This page covers the different methods used for explicitly depicting the absolute stereochemistry of chiral carbon atoms within drawings of molecules. The purpose of this page is to help students rapidly determine the absolute stereochemistry of a chiral center, regardless of which method has been chosen for its depiction. [Note: This page does not cover (in detail) the "Sequence Rules" that are necessary for assigning absolute configuration; if you need to review those rules in detail before starting on this page, click below on "Specifying alkene geometry: the sequence rules."]
|absolute configuration||achiral||anti conformation|
|asymmetric center||chiral||chiral center|
|eclipsed conformation||enantiomers||entgegen (E)|
|Fischer projection||gauche conformation||Haworth projection|
|Newman projection||optical isomers||optical activity|
|plane polarized light||R,S convention||racemic mixture|
|sawhorse structure||stereochemistry||stereogenic center|
A molecule -- or any object -- that is not superimposable on its mirror image is said to be "chiral." (Perhaps the most familiar examples of chiral objects are your hands.) Molecules that are superimposable on their mirror image are said to be "achiral". Any object that has a plane of symmetry -- a "mirror plane," dividing the object in half and reflecting each half into the other -- must be achiral. Nails, hammers, balls, vases, and many other objects are achiral. Another familiar example of an achiral object would be your body: although it has "parts" that are chiral by themselves (hands, feet, etc), each of these parts has a mirror-image counterpart located on the other side of a plane of symmetry dividing your body in half.
Pairs of nonsuperimposable mirror image molecules are referred to as "enantiomers" of one another. Enantiomers are just one class of "stereoisomers," i.e., molecules in which the atoms are connected to one another in the same order (same "connectivity") but have different spatial arrangement. Cis and trans isomers of an alkene are one obvious example of stereoisomers.
In the context of organic chemistry, the most common structural feature for a chiral molecule to have is a carbon atom bearing four different substituents, as shown in the diagram to the right. Note that the carbon atom does not have to be bound to four different types of atoms. The substituents (W, X, Y, and Z) need only be different from each other in any way in order for the carbon atom to which they are attached to be referred to as a "chiral center" (also called an "asymmetric center" or a "stereogenic center"). Any molecule having a carbon atom bearing four different substituents will be chiral, and could exist as a discrete pair of enantiomers.
a)What is the simplest alkane (lowest molecular weight) you can think of that has a chiral center?
b)What is the simplest hydrocarbon you can think of that has a chiral center?
Self-test question #2--
In the Web page on "Nomenclature," you learned about (or reviewed) the trivial names for twelve alkyl groups having five or fewer carbons. Consider the bromides derived from each of those groups. Which of these compounds, if any, possesses a chiral center?
One should note that molecules can possess any number of chiral centers. Usually, the presence of "n" chiral centers in a molecule implies the existence of (at most) 2n different stereoisomers. For any given stereoisomer, only one of the other stereoisomers will be its enantiomer; these two are an "enantiomeric pair." Stereoisomers that are not enantiomers of one another are referred to as "diastereomers." Interestingly, the presence of multiple chiral centers within a molecule does not guarantee that the molecule itself will be chiral. "Meso compounds" possess multiple chiral centers, but each of these is the mirror image of another chiral center located symmetrically across a mirror plane dividing the molecule in half. In this sense they are similiar to your body; they have chiral "parts", but overall they are achiral!
Many organic molecules have three-dimensional geometry. Nowhere is this more evident than in the existence of chiral molecules. No object possessing a plane of symmetry can possibly be chiral, and a planar molecule -- like benzene -- would have to possess a plane of symmetry; it's the plane in which each of the atoms lies!
Unfortunately, we are constrained to using only two dimensions when we write on a sheet of paper. As such, organic chemists have had to devise various clever ways for representing three-dimensional molecules on a two-dimensional sheet of paper. This Web page reviews four of those methods, i.e., the ones that are likely to be encountered by students early in their study of organic chemistry. These methods are:
Of these four, the first and the last have the greatest relevance in terms of representing chiral molecules, in particular.
Wedge and dotted-line representations
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Version 1.2.1, 2/21/97