Structural Formulae in Space
Carbon's four bonds point to the corners of a tetrahedron — so molecules have shape, and some come in left- and right-handed forms.
Why does your right hand never fit a left-handed glove — and what could that possibly have to do with a molecule of sugar?
The big idea
Chemists used to draw molecules flat, as letters joined by lines on a page. In 1874 a 22-year-old named van 't Hoff insisted that a carbon atom's four bonds actually reach out into three dimensions — toward the four corners of a tetrahedron, a little triangular pyramid, with the carbon at the centre.
That one move has a startling consequence. If the four things attached to a carbon are all different, you can build the molecule in two ways that are mirror images of each other — like a left and a right hand. No matter how you turn one, it will never sit exactly on top of the other. Such a carbon is called asymmetric, and the two mirror-image molecules are its two forms. Molecules, in other words, have a shape, and sometimes a handedness.
How it came about
The clue was a strange property of light. Some substances, in solution, twist the plane of polarised light — one to the left, another, otherwise identical, to the right. Louis Pasteur had even sorted two mirror-image crystals of a tartrate by hand under a microscope in 1848, but no one knew what, deep down, made a molecule handed. Johannes Wislicenus had sharpened the puzzle with two lactic acids that looked the same on paper yet behaved differently.
Van 't Hoff, still a student, saw that a tetrahedral carbon answered it exactly, and published a short pamphlet. In Paris, Joseph Le Bel reached the same conclusion within weeks, by a different argument. Not everyone was pleased: the eminent Hermann Kolbe publicly mocked the unknown youth for mounting "Pegasus, evidently borrowed from the Veterinary School," to fantasise about atoms in space. Van 't Hoff turned out to be right, and in 1901 received the very first Nobel Prize in Chemistry — though, as it happens, for other work.
Why it mattered
Handedness is not a curiosity; it is how life is built. The amino acids in your proteins and the sugars in your DNA are all of a single handedness, so your body tells mirror-image molecules apart with ease. That is why a medicine and its mirror twin can behave completely differently — one may heal while the other does nothing, or worse. Knowing that a molecule has a shape, and being able to predict and control which handed form you make, is now central to chemistry, biology and the entire pharmaceutical industry.
A way to picture it
Hold up your two hands. Same parts — four fingers and a thumb each — joined the same way, and yet they are mirror images: try as you might, you cannot lay your right hand on your left so that every finger matches. A carbon carrying four different groups is exactly like this. Make one of the four groups a repeat — give a hand six fingers, two of them identical and interchangeable — and the mirror trick suddenly works again: now the two versions are the same. Four different things at the corners; that is the whole condition for handedness.
Where it sits
Van 't Hoff arrived just as chemistry was learning the architecture of molecules: Kekulé had shown that carbon forms four bonds and chains into rings, and the tetrahedron now lifted those chains into space. What a bond actually is came later — Lewis's shared electron pair (lewis-1916) — and why it points where it does came later still, with quantum mechanics in Pauling's hands (pauling-1931). The thread runs straight on to the handed double helix of DNA (watson-crick-1953) and the folded shapes of proteins: the idea that a molecule's arrangement in space is the thing that matters begins here.
…in cases where the four affinities of the carbon atom are saturated with four mutually different univalent groups, two and not more than two different tetrahedra can be formed, which are each other's mirror images.