0:00:02.260,0:00:03.830 I made this video to 0:00:03.830,0:00:07.339 try and help some people understand how[br]cis and trans 0:00:07.339,0:00:10.590 geometric isomers work. 0:00:10.590,0:00:14.590 I'm hoping it will be useful to some people,[br]and 0:00:14.590,0:00:19.380 before we begin, some definitions.[br]Structural isomers have the same 0:00:19.380,0:00:22.900 molecular formula but a different[br]structural formula. 0:00:22.100,0:00:25.570 There are two examples of structural[br]isomers here. 0:00:25.570,0:00:29.439 If you look at them, the sequence of[br]atoms within the structures 0:00:29.439,0:00:32.950 are different, even though the molecular[br]formulas are the same. 0:00:32.950,0:00:35.950 Geometric isomers are different. 0:00:35.950,0:00:38.969 They will have the same molecular and[br]structural formula, 0:00:38.969,0:00:45.969 but a different arrangement of atoms in[br]space. 0:00:46.180,0:00:47.100 Before we start 0:00:47.100,0:00:50.260 talking about alkenes and their geometric isomers, 0:00:50.260,0:00:54.700 I think a good starting point is a simple 0:00:54.700,0:00:57.329 alkane. This is 1,2-dichloroethane. It's 0:00:57.329,0:01:01.140 got single bonds between the two[br]carbon atoms, 0:01:01.140,0:01:04.860 and if this was the structure of the molecule, 0:01:04.860,0:01:08.220 I could draw a picture of it. And, 0:01:08.220,0:01:12.900 it would look, in terms of fully expanded, 0:01:12.900,0:01:15.590 like the top diagram above. I've got two chlorines 0:01:15.590,0:01:19.550 up top, two hydrogens on the bottom and there's a hydrogen left and a hydrogen right 0:01:19.550,0:01:24.929 that are slightly obscured because of the geometry. 0:01:24.929,0:01:28.940 The top diagram is a expanded structural formula. 0:01:28.940,0:01:33.390 It shows exactly, all the bonds in the structure where the bottom one is a condensed 0:01:33.390,0:01:37.880 structural formula. It still shows us the[br]sequence of all the atoms in the molecule, but 0:01:37.880,0:01:39.399 it's much, much faster to write. 0:01:39.399,0:01:42.890 The thing about this model is that 0:01:42.890,0:01:47.490 the molecule could be viewed from many[br]different angles. 0:01:47.490,0:01:50.390 And, if I want to view it from say, this angle, I would actually draw 0:01:50.390,0:01:54.790 I would actually draw the 0:01:54.790,0:01:58.810 two chlorines left and right and the hydrogens up and down, up and down on each side. 0:01:58.810,0:02:03.200 It's still the same molecule, just viewed[br]from a slightly different angle, 0:02:03.200,0:02:06.570 and to show that on paper, 0:02:06.570,0:02:09.690 I could have gone those two chlorines in in two different arrangements. 0:02:09.690,0:02:12.910 It could also set possibly with the two chlorines 0:02:12.910,0:02:16.859 both looking downwards, in which case 0:02:16.859,0:02:21.180 I would draw the structural formula like that.[br]Every single one of these 0:02:21.180,0:02:24.660 are all equivalent. It's still[br]the same thing, 0:02:24.660,0:02:28.859 just viewed from different angles. But,[br]there's 0:02:28.859,0:02:33.490 one more trick that this molecule can do and[br]that is is that 0:02:33.490,0:02:36.620 whenever there's a single bond present, 0:02:36.620,0:02:41.870 the atoms on each end of it can rotate about the axis of that bond, 0:02:41.870,0:02:45.130 that means that 0:02:45.130,0:02:51.230 this one here can spin on this axis[br]of the bond, and this one here can also do 0:02:51.230,0:02:52.180 the same. 0:02:52.180,0:02:55.690 So, both of these atoms have that spin movement available to 0:02:55.690,0:02:58.728 spin around as they wish. 0:02:58.729,0:03:03.769 And that means that I don't necessarily[br]have to have the two chlorines 0:03:03.769,0:03:07.180 sticking out side by side. They could be in any 0:03:07.180,0:03:10.889 other possible position relative to[br]each other. 0:03:10.889,0:03:15.190 So, that means that this molecule could also[br]be drawn 0:03:15.190,0:03:19.850 like this, in which case 0:03:19.850,0:03:23.888 there is a chlorine 0:03:23.889,0:03:29.229 above and below on each side and hydrogen are taking up 0:03:29.229,0:03:33.490 the other remaining spots. Still the exact[br]same molecule 0:03:33.490,0:03:36.370 because all I've done to change that from 0:03:36.370,0:03:42.199 what it was originally to what it is now 0:03:42.199,0:03:47.300 is just rotated the molecule internally[br]within its own structure. 