Drawing Conventions

One of the major challenges for someone learning organic chemistry is to make the mental connection between a 2-dimensional drawing of a molecule, and the real molecule in 3 dimensions.  There are several standard conventions that help us communicate the spatial properties of a molecule.

The first is the wedge-and-hash convention.

The solid wedge implies the group at the wide end is above the plane; for the hash, the group at the wide end is behind the plane.  Thus, 1-phenylethanol can be depicted as:

(A 3-dimensional ball-and-stick model is on the right.)

Heavy/dashed bond lines
A somewhat related convention for acyclic compounds uses bold bonds in place of a wedge, and a dashed line in place of the hash marks.  The carbon chain is depicted as a zig-zag chain of C-C bonds (remember that a terminal line with no group designator implies a methyl group):

One thing to note is that the actual conformation of the molecule may be different than the simplest depiction.  Rotations about single bonds can often relieve steric congestion, and internal attractive interactions (H-bonds and Van der Waals attractions) will usually cause a linear chain to curl up somewhat.  The molecule above, when minimized at the MM+ level, looks like this:

The Stereochemical Dot
In complex cyclic structures, it may be difficult to completely depict the locations of all hydrogens.  A convention developed in steroid chemistry is the "stereochemical dot;"  this explicitly depicts a b-hydrogen (one lying above the plane of the carbon).  Two alternative depictions of cholesterol are shown below.

In the bottom convention, and exocyclic bond is assumed to be "up" unless specifically given a dashed line.  Ring junctions must be explicitly designated; the one stereochemical dot makes the B-C and C-D ring junctions trans.
The Diamond Lattice
See this separate page.

Newman Projections
Often, the arrangement of groups about one bond reveal important aspects of molecular behavior.  A useful method of examining such interactions was devised by Dr. Melvin Newman.  You imagine yourself looking down the bond, and represent the bond by a circle.  The atom in front has bonds radiating from the center of the circle.  The atom in back has bonds radiating from the circle.  Thus, the 1,2-bond of cis-dibromocyclohexane may be represented as follows:


Fischer Projections
Fischer projections are most commonly seen in carbohydrate and amino acid chemistry.  The historical source of this convention stems from the problem of assigning absolute stereochemistry; prior to the advent of X-ray diffraction techniques, it was possible to deduce relative stereochemistry in molecules with two or more stereocenters, but not absolute stereochemistry.  Professor Emil Fischer invented a convention that allowed depiction of relative stereochemistry, but was ambiguous about absolute stereochemistry.  A crossing of vertical and horizontal lines to represent tetrahedral carbon could have either of two meanings; once diffraction analysis gave us absolute stereochemical information, we recognize the convention as indicating the vertical bonds going behind the plane, and the horizontal bonds coming out of the plane:

Thus, the open form of D-glucose has the following representations (Fischer projection on the left, wedge-and-hash convention on the right):


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 Last updated:  8/5/97
Comments to K. Gable