Stereochemical Representations and Resolution

1. Wedge-Dash Structures

Wedge-dash notation depicts a tetrahedral center in three dimensions: a solid wedge is a bond coming out of the plane toward the viewer, a dashed (hashed) wedge is a bond going behind the plane, and plain lines lie in the plane. This is the most common convention for showing stereochemistry in skeletal and line-angle drawings.

2. Fischer Projections

A Fischer projection represents a tetrahedral carbon as the intersection of a vertical and a horizontal line. The convention is that horizontal bonds project toward the viewer (wedges) and vertical bonds project away from the viewer (dashes). The carbon chain is drawn vertically. Converting a Fischer projection to a wedge-dash structure means replacing each horizontal line with a solid wedge and each vertical line with a dashed wedge, then re-drawing as a normal tetrahedron; converting back reverses this. The configuration is preserved only if you keep the horizontal/vertical convention intact (an in-plane 90° rotation of a Fischer projection flips configuration).

3. Chirality Without Stereocenters and Resolution

Some molecules are chiral with no tetrahedral stereocenter at all. Allenes of the type abC=C=Cab are twisted so the two ends are in perpendicular planes, giving axial chirality when each end bears two different groups. Ortho-substituted biphenyls with bulky groups cannot rotate freely about the central single bond; the two rings are locked out of plane and are chiral (atropisomerism). In both cases chirality comes from a hindered axis, not a stereocenter. Because enantiomers share achiral properties, they cannot be separated by ordinary distillation or crystallization; resolution requires a chiral environment.

How it works

1. Choose a representation: wedge-dash for 3-D detail, Fischer for chains and sugars.
2. Apply the bond conventions (wedges out, dashes back; horizontal toward, vertical away) and convert while preserving R/S.
3. Check for axial chirality in allenes and hindered biphenyls when no stereocenter exists.
4. Resolve a racemic mixture by forming diastereomeric salts with a chiral agent, crystallizing them apart, and regenerating the pure enantiomers — or use chiral chromatography.

Common confusions

Memory aids

"Horizontal hands out, vertical back." In a Fischer projection the horizontal bonds wave toward you (wedges) and the vertical chain leans away (dashes). To separate mirror-image twins, make them "fraternal" first by turning them into diastereomeric salts.

Quick review

Topic Recap

Stereochemistry is communicated through conventions — wedge-dash (wedges out, dashes back) and Fischer projections (horizontal toward, vertical away) — that convert into one another while preserving configuration. Chirality can arise without any stereocenter through axial chirality in allenes and hindered biphenyls. Resolution uses chiral partners (diastereomeric salts or chiral chromatography) to obtain the single enantiomers demanded by modern pharmaceuticals.

Knowledge Check

1. In a wedge-dash drawing, what does a dashed wedge mean?
2. In a Fischer projection, do horizontal bonds point toward or away from the viewer?
3. Name one class of molecule that is chiral without any tetrahedral stereocenter.
4. Why can't enantiomers be separated by ordinary (achiral) crystallization?
5. What is the key to resolving a racemic mixture via diastereomeric salts?

Answers and Rationales

1. A dashed wedge means the bond points behind the plane, away from the viewer.
2. Toward the viewer — horizontal bonds are drawn as wedges pointing at you.
3. Twisted allenes (abC=C=Cab) or ortho-substituted biphenyls (atropisomers) are axially chiral with no stereocenter.
4. Enantiomers have identical achiral physical properties (melting point, solubility), so achiral separation methods cannot tell them apart.
5. React the racemate with a single enantiomer of a chiral agent to form diastereomeric salts, which have different solubilities and can be crystallized apart.