Chemistry · Ch 14 — Basic Principles of Organic Chemistry
Drawing the molecules in three dimensions
Drawing the molecules in three dimensions
Because most real organic molecules are genuinely three-dimensional in shape, chemists use four different conventions to project a 3-D molecule onto flat paper, each suited to a slightly different purpose. The WEDGE formula represents each individual bond with one of three line types depending on its direction in space: an ordinary straight line for a bond lying flat in the plane of the paper, a solid (bold, filled-in) wedge for a bond projecting up and OUT of the paper toward the person reading it, and a dashed (hatched) wedge for a bond going back INTO the paper, away from the reader. The FISCHER PROJECTION (or 'cross') formula instead imagines the molecule's main carbon chain held perfectly vertical, with every chain carbon drawn as a simple cross (a plus sign); by a fixed convention, the two horizontal arms of each cross are always understood to project toward the viewer, and the two vertical arms are always understood to project away from the viewer -- this convention is used especially heavily in carbohydrate (sugar) chemistry, where long vertical chains of stereocentres are common. The NEWMAN PROJECTION formula is built by looking straight down a chosen carbon-carbon single bond from one end: the nearer ('front') carbon of that bond is drawn as a single point at the centre of a circle, with its three other bonds drawn radiating outward from that centre point like the spokes of a wheel; the farther ('rear') carbon is represented by the circle itself, with ITS three other bonds drawn radiating outward from the circle's rim (circumference) instead of its centre -- this device is specifically built to show different rotational arrangements (conformations) about that one C-C bond, as illustrated for ethane. The SAWHORSE (also called andiron, or perspective) formula instead draws the C-C bond under examination as one long, deliberately-elongated diagonal line -- its lower end standing for the front carbon and its upper end for the rear carbon -- w …
What this figure shows. A generic tetrahedral carbon centre drawn with three different line types for its four bonds: one bond drawn as a solid (bold) wedge widening away from the carbon, meaning that bond projects up out of the paper towards the reader; one bond drawn as a dashed/hashed wedge, meaning it projects back behind the paper away from the reader; and the remaining bonds drawn as ordinary plain lines lying in the plane of the paper itself. This is the book's key legend figure explai …
What this figure shows. Shows the same two related compounds (labelled with COOH, CH3 and OH groups -- a generic vicinal-diol/hydroxy-acid pair used purely to illustrate the convention) drawn twice: once as a wedge-formula 3-D sketch, and once redrawn as a Fischer (cross) projection. In the Fischer form the vertical chain runs COOH at the top to CH3 at the bottom with OH group(s) shown branching horizontally off the cross at the relevant carbon(s); by the stated convention, each horizontal arm of the cross is understood to project towards the viewer and each vertical arm to project away, so the flat cross notation stands in for the same 3-D arrangemen …
What this figure shows. Two Newman projections of ethane (CH3-CH3) viewed end-on along the central C-C bond. In both, the front carbon is represented by a point at the centre of a circle with its three C-H bonds drawn as three lines radiating from that centre (spaced 120° apart, like spokes), and the rear carbon is represented by the circle itself, with its three C-H bonds drawn radiating from the circle's circumference. Representation (I) and (II) differ only in the relative rotational angle between the front and rear sets of three C-H bonds (i.e. two different conformations/rotational snapshots about the C-C single bond), illustrating that the Newman projection is specifically a too …
What this figure shows. Two sawhorse (andiron/perspective) drawings of ethane. In each, the central C-C single bond is drawn as one long diagonal line rather than a point, with its lower end representing the front carbon and its upper end the rear carbon; three C-H bonds radiate outward from the lower (front) end and three from the upper (rear) end, drawn spread apart (not intermingled) because the central bond is deliberately elongated. As with the Newman projections, (I) and (II) show two different rotational arrangements of the front three C-H bonds relative to the rear thre …