1 00:00:17,000 --> 00:00:19,000 In the early days of organic chemistry, 2 00:00:19,000 --> 00:00:22,000 chemists understood that molecules were made of atoms 3 00:00:22,000 --> 00:00:24,000 connected through chemical bonds. 4 00:00:24,000 --> 00:00:27,000 However, the three-dimensional shapes of molecules 5 00:00:27,000 --> 00:00:31,000 were utterly unclear, since they couldn't be observed directly. 6 00:00:31,000 --> 00:00:34,000 Molecules were represented using simple connectivity graphs 7 00:00:34,000 --> 00:00:37,000 like the one you see here. 8 00:00:37,000 --> 00:00:40,000 It was clear to savvy chemists of the mid-19th century 9 00:00:40,000 --> 00:00:44,000 that these flat representations couldn't explain 10 00:00:44,000 --> 00:00:46,000 many of their observations. 11 00:00:46,000 --> 00:00:49,000 But chemical theory hadn't provided a satisfactory explanation 12 00:00:49,000 --> 00:00:51,000 for the three-dimensional structures of molecules. 13 00:00:51,000 --> 00:00:57,000 In 1874, the chemist Van't Hoff published a remarkable hypothesis: 14 00:00:57,000 --> 00:01:01,000 the four bonds of a saturated carbon atom 15 00:01:01,000 --> 00:01:03,000 point to the corners of a tetrahedron. 16 00:01:03,000 --> 00:01:06,000 It would take over 25 years 17 00:01:06,000 --> 00:01:10,000 for the quantum revolution to theoretically validate his hypothesis. 18 00:01:10,000 --> 00:01:14,000 But Van't Hoff supported his theory using optical rotation. 19 00:01:14,000 --> 00:01:17,000 Van't Hoff noticed that only compounds containing a central carbon 20 00:01:17,000 --> 00:01:21,000 bound to four different atoms or groups 21 00:01:21,000 --> 00:01:24,000 rotated plane-polarized light. 22 00:01:24,000 --> 00:01:26,000 Clearly there's something unique about this class of compounds. 23 00:01:26,000 --> 00:01:29,000 Take a look at the two molecules you see here. 24 00:01:29,000 --> 00:01:34,000 Each one is characterized by a central, tetrahedral carbon atom 25 00:01:34,000 --> 00:01:36,000 bound to four different atoms: 26 00:01:36,000 --> 00:01:39,000 bromine, chlorine, fluorine, and hydrogen. 27 00:01:39,000 --> 00:01:41,000 We might be tempted to conclude that the two molecules 28 00:01:41,000 --> 00:01:45,000 are the same, if we just concern ourselves with what they're made of. 29 00:01:45,000 --> 00:01:48,000 However, let's see if we can overlay the two molecules 30 00:01:48,000 --> 00:01:51,000 perfectly to really prove that they're the same. 31 00:01:51,000 --> 00:01:55,000 We have free license to rotate and translate both of the molecules 32 00:01:55,000 --> 00:01:58,000 as we wish. Remarkably though, 33 00:01:58,000 --> 00:02:00,000 no matter how we move the molecules, 34 00:02:00,000 --> 00:02:04,000 we find that perfect superposition is impossible to achieve. 35 00:02:04,000 --> 00:02:07,000 Now take a look at your hands. 36 00:02:07,000 --> 00:02:10,000 Notice that your two hands have all the same parts: 37 00:02:10,000 --> 00:02:14,000 a thumb, fingers, a palm, etc. 38 00:02:14,000 --> 00:02:17,000 Like our two molecules under study, 39 00:02:17,000 --> 00:02:20,000 both of your hands are made of the same stuff. 40 00:02:20,000 --> 00:02:25,000 Furthermore, the distances between stuff in both of your hands are the same. 41 00:02:25,000 --> 00:02:27,000 The index finger is next to the middle finger, 42 00:02:27,000 --> 00:02:30,000 which is next to the ring finger, etc. 43 00:02:30,000 --> 00:02:33,000 The same is true of our hypothetical molecules. 