0:03:47.300,0:03:50.479 The same thing would apply 0:03:50.479,0:03:55.799 to any simple single-bonded carbon chain. 0:03:55.799,0:04:01.229 This is four carbons all in a line. But, I don't necessarily have to draw them like that 0:04:01.229,0:04:05.870 because with all these bonds having free rotation, I could draw three 0:04:05.870,0:04:09.320 in a row and one down. Or, I could draw 0:04:09.320,0:04:12.509 one up and one down, 0:04:12.509,0:04:15.798 or I could draw the whole thing in some sort of "n" shape, or as 0:04:15.799,0:04:19.470 some sort of "u" shape. 0:04:19.470,0:04:22.540 Exactly which one I draw is really 0:04:22.540,0:04:26.460 irrelevant because it's all the same[br]molecule, just twisted 0:04:26.460,0:04:30.270 internally. Of course, for clarity, 0:04:30.270,0:04:34.169 a structural formula will generally have all atoms in a row, or 0:04:34.169,0:04:38.000 if you draw the skeletal formulas, 0:04:38.000,0:04:45.000 generally, it will be drawn as a bit of a zig-zag. 0:04:50.510,0:04:51.300 Alkenes 0:04:51.300,0:04:54.700 are different 0:04:54.700,0:04:57.800 because they have a double bond between[br]the carbon atoms 0:04:57.800,0:05:01.350 and that limits what kind of rotation is[br]available 0:05:01.350,0:05:05.900 for those atoms. But, first of all, if we looked at this one here, it's 0:05:05.900,0:05:08.520 1,2-dichlorobutene (really 1,2-dichloroethene). 0:05:08.520,0:05:12.700 And, I could hold this 0:05:12.700,0:05:16.610 in a couple of different positions to get[br]the different perspectives. So, this one here 0:05:16.610,0:05:19.860 probably matches this, yes it does. So, 0:05:19.860,0:05:23.780 on the left, we've got the chlorine up top and on the right, we've got the chlorine on the 0:05:23.780,0:05:28.419 bottom. But, the exact time molecule flipped over, 0:05:28.420,0:05:32.500 we could now draw it as having the chlorine on one side down 0:05:32.500,0:05:36.300 on the left, and on the right hand side, it's[br]now up. 0:05:36.300,0:05:39.140 It's still the exact same molecule, just 0:05:39.140,0:05:43.110 one perspective versus another one.[br]However, 0:05:43.110,0:05:47.400 what this molecule can't do is 0:05:47.400,0:05:52.490 rotate on the axis of this double bond.[br]That means the two carbons on the 0:05:52.490,0:05:53.820 end of that double bond, 0:05:53.820,0:05:57.400 cannot twist, spin independently 0:05:57.400,0:06:00.500 of the other one. The double bond will not allow it. There is 0:06:00.500,0:06:04.200 no ability to twist around. That means that[br]I can't get this structure 0:06:04.200,0:06:08.390 twisted around and get both of my chlorine atoms 0:06:08.390,0:06:14.280 facing either both 0:06:14.280,0:06:18.710 up or both down. So, at no stage 0:06:18.710,0:06:21.870 can I turn this structure into 0:06:21.870,0:06:25.500 this one. They both still have the same structural formula, in terms of 0:06:25.500,0:06:29.560 the sequence of atoms in the molecule.[br]It has got a 0:06:29.560,0:06:33.330 hydrogen and chlorine on the left, 0:06:33.330,0:06:37.800 hydrogen and chlorine on the right, double bond between them. So, 0:06:37.800,0:06:39.630 in terms of the sequence of atoms and bonds, they are identical. 0:06:39.630,0:06:42.750 However, the arrangement in space of these atoms, 0:06:42.750,0:06:45.960 is different and 0:06:45.960,0:06:49.260 no amount rotation or manipulation 0:06:49.260,0:06:52.520 can ever get them to line up with each other. 0:06:52.520,0:06:59.520 So, these two here are two different[br]molecules. We call them geometric isomers. 0:07:00.900,0:07:04.429 Same structural formulas, but different[br]arrangements of atoms in space. 0:07:04.430,0:07:09.380 A requirement for this is that they must[br]have a double bond between 0:07:09.380,0:07:13.770 carbon atoms. Structures like this 0:07:13.770,0:07:17.400 many different ways up of drawing those chlorine atoms 0:07:17.400,0:07:20.679 around those carbon atoms within the same[br]molecule 0:07:20.680,0:07:24.650 because the bond can rotate to put them in 0:07:24.650,0:07:28.849 any position you wish. This double bond can't rotate 0:07:28.850,0:07:31.890 with means those two chlorines are forever one up 0:07:31.890,0:07:35.640 and one down. This one here, they're forever either both 0:07:35.640,0:07:40.260 that way, or they're both that way. So, 0:07:40.260,0:07:43.710 a geometric isomer must have a double bond for starters. 