0:00:01.968,0:00:03.214 - [Voiceover] Let's[br]say we're asked to draw 0:00:03.214,0:00:04.904 all the structural isomers that have the 0:00:04.904,0:00:08.413 molecular formula C5H12. 0:00:08.413,0:00:11.233 The word "isomer" means same parts. 0:00:11.233,0:00:13.869 And so we're talking about[br]the same number of atoms. 0:00:13.869,0:00:15.664 All of our structural[br]isomers are gonna have 0:00:15.664,0:00:18.709 five carbons and 12 hydrogens. 0:00:18.709,0:00:20.243 Our isomers are gonna differ 0:00:20.243,0:00:23.138 in how those atoms are[br]connected to each other. 0:00:23.138,0:00:25.866 So they differ in terms[br]of their structure. 0:00:25.866,0:00:28.597 And that's why we call[br]them structural isomers. 0:00:28.597,0:00:31.376 We can also call them[br]constitutional isomers. 0:00:31.376,0:00:33.214 So we need five carbons. 0:00:33.214,0:00:35.540 So for our first isomer we could just draw 0:00:35.540,0:00:37.380 five carbons in a chain. 0:00:37.380,0:00:40.266 So here are my five carbons in a chain. 0:00:40.266,0:00:41.643 And you should have already seen the video 0:00:41.643,0:00:44.901 on bond line structures[br]before you watch this one. 0:00:44.901,0:00:46.947 So let's draw those five carbons 0:00:46.947,0:00:48.471 and let's double check and make sure 0:00:48.471,0:00:51.000 we have the correct number of hydrogens. 0:00:51.000,0:00:53.909 The carbon on the far[br]left has three hydrogens, 0:00:53.909,0:00:55.945 so here we have our three hydrogens. 0:00:55.945,0:00:57.881 Next carbon has two, 0:00:57.881,0:00:59.502 same with the next carbons, 0:00:59.502,0:01:02.545 so two for this one,[br]two for the next carbon, 0:01:02.545,0:01:07.346 and finally three hydrogens[br]for the last carbon. 0:01:07.346,0:01:08.975 So let's count up everything 0:01:08.975,0:01:11.704 and make sure we have to[br]correct molecular formulas. 0:01:11.704,0:01:14.606 We have one, two, three,[br]four, five carbons. 0:01:14.606,0:01:16.137 So that's C5. 0:01:16.137,0:01:18.037 And then we should have 12 hydrogens. 0:01:18.037,0:01:21.110 Here's three plus two gives us five, 0:01:21.110,0:01:23.200 plus two gives us seven, 0:01:23.200,0:01:25.668 plus two gives us nine, 0:01:25.668,0:01:29.005 and then we have three[br]more for a total of 12. 0:01:29.005,0:01:34.005 So, C5H12 is the molecular[br]formula for this compound. 0:01:34.469,0:01:36.878 Let's draw another structural isomer 0:01:36.878,0:01:39.343 that has the same molecular formula. 0:01:39.343,0:01:41.445 So instead of drawing[br]five carbons in a chain 0:01:41.445,0:01:43.133 now we have to draw four. 0:01:43.133,0:01:45.177 So let's start by drawing four carbons. 0:01:45.177,0:01:47.109 We need a total of five carbons 0:01:47.109,0:01:48.776 so we need to show the fifth carbon 0:01:48.776,0:01:51.104 branching off of our chain. 0:01:51.104,0:01:52.667 So we could show the fifth carbon 0:01:52.667,0:01:54.976 branching off of our chain here. 0:01:54.976,0:01:56.881 Let's draw in those five carbons. 0:01:56.881,0:01:59.108 So here we have our five carbons. 0:01:59.108,0:02:00.408 Let's count up hydrogens. 0:02:00.408,0:02:03.076 Carbon on the left has three, 0:02:03.076,0:02:05.208 so three hydrogens here. 0:02:05.208,0:02:07.642 Three hydrogens on this top carbon. 0:02:07.642,0:02:10.461 There's only one hydrogen on this carbon, 0:02:10.461,0:02:12.434 two hydrogens on this one, 0:02:12.434,0:02:16.205 and finally three[br]hydrogens on this carbon. 0:02:16.205,0:02:18.281 So let's count up our atoms. 0:02:18.281,0:02:19.981 So let's use red for this one. 0:02:19.981,0:02:21.776 We have one, two, three 0:02:21.776,0:02:23.101 four, five carbons. 0:02:23.101,0:02:24.442 So that's C5. 