Chirality — Stereochemistry | Organic Chemistry

Two molecules can have the same formula, the same connectivity, the same bonds in the same places — and still be different substances, because they are mirror images that cannot be laid on top of each other. That is chirality, and it is the reason stereochemistry is not a footnote to organic chemistry but one of its central concerns. Living systems are built entirely from chiral molecules and can tell mirror images apart with total reliability.

The definition

An object is chiral if it is not superimposable on its mirror image — no amount of rotating or sliding will make every point of one coincide with the corresponding point of the other. If an object can be superimposed on its mirror image, it is achiral.

Every object has a mirror image. The question is never whether one exists but whether it is the same object. For most everyday objects it is; for hands, screws, seashells and a great many molecules, it is not.

The test that a flat drawing can only ask you to take on trust. The top row is one enantiomer at four rotations; the bottom row is its mirror image at the same four. Pick any frame on top and compare it with the one below: you can always bring two of the four groups into line, and the other two then come out swapped. That is what non-superimposable means — not that the two are hard to match, but that no rotation whatsoever will do it.

Hands: the classic example

Your left and right hands are mirror images, and you cannot rotate your left hand in space to make it match your right — try it, and the thumb always ends up on the wrong side. That is precisely what chiral means, and it is where the word comes from: Greek cheir, hand.

A plain coffee mug, by contrast, is achiral. Its mirror image is the same mug, just turned around. The handle does not have a handedness.

The test generalizes cleanly. A screw is chiral — right-hand threads do not fit a left-hand nut. A pair of scissors is chiral, which is why left-handed scissors are a real product. A sphere, a cube and a plain pencil are achiral.

The symmetry shortcut

Testing superimposability in your head is unreliable, so use this instead: if a molecule has an internal plane of symmetry, it is achiral. An internal mirror plane is a flat plane that slices the molecule into two halves that are reflections of each other. If no such plane exists in any conformation, the molecule is chiral.

The symmetry shortcut, which is far more reliable than trying to superimpose things in your head. Look for an internal mirror plane: a flat plane slicing the molecule into two halves that reflect each other. The right-hand pair has one — swap the two chlorines and nothing changes — so the mirror image is the same substance. The left-hand pair has no such plane in any conformation, so its two mirror images are genuinely different compounds, as different as your two hands.

This is fast and it is right the overwhelming majority of the time. Strictly, the complete criterion involves a slightly broader class of symmetry — a molecule is achiral if it has any improper axis of rotation, which includes a mirror plane and also a centre of inversion — but for everything in this course, "look for a mirror plane, and also check for a centre of symmetry" is sufficient.

Search for a plane of symmetry in the right conformation. A molecule that can rotate about its single bonds will have many shapes, and a mirror plane present in any accessible conformation makes it achiral. Conversely, a coincidental plane in one drawn conformation of a genuinely chiral molecule does not exist — if you think you have found one, rotate the molecule and check again. This is the most common source of wrong answers in this section.

Chirality is a property of the whole molecule's three-dimensional shape, not of any single atom. The next section introduces stereocenters, the specific atoms that most often cause a molecule to be chiral — but the two ideas are not the same thing, and the meso compounds section shows a case where a molecule with two stereocenters is nevertheless achiral.

Chirality without a stereocenter

Most chiral molecules in this course owe their chirality to a tetrahedral carbon with four different groups. But that is not the only route, and knowing that stops you from treating "stereocenter" and "chiral" as synonyms.

Allenes with the right substitution pattern are chiral: in R–CH=C=CH–R the two ends are held perpendicular to each other by the cumulated double bonds, producing a twisted, handed shape with no stereocenter at all. Atropisomers — biaryls such as substituted binaphthyls — are chiral because rotation about the bond joining two rings is blocked by bulky groups sitting immediately beside the joint, freezing a twisted shape. This is the basis of BINAP, one of the most important ligands in asymmetric catalysis. And helical molecules, including DNA itself, are chiral by virtue of being helices.

Why this matters biologically

Almost all of biology is built from chiral molecules: nineteen of the twenty proteinogenic amino acids are chiral and all are the L form; natural sugars are almost all D; DNA is a right-handed helix. Because the receptors, enzymes and transporters of a living system are themselves chiral, they can distinguish mirror images absolutely — the same way a right-handed glove fits only one hand.

The consequences are practical. Carvone's two mirror-image forms — a pair like this is called a pair of enantiomers, and the next two sections take them up properly — smell of spearmint and caraway respectively, because your olfactory receptors are chiral. Limonene's smell of oranges or of turpentine, depending on the enantiomer. Ibuprofen is sold as a mixture, but only the (S) enantiomer is active; the (R) form is slowly converted to it in the body. Naproxen's (S) enantiomer is the anti-inflammatory drug and the (R) form is a liver toxin, so it must be sold as a single enantiomer.

Thalidomide is the case everyone cites, and it is worth stating accurately: the (R) enantiomer is an effective sedative and the (S) is a teratogen, but the two interconvert in the body, so selling the pure (R) form would not have prevented the disaster. The real lesson is not "separate your enantiomers" but "know what each one does," which is now a regulatory requirement.

What carries forward

Chirality is the organizing idea for this entire chapter. The next six sections give you the machinery: how to spot the atoms that generate it, how to name the two forms, how to classify the relationships between several stereoisomers of the same compound, and how to draw them all unambiguously. And from Module 6 onwards, the stereochemical outcome of a reaction — inversion, retention, racemization — becomes a standard part of describing what a mechanism does.