0:07:43.710,0:07:48.719 However, there's one more requirement, and that is 0:07:48.720,0:07:51.870 that each carbon within 0:07:51.870,0:07:56.340 the double bond has to have 0:07:56.340,0:08:00.190 two unique groups attached[br]to it. So, that carbon there 0:08:00.190,0:08:04.270 has got a hydrogen and a chlorine, which are[br]different from each other, 0:08:04.270,0:08:07.380 and also, this one here has got 0:08:07.380,0:08:11.300 a chlorine and a hydrogen that are different from each other. 0:08:11.300,0:08:14.400 It doesn't matter whether or not these ones match or 0:08:14.400,0:08:17.489 these ones match, it's one carbon at a time. Are they 0:08:17.490,0:08:22.900 the same or are they different for both sides? 0:08:22.900,0:08:27.469 If there's ever an instance where 0:08:27.470,0:08:30.910 on either side of that bond, 0:08:30.910,0:08:33.979 there is two groups that are the 0:08:33.979,0:08:38.120 same as each other then it is not[br]possible 0:08:38.120,0:08:41.159 to have this molecule arranged 0:08:41.159,0:08:44.839 in any other 0:08:44.839,0:08:48.220 conformation other than this. 0:08:48.220,0:08:51.910 So, this one here. If I was to have this one 0:08:51.910,0:08:55.500 and this one. 0:08:55.500,0:09:00.410 On the surface they might look[br]possibly 0:09:00.410,0:09:05.199 a little bit different because this one[br]here has got the chlorine on the right 0:09:05.200,0:09:09.100 down and this one has the chlorine on the right up. 0:09:09.100,0:09:14.520 But, because the right-hand sides, sorry, left-hand sides of these 0:09:14.520,0:09:18.290 both have identical groups, two chlorines on that left-hand side 0:09:18.290,0:09:21.500 carbon that means that 0:09:21.500,0:09:26.480 even though I can't rotate the double-bond, 0:09:26.480,0:09:31.400 if I just take the whole structure and[br]twist it over, then I've 0:09:31.400,0:09:36.310 got the exact same arrangement 0:09:36.310,0:09:40.890 in space. So, these aren't geometric isomers. It's the same molecule. 0:09:40.890,0:09:43.920 Possibly just originally viewed from a[br]different angle. 0:09:43.920,0:09:49.140 So this one here can't have geometric isomers. 0:09:49.140,0:09:53.530 But, it does satisfy the double-bond requirements. One of the carbon atoms 0:09:53.530,0:09:56.740 has got two identical group attached. 0:09:56.740,0:10:01.100 That means that any different ways I[br]can try and arrange, 0:10:01.100,0:10:05.370 these groups still represent the same[br]molecule 0:10:05.370,0:10:14.990 from a different perspective. In this one[br]here, 0:10:14.990,0:10:18.600 can exist as geometric isomers because 0:10:18.600,0:10:22.489 number one, there's a double-bond present[br]and number two, each codon 0:10:22.490,0:10:28.500 has got two different groups attached to[br]it. So I can have this structure drawn like this 0:10:28.500,0:10:31.620 or I could draw it so 0:10:31.620,0:10:36.150 the two chlorines are on the same side of each other. 0:10:36.150,0:10:40.650 I cannot manipulate this molecule to get[br]their original arrangement back again. 0:10:40.650,0:10:44.890 No matter how I try because there's no rotation in here. 0:10:44.890,0:10:49.620 I can never get one chlorine up and one chlorine down. 0:10:49.620,0:10:53.560 So this is a different structure from the[br]other one. 0:10:53.560,0:10:58.250 And, finally the original molecule we had 0:10:58.250,0:11:02.220 was an alkane. An alkane 0:11:02.220,0:11:07.000 by default cannot have, when it's in a[br]single chain, 0:11:07.000,0:11:11.240 cis and trans geometric isomers because 0:11:11.240,0:11:14.650 this rotation means that 0:11:14.650,0:11:18.810 having the two chlorines on opposite sides or the same sides, 0:11:18.810,0:11:23.390 are actually just the same molecule with different amounts of internal rotation. 0:11:23.390,0:11:28.550 The lack of a double bond means any different ways of drawing this, 0:11:28.550,0:11:30.290 still represent the same molecule.