0:02:24.442,0:02:26.470 And then for hydrogens we have three here 0:02:26.470,0:02:28.435 plus three gives us six, 0:02:28.435,0:02:30.103 plus one gives us seven, 0:02:30.103,0:02:31.847 plus two gives us nine. 0:02:31.847,0:02:34.034 And three more for a total of 12. 0:02:34.034,0:02:38.964 So C5H12 is the molecular[br]formula for this compound. 0:02:38.964,0:02:42.104 So these two drawings represent 0:02:42.104,0:02:44.549 two different molecules. 0:02:44.549,0:02:48.989 Both these molecules have[br]the molecular formula C5H12. 0:02:48.989,0:02:52.646 But they differ in terms of[br]how those atoms are connected. 0:02:52.646,0:02:55.366 They differ in terms of their structure. 0:02:55.366,0:02:59.780 So we call them structural[br]isomers of each other. 0:02:59.780,0:03:02.406 All right, to draw[br]another structural isomer, 0:03:02.406,0:03:04.677 some students might say,[br]"We could start with 0:03:04.677,0:03:06.638 "four carbons in our chain again." 0:03:06.638,0:03:07.747 And this time, 0:03:07.747,0:03:10.709 instead of showing a[br]branch off of this carbon, 0:03:10.709,0:03:13.314 we could show a branch off of this carbon. 0:03:13.314,0:03:15.779 And so a student might draw this structure 0:03:15.779,0:03:19.070 and say, "Okay, there's a[br]different structural isomer." 0:03:19.070,0:03:21.475 But actually these are[br]just two different ways 0:03:21.475,0:03:24.409 to represent the same molecule. 0:03:24.409,0:03:27.672 If you analyze that second[br]structure that we just drew 0:03:27.672,0:03:29.867 the connections are the same. 0:03:29.867,0:03:31.770 We have a CH right here 0:03:31.770,0:03:35.000 bonded to a CH3, bonded to a CH3, 0:03:35.000,0:03:36.839 and bonded to a CH2. 0:03:36.839,0:03:39.912 And the CH2 is bonded to a CH3. 0:03:39.912,0:03:44.002 That's the same structure as[br]what we drew out over here. 0:03:44.002,0:03:46.403 So it looks like it's[br]a different structure. 0:03:46.403,0:03:48.836 It's a different drawing[br]than the one up here, 0:03:48.836,0:03:51.035 but actually this is[br]just two different ways 0:03:51.035,0:03:54.076 to represent the same molecule. 0:03:54.076,0:03:57.072 So we have two structural isomers so far. 0:03:57.072,0:03:59.173 Let's think about one more. 0:03:59.173,0:04:01.966 So we can no longer do[br]four carbons in our chain 0:04:01.966,0:04:03.947 so we go down to three carbons. 0:04:03.947,0:04:06.475 So we start with three[br]carbons in our chain. 0:04:06.475,0:04:08.735 We know we need a total of five carbons. 0:04:08.735,0:04:12.700 So we need to show two more[br]carbons added to our chain. 0:04:12.700,0:04:14.709 And these would have to[br]add those two carbons 0:04:14.709,0:04:17.112 to our central carbon like that. 0:04:17.112,0:04:20.265 Let's draw out all of our carbons here. 0:04:20.265,0:04:22.401 And let's add in our hydrogen. 0:04:22.401,0:04:25.146 So this carbon would have three hydrogens 0:04:25.146,0:04:27.508 same with this carbon. 0:04:27.508,0:04:29.736 And the same with this one, 0:04:29.736,0:04:33.435 and finally the same for this carbon. 0:04:33.435,0:04:36.679 The carbon in the center, this[br]carbon in the center here, 0:04:36.679,0:04:38.380 already has four bonds. 0:04:38.380,0:04:40.870 So it doesn't have any hydrogens on it. 0:04:40.870,0:04:42.676 Let's count up everything. 0:04:42.676,0:04:43.943 Let's count our carbons first, 0:04:43.943,0:04:48.602 one, two, three, four,[br]five carbons, so C5. 0:04:48.602,0:04:50.810 And then we have three hydrogens 0:04:50.810,0:04:53.745 plus three is six plus three is nine 0:04:53.745,0:04:55.303 plus three is 12. 0:04:55.303,0:04:58.407 So C5H12 is the molecular formula 0:04:58.407,0:05:00.311 for this compound. 0:05:00.311,0:05:03.197 And this is another structural isomer. 