WEBVTT 00:00:17.000 --> 00:00:19.000 In the early days of organic chemistry, 00:00:19.000 --> 00:00:22.000 chemists understood that molecules were made of atoms 00:00:22.000 --> 00:00:24.000 connected through chemical bonds. 00:00:24.000 --> 00:00:27.000 However, the three-dimensional shapes of molecules 00:00:27.000 --> 00:00:31.000 were utterly unclear, since they couldn't be observed directly. 00:00:31.000 --> 00:00:34.000 Molecules were represented using simple connectivity graphs 00:00:34.000 --> 00:00:37.000 like the one you see here. 00:00:37.000 --> 00:00:40.000 It was clear to savvy chemists of the mid-19th century 00:00:40.000 --> 00:00:44.000 that these flat representations couldn't explain 00:00:44.000 --> 00:00:46.000 many of their observations. 00:00:46.000 --> 00:00:49.000 But chemical theory hadn't provided a satisfactory explanation 00:00:49.000 --> 00:00:51.000 for the three-dimensional structures of molecules. 00:00:51.000 --> 00:00:57.000 In 1874, the chemist Van't Hoff published a remarkable hypothesis: 00:00:57.000 --> 00:01:01.000 the four bonds of a saturated carbon atom 00:01:01.000 --> 00:01:03.000 point to the corners of a tetrahedron. 00:01:03.000 --> 00:01:06.000 It would take over 25 years 00:01:06.000 --> 00:01:10.000 for the quantum revolution to theoretically validate his hypothesis. 00:01:10.000 --> 00:01:14.000 But Van't Hoff supported his theory using optical rotation. 00:01:14.000 --> 00:01:17.000 Van't Hoff noticed that only compounds containing a central carbon 00:01:17.000 --> 00:01:21.000 bound to four different atoms or groups 00:01:21.000 --> 00:01:24.000 rotated plane-polarized light. 00:01:24.000 --> 00:01:26.000 Clearly there's something unique about this class of compounds. 00:01:26.000 --> 00:01:29.000 Take a look at the two molecules you see here. 00:01:29.000 --> 00:01:34.000 Each one is characterized by a central, tetrahedral carbon atom 00:01:34.000 --> 00:01:36.000 bound to four different atoms: 00:01:36.000 --> 00:01:39.000 bromine, chlorine, fluorine, and hydrogen. 00:01:39.000 --> 00:01:41.000 We might be tempted to conclude that the two molecules 00:01:41.000 --> 00:01:45.000 are the same, if we just concern ourselves with what they're made of. 00:01:45.000 --> 00:01:48.000 However, let's see if we can overlay the two molecules 00:01:48.000 --> 00:01:51.000 perfectly to really prove that they're the same. 00:01:51.000 --> 00:01:55.000 We have free license to rotate and translate both of the molecules 00:01:55.000 --> 00:01:58.000 as we wish. Remarkably though, 00:01:58.000 --> 00:02:00.000 no matter how we move the molecules, 00:02:00.000 --> 00:02:04.000 we find that perfect superposition is impossible to achieve. 00:02:04.000 --> 00:02:07.000 Now take a look at your hands. 00:02:07.000 --> 00:02:10.000 Notice that your two hands have all the same parts: 00:02:10.000 --> 00:02:14.000 a thumb, fingers, a palm, etc. 00:02:14.000 --> 00:02:17.000 Like our two molecules under study, 00:02:17.000 --> 00:02:20.000 both of your hands are made of the same stuff. 00:02:20.000 --> 00:02:25.000 Furthermore, the distances between stuff in both of your hands are the same. 00:02:25.000 --> 00:02:27.000 The index finger is next to the middle finger, 00:02:27.000 --> 00:02:30.000 which is next to the ring finger, etc. 00:02:30.000 --> 00:02:33.000 The same is true of our hypothetical molecules. 00:02:33.000 --> 00:02:35.000 All of their internal distances 00:02:35.000 --> 00:02:38.000 are the same. Despite the similarities between them, 00:02:38.000 --> 00:02:40.000 your hands, and our molecules, 00:02:40.000 --> 00:02:43.000 are certainly not the same. 00:02:43.000 --> 00:02:46.000 Try superimposing your hands on one another. 00:02:46.000 --> 00:02:48.000 Just like our molecules from before, 00:02:48.000 --> 00:02:51.000 you'll find that it can't be done perfectly. 00:02:51.000 --> 00:02:54.000 Now, point your palms toward one another. 00:02:54.000 --> 00:02:56.000 Wiggle both of your index fingers. 00:02:56.000 --> 00:03:00.000 Notice that your left hand looks as if it's looking 00:03:00.000 --> 00:03:02.000 in a mirror at your right. 00:03:02.000 --> 00:03:05.000 In other words, your hands are mirror images. 00:03:05.000 --> 00:03:08.000 The same can be said of our molecules. 00:03:08.000 --> 00:03:11.000 We can turn them so that one looks at the other 00:03:11.000 --> 00:03:14.000 as in a mirror. Your hands - and our molecules - 00:03:14.000 --> 00:03:18.000 possess a spatial property in common called chirality, 00:03:18.000 --> 00:03:20.000 or handedness. 00:03:20.000 --> 00:03:23.000 Chirality means exactly what we've just described: 00:03:23.000 --> 00:03:25.000 a chiral object is not the same as its mirror image. 00:03:25.000 --> 00:03:30.000 Chiral objects are very special in both chemistry and everyday life. 00:03:30.000 --> 00:03:33.000 Screws, for example, are also chiral. 00:03:33.000 --> 00:03:37.000 That's why we need the terms right-handed and left-handed screws. 00:03:37.000 --> 00:03:40.000 And believe it or not, certain types of light 00:03:40.000 --> 00:03:42.000 can behave like chiral screws. 00:03:42.000 --> 00:03:47.000 Packed into every linear, plane-polarized beam of light 00:03:47.000 --> 00:03:50.000 are right-handed and left-handed parts 00:03:50.000 --> 00:03:55.000 that rotate together to produce plane polarization. 00:03:55.000 --> 00:03:58.000 Chiral molecules, placed in a beam of such light, 00:03:58.000 --> 00:04:01.000 interact differently with the two chiral components. 00:04:01.000 --> 00:04:06.000 As a result, one component of the light gets temporarily slowed down 00:04:06.000 --> 00:04:09.000 relative to the other. The effect on the light beam 00:04:09.000 --> 00:04:13.000 is a rotation of its plane from the original one, 00:04:13.000 --> 00:04:16.000 otherwise known as optical rotation. 00:04:16.000 --> 00:04:21.000 Van't Hoff and later chemists realized that the chiral nature 00:04:21.000 --> 00:04:24.000 of tetrahedral carbons can explain this fascinating phenomenon. 00:04:24.000 --> 00:04:29.000 Chirality is responsible for all kinds of other fascinating effects 00:04:29.000 --> 00:04:31.000 in chemistry, and everyday life. 00:04:31.000 --> 00:04:34.000 Humans tend to love symmetry 00:04:34.000 --> 00:04:36.000 and so if you look around you, you'll find that chiral objects 00:04:36.000 --> 00:04:38.000 made by humans are rare. 00:04:38.000 --> 00:04:42.000 But chiral molecules are absolutely everywhere. 00:04:42.000 --> 00:04:45.000 Phenomena as separate as optical rotation, 00:04:45.000 --> 00:04:47.000 Screwing together furniture, 00:04:47.000 --> 00:04:49.000 and clapping your hands 00:04:49.000 --> 00:04:53.000 all involve this intriguing spatial property.