WEBVTT

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In the early days of organic chemistry,

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chemists understood
that molecules were made of atoms

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connected through chemical bonds.

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However, the three-dimensional
shapes of molecules

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were utterly unclear, since they couldn't
be observed directly.

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Molecules were represented using
simple connectivity graphs

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like the one you see here.

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It was clear to savvy chemists
of the mid-19th century

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that these flat representations
couldn't explain

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many of their observations.

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But chemical theory hadn't provided
a satisfactory explanation

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for the three-dimensional
structures of molecules.

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In 1874, the chemist Van't Hoff
published a remarkable hypothesis:

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the four bonds of a saturated carbon atom

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point to the corners of a tetrahedron.

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It would take over 25 years

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for the quantum revolution
to theoretically validate his hypothesis.

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But Van't Hoff supported
his theory using optical rotation.

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Van't Hoff noticed that only compounds
containing a central carbon

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bound to four different atoms or groups

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rotated plane-polarized light.

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Clearly there's something unique
about this class of compounds.

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Take a look at the two molecules
you see here.

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Each one is characterized
by a central, tetrahedral carbon atom

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bound to four different atoms:

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bromine, chlorine, fluorine, and hydrogen.

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We might be tempted to conclude
that the two molecules

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are the same, if we just concern
ourselves with what they're made of.

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However, let's see if we can
overlay the two molecules

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perfectly to really prove
that they're the same.

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We have free license to rotate
and translate both of the molecules

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as we wish. Remarkably though,

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no matter how we move the molecules,

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we find that perfect superposition
is impossible to achieve.

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Now take a look at your hands.

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Notice that your two hands
have all the same parts:

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a thumb, fingers, a palm, etc.

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Like our two molecules under study,

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both of your hands are made
of the same stuff.

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Furthermore, the distances between stuff
in both of your hands are the same.

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The index finger
is next to the middle finger,

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which is next to the ring finger, etc.

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The same is true
of our hypothetical molecules.

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All of their internal distances

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are the same. Despite
the similarities between them,

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your hands, and our molecules,

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are certainly not the same.

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Try superimposing
your hands on one another.

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Just like our molecules from before,

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you'll find that it can't
be done perfectly.

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Now, point your palms toward one another.

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Wiggle both of your index fingers.

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Notice that your left hand
looks as if it's looking

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in a mirror at your right.

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In other words, your hands
are mirror images.

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The same can be said of our molecules.

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We can turn them so
that one looks at the other

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as in a mirror. Your hands
- and our molecules -

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possess a spatial property
in common called chirality,

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or handedness.

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Chirality means exactly
what we've just described:

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a chiral object is not
the same as its mirror image.

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Chiral objects are very special
in both chemistry and everyday life.

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Screws, for example, are also chiral.

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That's why we need the terms
right-handed and left-handed screws.

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And believe it or not,
certain types of light

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can behave like chiral screws.

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Packed into every linear,
plane-polarized beam of light

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are right-handed and left-handed parts

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that rotate together
to produce plane polarization.

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Chiral molecules, placed
in a beam of such light,

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interact differently
with the two chiral components.

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As a result, one component of the light
gets temporarily slowed down

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relative to the other. The
effect on the light beam

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is a rotation of its plane
from the original one,

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otherwise known as optical rotation.

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Van't Hoff and later chemists
realized that the chiral nature

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of tetrahedral carbons can explain
this fascinating phenomenon.

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Chirality is responsible for all kinds
of other fascinating effects

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in chemistry, and everyday life.

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Humans tend to love symmetry

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and so if you look around you,
you'll find that chiral objects

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made by humans are rare.

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But chiral molecules
are absolutely everywhere.

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Phenomena as separate as optical rotation,

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Screwing together furniture,

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and clapping your hands

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all involve this intriguing
spatial property.