0:05:03.197,0:05:06.067 So it's a different[br]molecule from the other two. 0:05:06.067,0:05:08.780 So we have a total of[br]three structural isomers 0:05:08.780,0:05:12.807 that have the molecular formula C5H12. 0:05:14.366,0:05:16.405 Now let's draw all of[br]the structural isomers 0:05:16.405,0:05:19.947 that have the molecular formula C3H8O. 0:05:19.947,0:05:21.340 And we'll start with the molecule 0:05:21.340,0:05:23.674 we talked about in the[br]bond line structure video, 0:05:23.674,0:05:25.772 so that molecule look like this. 0:05:25.772,0:05:27.905 We have three carbons[br]and then we have an OH 0:05:27.905,0:05:30.851 coming off of the central carbon. 0:05:30.851,0:05:32.613 Let's expand that out and make sure 0:05:32.613,0:05:35.110 that this has the correct[br]molecular formula. 0:05:35.110,0:05:37.235 We have our three carbons. 0:05:37.235,0:05:39.909 And on the middle carbon we have an OH. 0:05:39.909,0:05:42.546 So an oxygen bonded to a hydrogen. 0:05:42.546,0:05:44.012 I'll go ahead and put lone pairs of 0:05:44.012,0:05:46.710 electrons on this oxygen. 0:05:46.710,0:05:48.539 How many hydrogens do we need to add 0:05:48.539,0:05:50.673 to the carbon on the left? 0:05:50.673,0:05:52.108 Well, we need to add three hydrogen. 0:05:52.108,0:05:54.404 So we go ahead and draw[br]in those three hydrogens. 0:05:54.404,0:05:56.841 The carbon in the center[br]already has three bonds 0:05:56.841,0:05:59.143 so it needs one more[br]so we add one hydrogen 0:05:59.143,0:06:00.503 to that carbon. 0:06:00.503,0:06:02.898 And the carbon on the right[br]needs three hydrogens. 0:06:02.898,0:06:04.997 So let's count everything up now. 0:06:04.997,0:06:06.706 So we'll start with our carbons. 0:06:06.706,0:06:08.873 We have one, two, three carbons. 0:06:08.873,0:06:10.447 So that's C3. 0:06:10.447,0:06:13.679 We have three hydrogens[br]here and three here, 0:06:13.679,0:06:16.000 so that's six plus one is seven, 0:06:16.000,0:06:19.214 and don't forget about the[br]hydrogen on the oxygen for eight. 0:06:19.214,0:06:20.841 So we have eight hydrogens. 0:06:20.841,0:06:23.379 And obviously we have one oxygen here. 0:06:23.379,0:06:24.642 So I went ahead and 0:06:24.642,0:06:27.076 put in lone pairs of[br]electrons on that oxygen. 0:06:27.076,0:06:29.839 So the molecular formula for this molecule 0:06:29.839,0:06:32.833 is C3H8O. 0:06:32.833,0:06:35.334 And if I number this, if[br]I said this was carbon 1 0:06:35.334,0:06:38.546 and this was carbon 2,[br]and this was carbon 3, 0:06:38.546,0:06:41.875 that helps us to draw the[br]next structural isomer 0:06:41.875,0:06:42.974 because we could think about 0:06:42.974,0:06:45.846 instead of that OH group[br]coming off of carbon 2, 0:06:45.846,0:06:49.044 what if that OH group[br]came off of carbon 1? 0:06:49.044,0:06:51.366 And so let's draw out[br]our three carbons here. 0:06:51.366,0:06:55.600 And now we put our OH group[br]coming off of carbon 1. 0:06:55.600,0:06:57.514 And let's expand this out 0:06:57.514,0:06:59.401 and draw the Lewis dot structure 0:06:59.401,0:07:00.380 and make sure that this has 0:07:00.380,0:07:02.013 the correct molecular formula. 0:07:02.013,0:07:05.047 So we have three carbons, again, in a row. 0:07:05.047,0:07:06.469 And then the carbon on the left 0:07:06.469,0:07:08.046 is bonded to the oxygen. 0:07:08.046,0:07:09.933 The oxygen is bonded to a hydrogen. 0:07:09.933,0:07:12.975 I'll put in lone pairs of[br]electrons on the oxygen. 0:07:12.975,0:07:15.675 Now we need to add in[br]carbon hydrogen bonds. 0:07:15.675,0:07:17.967 So this carbon needs two. 0:07:17.967,0:07:20.010 The next carbon also needs two. 0:07:20.010,0:07:22.834 And the carbon on the[br]end would need three. 0:07:22.834,0:07:26.204 So that's one, two, and three. 