44 00:02:33,000 --> 00:02:35,000 All of their internal distances 45 00:02:35,000 --> 00:02:38,000 are the same. Despite the similarities between them, 46 00:02:38,000 --> 00:02:40,000 your hands, and our molecules, 47 00:02:40,000 --> 00:02:43,000 are certainly not the same. 48 00:02:43,000 --> 00:02:46,000 Try superimposing your hands on one another. 49 00:02:46,000 --> 00:02:48,000 Just like our molecules from before, 50 00:02:48,000 --> 00:02:51,000 you'll find that it can't be done perfectly. 51 00:02:51,000 --> 00:02:54,000 Now, point your palms toward one another. 52 00:02:54,000 --> 00:02:56,000 Wiggle both of your index fingers. 53 00:02:56,000 --> 00:03:00,000 Notice that your left hand looks as if it's looking 54 00:03:00,000 --> 00:03:02,000 in a mirror at your right. 55 00:03:02,000 --> 00:03:05,000 In other words, your hands are mirror images. 56 00:03:05,000 --> 00:03:08,000 The same can be said of our molecules. 57 00:03:08,000 --> 00:03:11,000 We can turn them so that one looks at the other 58 00:03:11,000 --> 00:03:14,000 as in a mirror. Your hands - and our molecules - 59 00:03:14,000 --> 00:03:18,000 possess a spatial property in common called chirality, 60 00:03:18,000 --> 00:03:20,000 or handedness. 61 00:03:20,000 --> 00:03:23,000 Chirality means exactly what we've just described: 62 00:03:23,000 --> 00:03:25,000 a chiral object is not the same as its mirror image. 63 00:03:25,000 --> 00:03:30,000 Chiral objects are very special in both chemistry and everyday life. 64 00:03:30,000 --> 00:03:33,000 Screws, for example, are also chiral. 65 00:03:33,000 --> 00:03:37,000 That's why we need the terms right-handed and left-handed screws. 66 00:03:37,000 --> 00:03:40,000 And believe it or not, certain types of light 67 00:03:40,000 --> 00:03:42,000 can behave like chiral screws. 68 00:03:42,000 --> 00:03:47,000 Packed into every linear, plane-polarized beam of light 69 00:03:47,000 --> 00:03:50,000 are right-handed and left-handed parts 70 00:03:50,000 --> 00:03:55,000 that rotate together to produce plane polarization. 71 00:03:55,000 --> 00:03:58,000 Chiral molecules, placed in a beam of such light, 72 00:03:58,000 --> 00:04:01,000 interact differently with the two chiral components. 73 00:04:01,000 --> 00:04:06,000 As a result, one component of the light gets temporarily slowed down 74 00:04:06,000 --> 00:04:09,000 relative to the other. The effect on the light beam 75 00:04:09,000 --> 00:04:13,000 is a rotation of its plane from the original one, 76 00:04:13,000 --> 00:04:16,000 otherwise known as optical rotation. 77 00:04:16,000 --> 00:04:21,000 Van't Hoff and later chemists realized that the chiral nature 78 00:04:21,000 --> 00:04:24,000 of tetrahedral carbons can explain this fascinating phenomenon. 79 00:04:24,000 --> 00:04:29,000 Chirality is responsible for all kinds of other fascinating effects 80 00:04:29,000 --> 00:04:31,000 in chemistry, and everyday life. 81 00:04:31,000 --> 00:04:34,000 Humans tend to love symmetry 82 00:04:34,000 --> 00:04:36,000 and so if you look around you, you'll find that chiral objects 83 00:04:36,000 --> 00:04:38,000 made by humans are rare. 84 00:04:38,000 --> 00:04:42,000 But chiral molecules are absolutely everywhere. 85 00:04:42,000 --> 00:04:45,000 Phenomena as separate as optical rotation, 86 00:04:45,000 --> 00:04:47,000 Screwing together furniture, 87 00:04:47,000 --> 00:04:49,000 and clapping your hands 88 00:04:49,000 --> 00:04:53,000 all involve this intriguing spatial property.