bond two three and four the carbon in red already has four bonds so the carbon in red doesn't have any hydrogen's on it at all and finally the carbon in blue the carbon in blue has three bonds one two three so three bonds already which means the carbon in blue needs one more bond and that bond is to hydrogen so now we have all of our hydrogen's that's a total of six hydrogen's so we can complete the molecular formula c4h6 and once again thinking about hybridization this carbon and this carbon right they're both SP hybridized and so we know the geometry is linear around those carbons and so that's why we draw this as being a straight line on our bond line structures it's because of the geometry we're trying to reflect the structure of the molecule the best that we can so practice your bond line structures because they're extremely important for everything that you will do in organic chemistry