0:07:26.204,0:07:27.607 When we add everything up 0:07:27.607,0:07:29.011 let's use blue for that, 0:07:29.011,0:07:30.900 that's one, two, three carbons. 0:07:30.900,0:07:32.148 We have C3. 0:07:32.148,0:07:36.008 We have three hydrogens[br]here, plus two is five, 0:07:36.008,0:07:38.799 plus two is seven, and one here is eight. 0:07:38.799,0:07:40.873 So C3H8. 0:07:40.873,0:07:42.534 And then, of course, our oxygen. 0:07:42.534,0:07:46.241 So C3H8O is the molecular formula. 0:07:46.241,0:07:47.348 Next. 0:07:47.348,0:07:49.497 Some students might think, "Okay, well, 0:07:49.497,0:07:51.864 "we put an OH coming off of carbon 1 0:07:51.864,0:07:54.765 "but what if I put an[br]OH on the other side?" 0:07:54.765,0:07:56.637 So, over here on the other side. 0:07:56.637,0:07:58.145 So let's see what would that give us. 0:07:58.145,0:08:00.734 If I put an OH coming off of that carbon, 0:08:00.734,0:08:02.927 hopefully it's obvious that these two 0:08:02.927,0:08:05.133 represent the same molecule. 0:08:05.133,0:08:06.677 There's no difference in terms of 0:08:06.677,0:08:09.300 how those two are connected structurally. 0:08:09.300,0:08:10.843 So this is the same molecule, 0:08:10.843,0:08:13.170 so two different ways[br]to draw the same one. 0:08:13.170,0:08:15.571 So this is not a new structural isomer. 0:08:15.571,0:08:18.949 Just a new way of[br]looking at this molecule. 0:08:18.949,0:08:20.609 Now let's draw one more. 0:08:20.609,0:08:23.512 So we can't put the OH[br]on the other carbon. 0:08:23.512,0:08:26.179 So now we have to figure out[br]something else that we can do. 0:08:26.179,0:08:27.644 Well, we could, this time, 0:08:27.644,0:08:31.544 put two carbons in a row and[br]put an oxygen in between, 0:08:31.544,0:08:34.903 so putting an oxygen to[br]break up our carbon chain. 0:08:34.903,0:08:37.613 So now this would be[br]carbon bonded to carbon 0:08:37.613,0:08:39.967 bonded to oxygen, bonded to carbon. 0:08:39.967,0:08:41.933 And then we fill in our hydrogen, 0:08:41.933,0:08:44.747 so there would be three on this carbon. 0:08:44.747,0:08:47.377 There would be two on this carbon. 0:08:47.377,0:08:49.998 There would be three on this carbon. 0:08:49.998,0:08:52.274 And I could put in lone pairs of electrons 0:08:52.274,0:08:55.172 on the oxygen like that, 0:08:55.172,0:08:56.540 and can everything up. 0:08:56.540,0:08:58.942 So we have one, two, three carbons, 0:08:58.942,0:09:00.533 so that's C3. 0:09:00.533,0:09:04.173 We have three hydrogens plus two is five, 0:09:04.173,0:09:05.808 plus three is eight. 0:09:05.808,0:09:07.133 So we have the H8. 0:09:07.133,0:09:09.043 And then, of course, the one oxygen. 0:09:09.043,0:09:12.178 So this is another structural isomer. 0:09:12.178,0:09:14.034 Again, some students might say, 0:09:14.034,0:09:15.366 "Well, we could go like this," 0:09:15.366,0:09:18.208 and this would be yet another[br]structural isomer like that. 0:09:18.208,0:09:21.410 But really this is just another[br]way to draw this molecule. 0:09:21.410,0:09:23.166 So it's not a new structural isomer. 0:09:23.166,0:09:24.907 It has the same connections. 0:09:24.907,0:09:27.739 So we have a total of[br]three structural isomers 0:09:27.739,0:09:31.768 that have the molecular formula C3H8O. 0:09:31.768,0:09:33.946 And as you go further in organic chemistry 0:09:33.946,0:09:37.036 you'll learn that the first[br]two isomers we talked about, 0:09:37.036,0:09:39.110 so this one and this one, 0:09:39.110,0:09:40.577 the ones that have an OH on it, 0:09:40.577,0:09:42.104 those are called alcohols. 0:09:42.104,0:09:45.740 And the last structural[br]isomer is called an ether. 0:09:45.740,0:09:49.508 So we'll worry about that[br]more later in other videos.