5.2 The Reason for Handedness in Molecules: Chirality

Chirality and Molecular Handedness

Chirality describes molecules that can't be superimposed on their mirror images, much like your left and right hands. This property determines how molecules interact with polarized light and with other chiral molecules, which has enormous consequences in biology and pharmacology.

Chirality and Molecular Mirror Images

A molecule is chiral if it lacks an internal plane of symmetry and is non-superimposable on its mirror image. The mirror-image pair are called enantiomers.

Enantiomers share identical physical properties like melting point, boiling point, and solubility. Where they differ is in two specific ways:

• They rotate plane-polarized light in opposite directions
• They interact differently with other chiral molecules (this is why biological systems can distinguish them)

Each enantiomer is designated R (right-handed) or S (left-handed) using the Cahn-Ingold-Prelog (CIP) priority rules, which rank the four groups attached to the chirality center by atomic number.

Chirality is everywhere in biology. Amino acids, sugars, and DNA are all chiral. Because biological receptors are themselves chiral, enantiomers of the same drug can have completely different effects. Thalidomide is the classic example: one enantiomer treated morning sickness, while the other caused birth defects. A less dramatic case is carvone, where one enantiomer smells like spearmint and the other like caraway.

Identification of Chirality Centers

A chirality center (also called a stereocenter) is an atom bonded to four different groups. In organic chemistry, this is almost always a tetrahedral carbon, sometimes called an asymmetric carbon.

To determine whether a carbon is a chirality center:

1. Look at the four groups attached to the carbon
2. Check whether all four are different from one another
3. If any two groups are identical, that carbon is not a chirality center

Common molecules with chirality centers include amino acids like alanine and serine, sugars like glucose and fructose, and lactic acid.

Molecules can have more than one chirality center. The maximum number of possible stereoisomers follows the formula $2^n$, where $n$ is the number of chirality centers. So a molecule with 2 chirality centers can have up to 4 stereoisomers, and one with 3 can have up to 8. (The actual number may be lower if meso compounds exist, as discussed below.)

Chiral vs. Achiral Molecular Structures

• Chiral molecules have at least one chirality center and lack an internal plane of symmetry. They are non-superimposable on their mirror images.
• Achiral molecules either have no chirality centers or possess an internal plane of symmetry. They are superimposable on their mirror images.

To determine whether a molecule is chiral or achiral:

1. Identify any potential chirality centers (carbons with four different groups)
2. Look for an internal plane of symmetry
   • If a plane of symmetry exists, the molecule is achiral
   • If there's no plane of symmetry and at least one chirality center, the molecule is chiral

Meso compounds are a special case worth paying attention to. These molecules do contain chirality centers, yet they are achiral overall. The reason is that an internal plane of symmetry bisects the molecule, so the chirality of one center is effectively canceled by the opposite configuration at the other center.

The tartaric acid / meso-tartaric acid comparison is one of the best ways to see why you can't just count chirality centers and assume a molecule is chiral. You always need to check for that internal plane of symmetry.

Optical Activity and Stereochemistry

Optical activity is the ability of a substance to rotate the plane of plane-polarized light. Only chiral molecules are optically active. One enantiomer rotates light clockwise (designated +, or dextrorotatory), and the other rotates it counterclockwise (designated −, or levorotatory) by exactly the same amount.

Molecules with a plane of symmetry are achiral and therefore optically inactive. This includes meso compounds: even though they contain chirality centers, their internal symmetry means they produce no net rotation of polarized light.

A racemic mixture (equal amounts of both enantiomers) is also optically inactive because the rotations cancel each other out. This is worth remembering because it's a common exam question: optical inactivity doesn't always mean the molecule itself is achiral. It can also mean you have a 50/50 mix of enantiomers.