Common Sense Organic Chemistry

Chirality

Chirality - Asymmetry and Its Consequences

Chirality

Chirality, or more precisely optical chirality, is a particularly important concept in organic chemistry and by extension, biochemistry and biology. It is central to understanding stereochemistry—the study of stereoisomers (IUPAC: Configurational Isomers) or compounds that are constituted the same but have differ in their arrangement of atoms in 3-dimensional space. Both terms, however are used interchangeably and will likely never change since they refer to precisely the same relationship between compounds.

Handedness, is a more practical or familiar way to envisage the concept and the basis of its etymology (Gr. χειρ= “hand”). While the terms chirality center, chiral center, and asymmetric center all refer to the same thing, many people confuse these with the terms stereogenic center or simply stereocenter and seemingly credible sources imprecisely claim all these terms are synonyms. Chiral centers are a subset of stereogenic centers.

A stereogenic center and the stereogenicity that results in stereoisomers are broader terms. Since chirality is among the more abstract concepts in chemistry, indeed in science, finer distinctions will be made later or as needed for the following.

Major Types:

  • Optical – What organic chemists usually refer to when discussing the concept.
  • Axial – A special case involving sp-hybridized carbon centers (=C=) covered in college organic.
  • Helical – Refers to larger twisting bioorganic macromolecules.

What Is It?

In terms of definitions, the most concise statements of chirality refer to an object with regard to its mirror image. For example, hands (and feet) are chiral. If one holds their left hand up to a mirror what one sees is not a left hand because if the image in the mirror materialized, it would instead be a right hand. What’s more, a left hand and a right hand are not able to superimpose on each other. When aligned correctly, one cannot put one hand in the other’s place and have thumbs and little fingers correspond or map onto one another without breaking existing bonds.

Therefore, a real and not just perceived difference exists between them based on this asymmetry.  Logically, enantiomers must come in pairs of one of each. Two left gloves or two right gloves are useless practically. A right hand and a left hand, proper shoes and gloves, etc. all constitute pairs of enantiomers.

enantiomers: nonsuperimposable mirror images

As it relates to organic molecules, such relationships arise when one or more tetrahedral atoms is attached to four different groups. Any tetrahedral atom attached to 4 different groups is known as a chiral center. (Some metals form square planar complexes to four groups…they are not chiral centers). The first Nobel Prize in chemistry was awarded to Jacobus Hendricus Van’t Hoff for providing evidence that sp3 carbon is tetrahedral and not square planar and the source of its optical activity.

What You Need to Know

Despite the fact that chirality is a property of the molecule as a whole, one must be able to determine if individual carbons are chiral. Just like being able to determine how many hydrogens atoms are present on a skeletal structure even though they aren’t drawn in, with practice, spotting chiral centers will become like second nature.

©2017 Organic-ese. All rights reserved.